Shipping companies and shipyards are assisted in evaluating the technical implementation of their ideas. The result is a tailor-made solution based on sound naval architectural principles and integrating all design aspects from the operational and shipbuilding side. 

Ship design and corresponding support is one of the core business areas of SDC and includes: 

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               e.g. weight estimation, speed-power estimations, freeboard, fuel oil               protection calculations etc. 

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Since the very beginning SDC has been known as a specialist for any kind of stability calculations for ships. Providing stability calculations for designers, shipyards and ship owners is a basic business of SDC. Our services cover calculation of all kinds of regulations: 

SOLAS and MARPOL-Conventions HSC-, IBC-, IGC-, MODU-Codes IMO Res.A265, A749, ADNR, etc... Deterministic and probabilistic damage stability of all kinds of regulation e.g. Water on deck Crane stability and optimization (more information: Services - > Other services -> Crane stability + Optimization) Loading and discharging manual Longitudinal strength Grain stability Ballast water management Mobile offshore drilling units Inclining and deadweight tests

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Hull Design Defects Part I

by David PascoeThis series of articles is written exclusively for marine surveyors to help identify the wide range of structural defects that can be found in boats and yachts. Because there is such a diversity in types of hulls, design styles and an ever-expanding array of new construction materials, it is difficult for surveyors to keep up to date on cause-and-effect evaluations.Related Reading:Hull Design Defects Part II

Whether the surveyor deals exclusively with prepurchase surveys, insurance

claims or marine expert related matters, learning how to locate, detect and

evaluate is a critical factor in the surveyor's work. This essay deals with basic

principles of hull design, along with cause and effect analysis of hull failures. It

will set the necessary foundation for this continuing series of essays.

Improper design and the improper selection and use of materials is the primary

cause of most non-damage related structural failures. Contrary to common belief,

actual manufacturing defects only rarely figure into structural failures. It should

come as no surprise to any surveyor that the boat building industry, much like the

automotive industry which, after more than 70 years of mass production, backed

up with their enormous financial resources, is still fraught with frequent design

defects. But unlike the automotive industry, boats are not manufactured in units

numbering millions, rather 10's and 100's at best.

Because of this, design faults are spread over a very wide array of different

builders and tens of thousands different models over the years so that rarely do

major design errors ever become widely documented. To make matters worse,

there are very few avenues for dissemination of information, and virtually no one

who maintains any kind of database on hull failures. This essay will attempt to

illustrate the most common defects, the cause and the visible effects that the

surveyor can use as a basis for conducting a thorough structural survey.

Structural Principles

Before we go directly into reviewing problems, its important that we first review

the major principles of hull design. From and engineering standpoint, fiberglass

boats have similarities to both bridges and aircraft airframes. A discussion of

these similarities will help us to better understand the forces that act on a boat

hull, and the structural principles required to build one.

Boats are similar to bridges in that the hull must have a framing system to

support it because the hull itself, like a bridge, spans a fluid substance. Whereas

a bridge spans air, a hull spans water, and while water is more dense, it is still a

fluid and offers lesser means of support that solid ground. Further, when a boat is

hauled out and set on blocks, often only one at each end of the hull, that hull then

literally becomes a bridge spanning open air. Unless the hull has an adequate

system of framing and girders to span the unsupported sections, like a bridge it

will buckle and collapse.

We can add to this the fact that boats are dynamic objects; they often travel at

high speeds over rough water and even occasionally, if not frequently, become

airborne. Thus, the stresses on a boat hull are far more than a matter of just

gravity and mass, but are multiplied by velocity and compounded by slamming.

And as anyone who has ever done a belly-flopper off a diving board knows,

water becomes hard as a rock when a wide, flat object falls upon it squarely.

Most bridges do not consist of a flat deck supported by girders underneath.

Rather, most bridges are either in the form of a truss, or they are suspended from

above by a combination of rigid and flexible supports. A boat is also similar to this

principle since the hull bottom and sides do not alone constitute the entire

structural framework. Boats that lack weather decks and superstructures, for

example, are far weaker than their cousins who do have these additional

structures. Thus, decks and superstructures also constitute major structural

elements of most boats and ships.

And here it is that fiberglass boats develop similarities to modern jet aircraft.

Aircraft utilize the principle of monocoque construction. That is, the body of the

aircraft does not have a frame but essentially is the frame. The skin of the aircraft

and the framing system are so closely integrated that they essentially become

one structure and its hard to tell where one ends and the other begins. Modern

jet aircraft are essentially flying pipes with wings, and it is from this engineering

principle that they gain their strength, despite the extremely light construction.

Modern fiberglass boats make use of this principle of monocoque construction

and in this way are more closely related to aircraft than they are to their wooden-

boat ancestors from which they evolved. A wood boat is the sum of its many

parts while a fiberglass boat hull is essentially one component. The combination

of molded hull and deck joined together creates a unified whole that is much

stronger than the sum of its parts. But boats are proportionately far heavier than

aircraft and are subjected to different stresses. Aircraft don't fly off the tops of

waves; boats do. While the bottoms of hulls take the major brunt of stresses, and

must be designed to withstand them, the monocoque construction still plays a

major role in providing strength to the overall structure.

There is no better illustration of this than the offshore racer type boat, a long

skinny hull equipped with tremendous horsepower. In the so-called "cigarette"

type boat, the deck provides a major part of the hull strength that, lacking a

strong deck, the hull would buckle. These decks are not "hull covers" but

designed as structural elements. These race boats are true monocoque

structures because the hull and deck structures are not screwed or bolted

together, but literally bonded together to become one piece.

Here's a good example of poor design and construction detail. Utilizing a glass over plywood framing system, there are no fillets under the frames or stringers which are butted hard against the hull. This creates hardspots with the propensity for stress cracking. In addition, the length-to-height ratio of the tall stringers creates instability where the stringers are likely to buckle under inpact loading. Additional framing between the stringers is needed to stabilize them. Also note that there are only two hull side stiffeners so that flexing of the sides is likely to cause hull/deck joint breakage. In the forward section, a dog leg in the stringer profile can be seen.

Dynamics of Hull Stress

Power boat hulls are essentially modified rectangles with a shallow vee on the

bottom. When a boat falls off, or slams down off a wave, the bottom impacts the

water and suddenly stops its downward movement. This sudden stop sends

shock waves up the hull sides that are then transmitted to the deck and any

upper structures that may exist. In the meantime, while the hull suddenly stops its

downward movement, everything inside the hull wants to continue on downward,

creating even more stress.

When the hull impacts the water, the resultant stresses work to cause the hull to

want to buckle transversely and longitudinally. The impact with the water is never

uniform along the length of the hull so that one end, or one side, of the hull is

more stressed than the other. One effect is to try to break the boat in half like

snapping a stick in half. The other effect is to bow the hull sides inward or

outward, the effect of bending along the horizontal plane. Yet another is twisting

or torsional stress along the entire length of the hull.

In actual operation under heavy conditions, the hull sides of most boats will

deflect to greater or lesser degrees depending on how well it is designed. This is

the result of impact loading, bending and torsional loading on the hull caused by

high velocity over waves, porpoising and so on. If you've ever wondered why so

many boats have rub rails falling off and weak and damaged hull/deck joints, you

probably thought that this was primarily due to hitting up against dock pilings. But

the real reason is that many boats have poorly designed hull/deck joints that are

simply lap joints screwed together. It is the stress transferred from the hull bottom

to the hull sides and thence to hull/deck join that causes the screws that join

these parts together to break loose. Putting screws into fiberglass is a terrible

means of making connections. Screw joins are simply too weak to work

effectively.

So it is that the deck - and the superstructure that is often integral with the deck,

i.e., are molded as one piece - are not only part of a unified structure, but also

absorb much of the load initially induced on the hull. This also accounts for much

of the damage and cracking found in and around deck structures, and why on

many boats windows, doors and hatches and portholes just never seem to stop

leaking. The whole structure is working so that no amount of caulking, bedding

and gasketing can ever stop the leaks because they just open up again

These are the effects of stress on the exterior boat hull and structure. But the

stress doesn't end there for we've not yet considered the hull framing system.

The framing system consists of stringers, bulkheads and frames in more

conventional construction. Yet increasingly builders are seeking to reduce costs

and streamline production by eliminating much of the detail work involved in the

framing system. They are doing this by again utilizing the principle of monocoque

construction which takes the form of premolded "liners" or so-called 'grid liners,"

a premolded combination internal framing system and accommodation

components. And rather than bonding these parts together with conventional

tabbing or taping, instead they are being glued together with some sort of

adhesive putty.

Although the use of liners has been around for a long time, the combining of a

framing system with a liner is new. And as any experienced surveyor can see, it

poses some obvious problems, but that's a subject I'll deal with in Part II. In the

meantime, the conventional stringer, bulkhead and frame system is the method

used by about 98% of all boats over 30 feet.

Stringers

In power boats, stringers provide the majority of the longitudinal hull resistance to

bending in the vertical plane. The apex of the vee at the bottom or keel adds

additional strenght. This is qualified by whether the deck is also designed to give

the hull longitudinal rigidity. Depending on design, some decks, particularly on

motor yachts with very short decks and lots of windows, are so small as to add

very little additional strength. On the other hand, the typical flybridge sport

fisherman with its long foredeck, relatively small windows and strong house

sides, adds a great deal of rigidity to a hull. So it is that we can now understand

why there is a lot more to the strength of hull than just the framing system. In

monocoque, or semi-monocoque construction, the whole structure must be

considered. And it is precisely here that so many untrained "designers" who lack

a solid background in engineering, make their mistakes.

Mistakes involving stringer design and installation are legion, about which a

whole book could be written. And yet the principles for creating an effective

stringer system are very simple and easy to achieve. Surely there are not many

designers or builders who do not understand this. Or are there? Problems usually

arise as a result of other design and marketing considerations. Typical examples

are when a designer wants to create a small boat with 6'6" headroom or wants to

install unusually large engines. The machinery spaces, which are not subject to

appearance and marketing considerations, are usually sacrificed.

In order to get the 6'6" head room or make high profile engines or other

equipment fit, the principles of proper stringer design are often sacrificed. In other

words, the principles of sound hull design get sacrificed for marketing

considerations and the surveyor needs to be constantly aware of this fact. Its the

primary reason why, in this day when all is known how to build a good boat, bad

boats are still being built. Give the customer what he wants, even if the product is

going to fall apart.

The principles of good stringer design are simple. They must run uninterrupted

from one end of the hull to the other. They must be of adequate height to width

ratio, i.e., structural modulus, to resist impact loading on the hull skin, be of

sufficient strength to carry the engine load, be stabilized against lateral

movement if high profile, and be securely attached to the hull so that they don't

break loose. The profile, or top of the stringer, should run in a straight line. If

there are any changes in the profile, then special design reinforcements must be

added.

Dog leg in stringer which was cut down to make the engine fit. The stringer proved to be so weak that the engine bounced every time the hull hit a wave, ultimately bending the shaft and wrecking the transmission. Also notice the hard spots created by the fuel tank mounting pads at top of photo that caused stress cracks in the hull.

These principles are often compromised by designs that utilized dog-legs, step

downs, step ups (meaning an inconsistent profile along their length), perforations

with large and ill-placed holes, inadequate section modulus and numerous other

faults. In nearly all the cases that I have seen, there is no compelling reason why

these faults should have occurred. What these design faults unfortunately

suggests is that the designers really don't understand the basic engineering

principles. Yet in most cases of failure that I have seen, the builder could have

had his cake and eat it too by giving a little more thought to the problem. What is

compromised in one way can always be built up in another. There's always an

alternative solution. The builder just didn't take the time to consider it.

Bulkheads

serve two very distinct functions. First, bulkheads act as transverse frames. More

importantly, the bulkhead is the structural element that prevents torsional stress

or twisting of the hull. Unified with a stringer system, they form a structural web

and a truss. Remove the bulkheads and its rather like removing the trusses from

a bridge or a roof. The overall strength can be reduced to the point of structural

failure. And because of the efforts of interior designers to produce small boats

with the appearance of wide open interior spaces by the elimination of full, and

even partial bulkheads, that hull structures begin to fall apart.

Here's what often happens when a large cut out is made in a structural bulkhead. In this case, the 3/4" plywood was fractured in three places.

One builder that produced a 34 footer which had only one partial internal

bulkhead - an engine room bulkhead that was only slightly more than half the

height of the freeboard of the hull - resulted in severe structural failures in much

of the model line. You probably know the boat, the 34 Wellcraft Grand Sport. In

this model line, not only did major hull skin and stringer failures occur, but in

many cases the single plywood bulkheads fractured from side to side.

Even companies with reputations for building very rugged hulls occasionally

make silly mistakes. In a nearby photo you will see the result when Bertram

decided to make very large cut outs in the centers of plywood bulkheads to save

weight. They unthinkingly removed all the strength from the plywood bulkhead

with predictable results; the bulkheads fractured.

And we know how engine room fore and aft bulkheads constitute one of the

foremost structural elements of medium size yachts, and we've witnessed what

happens the builder unthinkingly decides to cut a big hole in the bulkhead and

install a door. For whatever reason, it did not occur to the builder or designer that

he was destroying the structural integrity of the bulkhead.

This is another good example of the structural integrity of a bulkhead being defeated by cutting it full of holes. It is perforated like a postage stamp and is destined to fail.

To do their job, bulkheads must be adequately secured to the hull bottom, sides

and underside of the deck. Judging by 30 years of inspecting fiberglass boats, its

a fair statement to say that many builders don't think that this is very important

considering the large number of bulkheads that surveyors find to be broken

loose. Probably at least half of all boat builders don't tie the bulkhead to the deck,

and often for good reason. The bottoms of their boats are so flexible that the

bulkhead will telegraph the deflection of the hull into the deck, causing damage to

the deck. Therefore, if they leave a gap at the top, at least it won't tear the deck

apart, just everything else that the bulkhead is attached to, or is attached to it.

While we've been talking so far about structural bulkheads, bulkheads come in

several varieties, including full, partial and nonstructural partitions. While I know

of no published rules on the subject, my own rule is that to be classified as a full

bulkhead (1) it must span the width of the hull, (2) span no less than 75% of the

depth of the hull and be attached to the bottom, (3) have no openings larger than

50% of the height of the bulkhead, and (4) such openings must be centered in

the vertical plane and be adequately strengthened to compensate for the cut out.

An opening that effectively cuts the bulkhead in half is not a full bulkhead but a

partial. For maximum effectiveness, the bulkhead must be attached to all four

sides of the hull.

Floor frames under main mast of large sail boat. Properly designed by the designer, the builder apparently saw nothing wrong with drilling the frames full of holes. Here you can follow the fracture along the perforated effect of the holes at right and left sides. Frame was so weakened that ply separation also occurred. A marine surveyor got sued because he either did not find or report this condition, which was far more extensive than this photo shows.

Partial bulkheads are really nothing more than frames and do not serve any

greater function than frames. It is a mistake to call a hull partition with two doors

in it a bulkhead, for it is really only a partition, or a partial bulkhead at best.

Surveyors often mistake partitions for bulkheads. Remember that to be classified

as such, a bulkhead must be serving the purpose of tying the four sides of the

hull together (bottom, deck and sides). If its shot full of holes and openings, its

not achieving that purpose.

Partitions simply serve the function of separating one space from another while

providing little, if any major structural strength. Builders often make the mistake

of thinking that partitions are structural bulkheads and this is because they don't

have any trained engineers or designers on staff. And just because a partition

may be taped into the hull does not mean that its structural; the taping is usually

there just to hold the partition in place, not the partition to hold the hull together.

Sail boats and some smaller power boats often have plywood partitions that are

screwed to bosses on an inner liner. Again, these should not be mistaken for

bulkheads.

Frames

Frames serve the purpose of stiffening panels between bulkheads and stringers.

Fiberglass boats often lack frames where they are needed. Obviously, if a panel

is flexing too much, additional framing would prevent that condition. Some

builders scrimp on frames because frames create additional detail work and adds

more to labor cost. Fortunately, where excessive panel weakness is discovered,

adding frames after the fact is usually fairly easy to accomplish. So long as there

is accessibility, correcting panel weakness is usually not difficult or costly.

Rigid or Flexible Hulls

Aluminum and steel boats are examples of vessels built to be completely rigid.

By the nature of the material, these hulls will not tolerate flexing. Fiberglass

boats, however, are another story. Fiberglass boats can be designed to be either

flexible or rigid. For example, if you examine Bertram hulls built over the years

one can see a very abrupt change in hull design philosophy. Somewhere in the

mid 1980's, Bertram made a transition from very rigid hulls to fairly flexible hulls.

And as the Bertram engineers have proved from years of extensive R&D (they

were one of the few boat builders that took R&D seriously) you can build light,

floppy hulls without danger of them falling apart. Moreover, there is a legitimate

need to attempt to reduce costs by reducing the weight of the most costly

materials. All you have to do to see how this is possible is to look at the aircraft

industry which has invested billions in R&D.

In recent years, boat builders have been observing and borrowing some of the

fruits of this technology. Unfortunately, aircraft and marine design principles,

while having similarities, are not the same. Equally unfortunate is the fact that

some boat builders attempt to incorporate this new technology directly into their

products without any R&D of their own. And herein lies the problem.

It is entirely possible to take just about any hull and reduce its glass/resin content

by 25-35%. In fact, back in 1985 I undertook such a project by taking the plans

for a 55' Hatteras with a design weight of 72,000 lbs and redesigned to come in

at 42,000 pounds, including a huge 50% reduction in the weight of the basic hull

structure. This was done by applying basic airframe design with modifications for

marine. The end result had two serious problems that were anticipated. First, the

hull weight was reduced by means of an intricate framing system. The problem

with that was that anything that was saved on materials cost was more than

offset by increased labor costs of achieving the detail work.

Even less did I anticipate the effect on how the hull would handle with a 41%

overall weight reduction. Scale model testing revealed the boat to be so light that

it would pitch and roll so violently that it would be uninhabitable to human beings.

It developed a whip-snap roll in a 3' sea that would literally throw people off the

deck. Or when pitching, launch them like a trampoline! So much as for ultra light

boats. Weight is a factor that provides stability.

But the project did prove the viability of ultralite, flexible hull construction. Rather

like the old Cleveland Browns Rubber Band Defense, designed to bend but not

break. The point here is that builders can get away with a lot of shortcuts if they

know how to do it right, and if the increased labor costs don't make it impractical.

Its easy to design a flexible hull that flexes without breaking. What do I mean by

flexible? Well, if on a sea trial you run a tape measure between the top of the

engine stringer and the underside of the deck, you'll probably be surprised to see

the stringers flexing by as much as 1/2" even on what you consider to be a well

made boat. If you were to string diagonal measures from one corner of a large

compartment to another, in the manner used to measure squareness of square

or rectangular structures, you will find that when you put a boat into a hard turn,

one of those measures is going to go very slack. That's because the hull is being

twisted by the torsion of the turn.

The early models of the 60' Hatteras Convertible were a prime example of a large

hull that was inadequately bulkheaded. These hulls would twist so badly that

when you put it into a hard, full speed turn, the propeller shafts would bind up in

the bearings. And you can just imagine the effects on shafts, engines and

transmissions! This was not so much a matter of a boat with not enough

bulkheads, but rather the bulkheads that it did have were poorly designed and

executed.

Design-wise, rigid hulls are easier to design and build. With a flexible hull, very

rigid attachments of internal components becomes a problem because the flexing

starts to tear everything loose. The designer overcomes this by making the

interior sort of "free floating." For example, in designing a flexible hull, you do not

use the hull or framing system (stringers and structural bulkheads) as a

foundation for the interior components such as the sole and cabinetry work.

Instead, you build a shelf on the upper hull sides and literally suspend the interior

from the shelf. That way when the bottom flexes and the hull sides pant, it

doesn't work so hard to tear the interior apart.

Conversely, if the designer is confidant that the hull is rigid, he can go ahead and

place the soles on top of stringers (although this is never a really good idea) and

attach components to bulkheads or hull sides. For slow speed boats that don't

skip across the tops of waves, this is the way its usually done. The hull isn't going

to flex that much that its going to rip the interior apart. Whereas the slow boat

builder can get away with all sorts of haphazard design, the fast boat builder

cannot.

There are limits, of course, to just how far a designer can go with flexibility. In

terms of rigidity, we're talking about the difference of the bottom flexing 1/4 to

1/2" or not at all. With the increasing lust for speed and advent of high

performance diesels, flexibility causes serious problems. Flexibility is okay for

slow or moderate speed vessels, but becomes disastrous to high speed yachts.

The reason is not so much inherent in the hull structure itself, but rather in the

drive train. Delivering a thousand or more horsepower through a long and large

diameter shaft demands higher tolerances of the drive system, and therefore

mandates more rigid hulls, not less. Along the length of a 30' drive train, the hull

must be absolutely rigid; it cannot deflect or twist lest the whole drive system be

thrown out of alignment.

To gain an appreciation for the significance of this, just look at the massive

structural system found in high performance Hatteras or Vikings, shown below.

When you're dealing with a quarter million dollars or more worth of engines and

transmissions, it doesn't pay to fool around. Mistakes are just too costly. On

recent survey of a high performance 48 Hatteras and I was absolutely astounded

at the massive stringer system in this boat. Although I had seen it before, I didn't

really appreciated how large it was. The width of the top hat bottom supports

actually covered nearly 50% of the bottom panel area.

Stringer system of a 48' Hatteras Hi Performance Convertible. Note that the width of the top hats are about the same as the width of the bottom panel spans. This is a good example of structural overkill, yet demonstates the builder's concern with strength. Also note the webs between stringers under the engine mounts that provide extra stability. Despite the appearance, these top hats are actually quite thin. When slamming occurs, the thin sections will absorb much of the impact, hence the web sections to increase stability and insure that the engine beds do not move.

Now, did it cost the builder more to do it this way than in the usual way? Not

likely, they just had to spend some extra time thinking about what to do. The

actual execution and materials cost was probably no higher than any other

design. The point here should be painfully obvious; ultimately it costs more to do

it wrong that to do it right.

The bottom line is that whether a hull is successfully flexible or rigid is dependent

on design and function. In a high speed vessel, everything else about a hull can

be flexible, but the foundation of the drive system must be absolutely rigid.

Another point to remember is that the smaller the diameter of the shaft, the more

bending it can tolerate. Shafts from 1" to 1-1/2" can tolerate a heck of a lot of

bending caused by a flexing hull. But when you get up to 2" diameter, these

powerful systems will not tolerate movement of the foundation and the systems

will begin to self-destruct.

The importance of stringer stability is revealed by this stabilizing strut, in addition to the mounting frame above it. Yacht: 56' Magnum, 2600 HP. With this kind of horsepower, the mounting system and shafts will not tolerate movement.

Material Trends

If you read industry magazines like Composites and Professional Boat Builder, its

hard not to be impressed by these advertising vehicles efforts to influence the

use of aerospace composites and techniques into boat building. Every issue of

these two magazines devotes a major part of its space to promote the use of

exotic materials and very complex technology for building pleasure craft. In an

industry known for its trial and error, seat of the pants methods of development,

one could effectively argue that high technology is probably the last thing this

business needs to become involved with.

In my estimation, what they are attempting to do, is to promote and transfer these

high tech materials from the aerospace industry, which was backed up by the

bounteous source of federal tax dollars, to an industry well known for its critical

capitalization problems. They are promoting the very same technology utilized in

the production of military war planes such as the F117 and B2 bombers (the later

of which has a $2 billion per copy price tag) to the construction of pleasure craft.

Viewed in this light, the economics of this trend don't look very promising.

Currently the experimentation with these materials is largely confined to custom

boats with very wealthy patrons who are willing to foot the bill in order to posses

the latest and greatest. However, there has been some extension into production

building, mainly so-called niche markets such as race boats, both power and sail.

And to the extent that it is clear that the production boat building industry does

not possess the necessary capital resources, nor the profit margins to sustain

them, their incorporation of this technology into production building is very likely

to continue along the lines of trial and error. What this portends for the surveyor

are the risks of failing to locate design failures during surveys, failures involving

design, materials and construction techniques that fall into the realm of the

experimental. Make no mistake about it, experimentation with new materials

directly into a product is the norm, not the exception.

With this basis understanding of the principles of good hull design, we can now

begin to study the effects of what happens when these principles are violated. 

Related Reading: Hull Design Defects Part II

HOME>MARINE SURVEYING>

 

David Pascoe - Biography

David Pascoe is a second generation marine surveyor in his family who began

his surveying career at age 16 as an apprentice in 1965 as the era of wooden

boats was drawing to a close.

Certified by the National Association of Marine Surveyors in 1972, he has

conducted over 5,000 pre purchase surveys in addition to having conducted

hundreds of boating accident investigations, including fires, sinkings, hull failures

and machinery failure analysis.

Over forty years of knowledge and experience are brought to bear in following

books. David Pascoe is the author of:

"Mid Size Power Boats" (2003)

"Buyers’ Guide to Outboard Boats" (2002)

"Surveying Fiberglass Power Boats" (2001, 2nd Edition - 2005)

"Marine Investigations" (2004).

In addition to readers in the United States, boaters and boat industry

professionals worldwide from over 70 countries have purchased David Pascoe's

books, since introduction of his first book in 2001.

In 2012, David Pascoe has retired from marine surveying business at age 65.

Biography - Long version

 

Hull Design Defects Part II

by David PascoeAnyone who has ever seen airframe construction, particularly jet aircraft, understands why aircraft can be built with skins that are extremely thin. And while an aircraft isn't subjected to the same type of forces as a boat hull, the fuselage is the hull and must be strong in different ways. Rather than being framed, one could correctly say that an airframe is corrugated, for that's exactly what it is. The skin can be extremely thin because the frames are so close together.Related Reading:Hull Design Defects Part I

Boat hulls, of course, are not built that way, although they could be. Wooden

canoes or clinker construction is similar. Instead, modern fiberglass boat hulls

relay on a limited number of major girders and frames. Girders, or stringers as

they're called in yacht construction, serve a dual purpose of both supporting the

bottom and providing longitudinal rigidity to the hull. Frames provide lateral

support but very limited transverse stability so that they have only one purpose

and that is to support the bottom.

It is very helpful to think of a boat bottom as an upside down bridge. The main

difference is that bridges are not subjected to any force from the under side. But

boat hulls are subjected to forces from both sides. It is also helpful to think of a

boat hull not as a continuous, single skin, but as being made of panels that span

the stringers and frames. In hull design terms, the span between supports are

referred to as panels. These are the unsupported distances between supports.

When designing a hull, it is the thickness and strength of the unsupported panel

to resist bending forces that is of critical importance, precisely because the panel

is not supported. Our previous discussion talked about the differences between

flexible and rigid hulls. The amount of flexibility of the hull panel is dependent on

frame spacing and strength. For the purposes of this discussion, we'll assume

that the framing system is completely rigid.

There are very few pleasure yachts built in which the framing is so close and the

panels so thick that some bending does not take place under heavy load

conditions. In fact, flat fiberglass panels have a high modulus of elasticity,

meaning that they can bend a lot without damage to the panel. This is one of the

features of reinforced plastic that makes it so forgiving. But that forgivingness

induces the tendency for designers to stretch things a little too far in terms of

what they can get away with. If that weren't true, we wouldn't have so many boat

hulls with structural failures.

Fiberglass laminates, because they're not a rigidly controlled, machine made

substance, are subject to human error and variance in their uniformity. Neither

the thickness nor the quality of the lamination are subject to much control. This

means that while the same laminating schedule may be maintained throughout a

model line, the resultant strength of fiberglass hulls can vary widely from boat to

boat. Tests have shown that laminate strength on nominally "good" laminates

can easily vary by +/-33%. By "good" it is meant that the laminate has no major

defects, but rather simply variance in resin/glass ratios. This also explains why

one of an apparently same series of hulls fails while others don't. All surveyors

who are serious students of hull failures have encountered this anomaly that

often seems to defy explanation.

 Example of hard spot caused by improper stringer design and installation. Bottom hinges around hard edge of stringer wood core. At right (or bottom), wood core is elevated by a soft material so that it does not touch the hull skin and the load is bourn by the more flexible tabbing.

When investigating bottom panel failure, it is economically unfeasible to attempt

to evaluate the strength of a large panel, particularly one without a uniform

shape. To do so, every square inch of the panel would have to be analyzed.

However, if we could, there can be no doubt but that we'd find all sorts of

imperfections and defects. To illustrate an extreme example, in one case I found

two candy bar wrappers laminated into a hull. Accumulated saw dust caused by

the lay-up shop being located near the carpenter shop is yet another, not to

mention the fact that lamination is a sloppy, dangerous job that often entails a

very high turnover rate in workers that makes training very difficult and costly.

The point here is that final product values usually end up considerably below

design strength unless the designer leaves a healthy margin of error. Assuming

of course, that a degreed engineer is involved which, in many cases there isn't.

So what we end up with is many builders who utilize trial and error and

experience as a means of determining the lay-up schedule. The conscientious

builder will usually slightly over build, while the profit-minded builder will skimp

where ever possible. And here is where the problems begin.

Panel or Laminate Failure

There are three primary causes of panel failure: inadequate design strength or

thickness, design shape error, and lay-up faults. These can be stand alone

problems, or may appear in any combination of the three, including all together.

Assuming that a hull is properly framed out, and that the laminate does not have

serious imperfections, panel damage and failure can occur when the panel is too

thin. While fiberglass is flexible, there are limits on how much it can bend before

structural deformation causes the plastic to start disbonding or shattering.

Bending, as we know, causes tension on one side of the laminate and

compression on the other. Compression causes the plastic to crumble around the

glass fibers. Tension causes interlaminar sheer that works to separate the plastic

from the fibers, or ply from ply.

Now, in a typical laminate, particularly one using weaves, we have fibers running

in all directions. On the tension side, the fibers prevent the plastic from deforming

up to the limit of the strength of the fibers. When the stress exceeds the strength

of the bond of the plastic to fibers, these fibers then pull loose. When the bending

is repeated hundreds or thousands of cycles, this process then results in

significant weakening of the panel. You can't see this damage, but it is there.

This weakening becomes progressive and so the panel starts bending more and

more. Eventually, stress cracks begin to appear, usually first on the exterior, but

also on the interior particularly, if the inside of the hull is gelcoated or painted so

as to show up the cracks.

Hard Spots or Hinge Effect

If this condition continues long enough unchecked, it can eventually result in

fatigue failure of the panel. In the real world, this description of factors is rarely

this simple. All sorts of design defects and other faults may exist to compound

the situation.

Simple panel failure caused by inadequate thickness is both common and easy

to detect. Panel failure unrelated to any other factors always occur near the

center of the panel, or the periphery of the dimple caused by deflection. That is

because the center is the area least supported by frames.

Panel failures that occur close to, or exactly at the intersect of a frame (here a

frame is meant to be any structural member), then there is a contributory cause.

This is known as the "hard spot" or "hinge effect." Obviously, were the panel thick

enough, no hinging would take place, so hinging is always compounded by other

faults.

When a panel bends, at some point near a frame that bending is going to be

resisted by the frame. If the bending occurs exactly at the intersect of the panel

and frame, there exists at this point an abrupt resistance to bending. This sudden

resistance causes a bend to occur with a very short radius, and it is the radius of

the bend that has everything to do with how much bending can occur without

damage or failure.

The shorter the radius of the bend, the greater the compression and tension load,

the sooner structural deformation begins. This is why frames and bulkheads

should never make sharp intersects with either bottom or side panels. Short

radius bends are prevented by adding bosses or fillets in way of the panel/frame

intersect. This spreads out the load over a wider area, increases the radius and

reduces compression/tension loading. As previously mentioned, panel defection

in itself is not a bad thing.

Stress cracking is one of the visible effects of panel deflection. In this case, the panel was dimpled or "oil canning" as the eliptical array of the stress cracks indicate. The number of cracks tends to indicate the severity of the deflection or bending.

This short radius or sudden change in direction is what is referred to as hard

spots or hinge effect. It means the panel is bending sharply around the frame or

anything else inside the hull that is rigid such as a deck support post or a fuel

tank bed in contact with the hull.

Stress Cracks

Stress cracks are the warning signal that a panel is bending beyond the limits of

its strength. Stress cracks can appear either as a result of a one-time event such

as slamming hard off of a wave, or it can be the result of repetitive stress cycles.

This is one of the things that makes the evaluation of stress cracking so difficult.

My 30 years of experience suggests that fairly large numbers of boats sustain

single incident stress cracking with no evidence that the damage becomes

progressive. On the other hand, this can be extremely hard to know with any

certainty because no one has the ability to follow the life history of a boat. Yet,

because the surveyor encounters so many boats with stress cracks on the

bottom, it is the surveyor's task to make that evaluation.

 Natural hinge points such as chine flats and other angular surfaces require the build up of extra laminations called fillets which add extra strength. Lacking these, bending and stress cracking is likely to occur.

Most stress cracking occurs as a result of repetitive panel bending or hard spots

caused by improper design of internal components, combined with inadequate

panel thickness. This type of stress cracking is usually progressive because the

bending is not intermittent, but occurs nearly every time the vessel is used. The

great difficulty, and therefore the great danger to surveyors, is that most boats

get used more often in calm water conditions so that a potentially dangerous

condition can exist for years without ever resulting in a failure. It can happen that

a boat with a weak bottom is used only in protected waters where pounding

almost never occurs. Then, suddenly, the boat is moved to another location

where the conditions are different, and now the hull is subject to frequent

slamming and stressing.

The hinge effect, or stress cracking initiated by hinging off of an internal structural such as a stringer, produces parallel cracks such as these. The deposits made by weepage of styrene based fluids indicates that the cracks penetrate well beyond the gelcoat.

Evaluating stress cracks is very difficult and there are no clear-cut rules. Every

case has to be evaluated independently. Yet hard spots are easy to identify

because the area of cracking is usually small. It may show as a star burst

pattern, as concentric circles of cracks, or a short series of parallel, arcing

cracks. These are easily evaluated simply by going to that exact point on the

interior to see what is causing it, and then to recommend a method of eliminating

the hard spot. Moreover, reinforcing an apparently weakened panel is usually not

difficult to accomplish.

Cracking due to overall panel deflection

Cracking due to overall panel deflection is also easy to detect, but much more

difficult to evaluate. On bottoms painted with anti-fouling paint, remember that the

paint is very brittle and does an excellent job in magnifying cracks. Cracks will

usually appear in the paint long before they will show up in gelcoat which is

usually a bit more pliable. By removing the bottom paint, we can usually find out

if the cracks also show up in the gelcoat. Be careful not to obliterate cracks in

gelcoat by scraping as this is easy to do. If cracks don't show up in the gelcoat,

I'll use my finger and try to rub dirt over the surface to try to get invisible cracks to

show up, then wipe clean with a rag. If the cracks are serious or old, they'll stand

out. If nothing shows up under the paint, it would be a fair assessment to assume

that the weakening is not serious.

The age of the boat

The age of the boat plays a very important role in evaluating the significance of

stress cracking. This is because older boats have been subjected vastly greater

number of stress cycles than newer boats. If cracks show up in the paint on an

older boat, but don't appear visible in the gelcoat, or are only faintly visible, I

usually dismiss them. If the condition has existed for a long time and there's no

evidence that it is highly progressive, I feel safe with that judgment.

The prominence of cracks

The prominence of cracks is another indicator of their significance. When cracks

initiate, they usually start out as a very fine fissure. As cracks age, the very sharp

edges of the crack will erode over time. That means that the appearance of the

crack will be wider or more prominent. Cracks that stand out prominently should

be regarded as a red flag. Cracks that are old and progressive will stand out

clearly, even after you've scraped bottom paint away. At this point, the cracks will

appear as a clear black lines. If bad enough, they will clearly reveal a fissure.

Remember that stress cracks that appear on the exterior bottom are the result of

the tension side of the bend since compression loading tends not to produce

cracks.

Examining the same area on the interior is likely to tell us much more about the

significance because the tension loading may appear on this side also,

depending on how the panel is bending. However, this is only true if the interior is

coated with gelcoat or paint. If it is a raw laminate surface, stress cracking may or

may not show up, especially if the surface is very dirty and permanently stained.

Dirt and oil may work its way into the stress cracks and completely obscure them,

even if you wipe the surface clean.

The number of parallel cracks

The number of parallel cracks is another indicator of how serious the condition is.

When there are 4 or more parallel cracks, there is good reason to believe that

panel bending is going beyond load limits. But, again, we have to evaluate in

terms of age. If its a fairly new boat with a three or more parallel cracks, odds are

that this is a progressive condition that could ultimately lead to panel failure.

Sail and power boats tend to exhibit different cracking patterns. This is because

sail boat bottoms are usually curved while power boat panels tend to be flat.

Flexing convex curves result in the condition known as oil-canning which

produces large dimples that can reveal circular patterns of cracking, or cracking

that appears in a parallel series of arcs. This condition should be considered as

dangerous with a high potential for ultimate failure.

Power boat panel defection tends to parallel either hull stringers, bottom strakes

or bulkheads. It will only show up as curving arcs if the panel defection is severe

in conjunction with oil-canning. This happens rarely, so when it does, beware that

the problem is very serious indeed. The most common cracking is found inside

the concave curve of a strake. This is because the strake forms a natural hinge

point, or a hard spot. Strakes that are not filled and filleted almost invariably end

up causing stress cracking. This is easy to determine by looking to see if there is

a strake depression on the interior of the hull. If there is, the strake has not been

filled and filleted and is the cause of the cracking.

Stress cracking appearing transversely across a bottom strake. In this case, it was caused by a 27' boat having only one structural bulkhead, located in the wrong place and no transverse framing. Torsional twisting of the hull caused the cracking which is on the verge catestrophic failure.

Improperly installed stringers often cause hard spots, particularly in smaller

boats, that results in cracking and possible ultimate long term failure. This is

usually very easy to detect by sounding the bottom and locating the stringers.

Again, it is the age of the boat and the severity of the cracking that determines its

significance.

These stress crack appearing on the interior bottom under an engine were nicely shown up by black diesel oil via the capillary effect. The 11 parallel cracks indicate that the degree of panel bending is severe and the possibility of failure must be considered. In this case, there were an insufficient number of transverse frames.

Delamination

Delamination, contrary to what one might expect, is not a common occurrence in

conjunction with panel deflection and stress cracking on solid laminates. In fact, it

is very rare. Out of 3600 surveys, I can't ever recall having found any. Panel

bending does not produce enough interlaminar shear to cause ply separation

unless the panel contains defects and the bending is very serve. Still, its a good

idea to tap around a bit when cracks are visible. However, delamination is often

found after complete panel failure occurs - i.e. the panel splits open - but this

happens as a result of the final fracturing and stress initiated during the final

failure mode, so it is not wise to use the absence of delamination as a positive

evaluation factor.

Cored Bottoms

Cored bottoms are an altogether different story. But then a cored bottom is a

problem just waiting to happen anyway. Coring a hull bottom is just plain foolish,

no matter what any builder or the glowing reports in the magazines may tell you.

Remember that these people get their income from advertising revenue derived

from the people who advance these materials. In other words, they are biased.

Core materials are simply too weak and hull bottoms take too much of a beating

for cores to survive. When we find cored bottoms, the presence of stress

cracking should be regarded with the same reaction as to skin cancer. Horror!

Here, stress cracking raises the potential for water ingress into the core, with all

the attendant problems that poses, including the potential for delamination. When

cores are involved, ply separation or delamination is highly likely. Consider the

hull guilty until proven innocent.

We should be especially wary of sailboats with cored bottoms. If you get sued

after a sailboat core fills with water or its keel falls off, take your lumps because

you deserve to get sued for not finding the problem. There is only one safe way

to handle a cored sailboat bottom, and that is to declaim all knowledge of what is

going on inside that core. You can't see it, test it or know what is happening

inside unless a failure is already well advanced. Failures involving cored bottoms

are legion. Even worse, it can happen that there are no visible, outward signs of

trouble before failure occur. Failures can occur suddenly, and without

warning.Disavow all responsibility, in writing, in detail, and all ability to determine

the condition of the hull.

To determine whether a hull is cored or not, look for those areas on the interior hull where the core terminates. In this photo, the core can clearly be seen standing out around the bow of this yacht.

Hull Sides

Stress cracking on hull sides is something that generally did not happen until the

mid 1980's when builders began skimping on hull side laminate. Hull sides have

gotten so flimsy in the last decade that its almost laughable if the construction of

some of these boats wasn't so pathetic. Hull side cracking is a problem that

shows up almost exclusively in low to mid price range boats. The cheaper the

boat, the more likely you'll find it.

This is rarely a problem to the hull side itself. The cracking usually occurs as a

result of severe panting because the sides are thin and unsupported. The sides

themselves don't fail because the panels are so large and the flexing occurs over

such a large area that the radius of bend is too large to cause damage to the

laminate strictly as a result of panting. However, because the stress is

transmitted vertically up the hull side, the forces of interlaminar shear are very

high. Therefore, delamination becomes a distinct possibility. Nowhere is this

more true that in vessels that have been repowered with more powerful engines.

In this case, total hull side failures have been known to occur even with high

quality boats.

Hull side panting can be a problem because there are other things attached to

the hull sides. Panting of the sides can cause disturbance not only to internal

components, but also to the hull/deck joint. A boat with floppy hull sides is not

very likely to have the deck bonded to the hull, but rather just be screwed on.

Panting hull sides almost invariably results in shearing of the hull/deck joint and

complete loosening of the fasteners. This is why we find so many boats with the

rub rails falling off. The screws holding the rub rails on are set into the hull/deck

joint and the shearing load applied to these screws breaks them loose. See

AlsoSCREW IT!

Hull sides should be generally sounded (cored or solid) and closely examined for

stress cracking. Stress cracking is most easily telegraphed through painted on

boot stripes because the paint is more brittle, so look for it there. Because side

failures are rare, the cracking needs to be evaluated in terms of the whole

structure. The cracking is more likely to be a sign of other problems. Suspect bad

deck joins and look for broken bulkhead tabbing. Also watch out for through hull

fittings that may be loosened or damaged as a result of panting.

Liners

Builders are always searching for ways to reduce labor costs, and one of these is

the use of interior liners. Experienced surveyors are all too familiar with the

problems that liners present. First is that a liner tends to obscure all internal

structural members so that the surveyor cannot make an evaluation of the hull

structure. Secondly, liners tend to preclude the use of proper bulkheads and

frames because a liner can't be placed where a structural bulkhead exists.

Thirdly, the design of the liner needs to substitute for the structural members is

has displaced. Fourth, the liner usually affords no access to examine its structure

and how it is attached.

Liners are most commonly used in boats up to about 32' but have been found in

boats up to 42'. The larger the boat, the greater the potential for trouble because

of this tendency of liners to displace or eliminate traditional framing methods.

Boats with liners over 30' are known to have a disproportionately larger number

of structural problems, a situation that is entirely predictable.

When liners displace bulkheads and frames, several things happen. First, the hull

becomes highly prone to twisting or wracking. When bulkheads are eliminated,

unsupported panel size naturally increases. And when that happens, panel

deflection and failures increase. Because the liner usually covers up so much of

the interior, this makes the surveyors job doubly difficult and exposes him to

more risk of failure to locate serious problems.

Grid/Liners

Grid/liners are a new development in which the designer attempts to include all of

the vessel's framing system into a full, complete hull liner. So far, the use of grid

liners is limited to only a few builders of small boats, but the idea is likely to

spread because it presents the possibility of eliminating all the difficult laminating

detail work of bulkheads and stringers inside the mold. With a grid liner, the detail

work can be transferred to a low profile mold on the shop floor that is more

accessible and easier to work.

While this may streamline production, this method has a number of problems.

One is that the liner has to be bonded to the hull, and obviously the builder

cannot laminate it to the hull once it is set into place. The only solution, of course,

is to glue the liner into the hull. The problem with adhesives is that they only work

perfectly under perfect conditions, something we don't see much of in boat

building.

The only things that glue together well are parts with identically uniform surfaces.

For example, gluing two pieces of wood together that are perfectly flat makes for

a very strong joint. But allow the slightest surface irregularity and the joint

becomes very weak. That's not just true of wood but any material. Unfortunately,

the interiors of laminated hulls can hardly be called uniform. Will the grid/liners

remain bonded to the hull? Only time will tell. Our experience with bonding putty

in cored hulls tells us that there's not likely to be any better level of success in

this application than for foam cores. See related article Hi Tech Materials.

Essentially what they are doing is spreading the glue on the interior of the hull,

and then dropping the liner in and hoping that a complete bond takes place. The

builder will never know because he can't see the results. The bonding surface is

just as likely, indeed, probably more so, to be full of voids or gaps where the two

parts are not bonded together. And a void in a glue joint or laminate is a stress

initiator that propagates delamination.

Sail boats utilize grid/liners more frequently. Fortunately, a sailboat hull is

considerable more amenable to this design, both by its shape and the fact that

they are not subjected to the forces of high speed. Even so, one of the largest

boats built with a full grid/liner was a Hunter 60 that experienced total liner

disbonding and failure. Yet even their smaller models were widely known for liner

failures.

If this method gains wider acceptance, its going to pose a whole new range of

problems for surveyors.

Interior Effects of Weak Hulls

When liners are used, they either have to sit on top of stringers, the bottom, or be

suspended from the hull sides. In either case, the liner is not completely isolated

from the hull, and if the hull is experiencing problems with excessive panel

deflection or panting, that deflection is most likely going to be transmitted to the

liner in one way or another.

In sailboats, liners are either tabbed, glued to the hull or both. In powerboats,

liners usually rest on top of stringers and are usually joined to the hull at the

deck, whether by bonding or mechanical fasteners. Flexing of the hull is usually

transmitted to the liner. Linered boats usually have a number of wood

components inside such as cabinets, trim seating and the like. These

components are usually fastened to the liner with screws. If both the hull and

liner are flexing, then it is common to find evidence of this. Look for screws

backing out, misaligned parts, cracked moldings and little piles of wood dust that

indicates friction against the wood. Unusually large gaps between parts or things

like built-in refrigerators backing out of their holes are often indicators of trouble.

Extensive stress cracking in liners is another indicator. Theoretically, the liner

should not be subjected to much stress, so when cracks appear the condition

requires careful evaluation. Be particularly alert to stress cracks around

companionway doors and at the bottom corners of the sole or foot well. Serious

cracks in these areas are a strong indicator of serious working. Another critical

area in power boats is the coaming around the windshield area where the fore

deck terminates into the cockpit. Cracks in all three locations indicate serious

trouble. If that's the case, also examine the hull/deck joint. If the screws are loose

at the mid section, the hull is probably bending excessively and may indicate a

serious structural design flaw.

The Effects of Speed

The faster a boat goes, the more stress it is subjected to. It follows, then, that

high speed boats are considerably more vulnerable to design defects. This also

means that that any evidence of stress cracking or other problems needs to be

evaluated relative to the vessel's speed, as well as it's age.

Unless you've had experience with high speed vessels in rough water conditions,

its hard to appreciate the extreme forces involved. Considering how hard it is on

the human body, its a little easier to imagine the stress on the hull. High

performance boats, those which are intended to give the impression that they're

capable of being operated fast and hard, are those that are most susceptible to

problems simply because they are used harder. And because of that, they are

substantially less tolerant of design flaws.

A good example of this is a 41' Cigarette race-style boat which was really a

tripple engine, 1400 HP, luxury go-fast boat. This boat was designed with

stringers that steps in them. That is, that the stringers had different heights at

different locations in the hull. At the only full bulkhead in the vessel - the

cockpit/cabin bulkhead, the stringers stepped down from 24" high to 12" high.

This created a serious stress initiator point which caused the stringers to fracture

at this point. Not only that, but the bulkhead had broken loose because the hull

was bending longitudinally so bad that the hull sides were bowing outward and

the hull/deck joint popped open.

This boat had a full interior liner and no part of the hull other than the aft engine

room was visible. The failures were foretold by serious cracks in the cabin liner,

around the companionway door, as well as the very loose guard rails. These

cracks were sufficiently severe that it was clear that they were not the result of

normal stress or improperly designed curves or a generally weak liner. The

combination of all these indicators pointed to a hull that was starting to break in

half.

In another case, also a Cigarette, the builder had tried a three, rather than four

stringer arrangement, with one stringer on the centerline. Apparently the designer

did not know that no stringer was needed at the vee of the bilge because this

was a natural strong point. Yet stringers were needed outboard on the bottom

panels where there was now only one on each side instead of two. The stringers

on both sides fractured and the bottom split open.

Yet another was the case of a Wellcraft 40 footer which had only one transverse

bulkhead, and in which the transverse frames were not bonded to the stringers.

These stringers were very tall, glass over plywood, and when pounding occurred,

the stringers buckled because, lacking any bonding to transverse members, they

had no lateral stability because they were too thin.. This hull began to self-

destruct during the delivery from builder to the owner in less than 30 hours

operating time.

Another builder designed and built perfectly good stringers, but then proceeded

to drill them full of three inch diameter holes for reasons known only to the

builder. The degree of ignorance displayed by these builders was truly

astonishing.

Examples like this should lay to rest forever any assumption that boat builders

always know what they're doing. All too often they don't. Surveyors should be

mindful of the fact that, more often than not, boats are designed not by naval

architects, but by people with no formal training whatsoever. This is not to say

that unschooled designers are not qualified. Many are if only by experience of

trial and error. Unfortunately, too many people from the marketing department

are actively involved with structural design when they shouldn't be.

These issues are raised, not to be gratuitously critical, but to point out how easy

it is for a surveyor to take structural design for granted, and to fall into the trap of

not looking closely. Luxury and high speed are rather like trying to mix water and

oil. The mix is not easy to achieve. The effects of speed multiply relative to the

mass or weight of the vessel. To create a high speed yacht that does not start to

fall apart when abused - as they are likely to be - requires some serious

engineering, engineering that as often as not is lacking. Paying close attention to

these warning signs will go a long way toward keeping the surveyor out of

trouble.

Related Reading: Hull Design Defects Part I

HICE STRENGTHENED SHIPS

Antarctic Ships: Overview  Historical: L'Astrolabe & Zelee | Aurora | Belgica | Discovery | Endurance

Erebus / Terror: Ross-Antarctica Franklin-Arctic Franklin time-line Franklin-Map | Fram Fram 2 | Nimrod | Terra Nova

Modern: Ice-strengthened and icebreakers | James Clark Ross | Kapitan Khlebnikov | Yamal

Ships then and now - a comparison

In the very earliest days of polar exploration, ice-strengthened ships were used. These were originally wooden and based on existing designs, but beefed up. Particularly around the waterline with double planking to the hull, strengthening cross members inside the ship and bands of iron around the outside and / or metal sheeting at the bows, stern and along the keel.

Such strengthening was designed to help the ship push through ice and also in case the ship was "nipped" by the ice. Nipped is an innocuous sounding word to describe a terrible and powerful event when ice floes around a ship driven by winds and tides (often many miles distant) push against the ship trapping it as if in a vice and causing damage - sometimes damage enough to reduce the ship to match-wood.

Such damage might be survivable, it might cause the loss of the ship when the ice finally relents - the ship now no longer being able to float as happened to Shackleton's Endurance or it might cause the loss of the ship in as little as 15 minutes from first pressure being exerted. In the days of wooden ships, the only vessel that could survive such treatment was the Fram, built for Fridtjof Nansen. The Fram was prodigiously strong, but it's chief defence was that when squeezed from the sides it would respond by rising up due to a rounded hull shape. Even the mighty Fram at one point looked to be be in danger when ice floes built up to such an extent that they might fall on it and prevent it rising when squeezed.

These days, ships that go to the polar regions are of course no longer made of wood, but of steel. They still need to be specially strengthened to work in ice conditions. An ordinary ship with no strengthening will not risk touching ice at all, no matter how gently. A modern ship weighing thousands of tonnes meeting an iceberg weighing perhaps as much again or up to thousands of times more can easily sustain enough damage to require major repairs or to sink her. Ice will easily hole a non-strengthened ship.

Ships therefore that have any chance of contacting ice are at least ice-strengthened if not being designed to plough through the ice as do ice-breakers.

Icebreakers are needed if there is a trade route to keep ice free, if there are military reasons for patrolling in areas with heavy sea ice or if you need to work in heavy ice conditions, particularly in winter. Icebreakers are expensive to build and very expensive in fuel to run (sometimes powered by gas turbines or a nuclear generator). They are uncomfortable to travel in on the open sea. All ships designed for the ice have rounded keels with no protuberances, these things provide stability in normal ships and result in ships that are designed to contact ice rolling heavily in a even a light sea.

Rounded keels and a lack of stabilizing fins means that progress is quicker and smoother through ice and that there aren't any parts to be ripped off. A further discomfort comes from breaking through continuous thick ice with constant vibration, noise and jarring against the ice.

Icebreakers are generally owned by those countries with an interest in the north-east and north-west passages in the Arctic or that have other shipping lanes and ports that need to be kept open during the winter months.

Ice strengthening on the other hand is found much more commonly in ships designed for Arctic or Antarctic work. There is no actual universal definition of what needs to be done to a ship to be "officially ice strengthened" and it can be applied to all manner of ships, whether supply ships, tankers, container ships, warships etc. Commonly ice-strengthened ships can cope with continuous one year old ice about 50cm - 100cm thick.

Breaking ice by any ship is not a case of forcing the ice aside, but by the ship riding up and over the ice in front of it, with the weight of the ship then breaking the ice, this may be a continuous process or can result in a lot of back-and-forth in particular thick places.

Characteristics of Ice BreakersIce breakers have the features of ice strengthened ships and then some of their own too.

Heavy for their size, to make them more effective at breaking through ice when they are pushed up above it by their engines.

Very gradual upwards slope at the bow, particularly at the water line to allow the bow to ride up over ice before the weight breaks through.

Hull made from special steels designed for optimum strength at low temperatures Air bubbling systems to assist ice-breaking. Air is forced under pressure from 2m or so below the water line where ice is met, helping to break it and move it out of the way.

Heated water jets below the waterline to help when breaking through ice.

Ability to rapidly move large amounts of water ballast within the ship to shift the weight when needing to break ice. The ships can be rocked from side to side in this manner.

Hull divided by bulkheads into a series of watertight compartments in case it is holed.

Extra thick steel at the bow, the stern and at the waterline.

An "ice horn" to protect the rudder and propeller when in reverse, and an "ice knife" in front to protect it when in forwards motion.

Electric propulsion to the propellers. Electric motors can apply torque when not actually turning or when only turning slowly, so hitting a large piece of ice will not stop the engine.

Extra strong propellers with replaceable blades. There may also be a propeller inspection well to examine them in operation and the facility to change blades while at sea.

Very powerful engines. The engine may be diesel possibly with extra power supplied by gas turbines for ice breaking or be nuclear powered.

Powerful searchlights for use in dark winter conditions.

Characteristics of ice-strengthened shipsMost of the ships that supply Antarctic bases are ice strengthened rather than full blown ice breakers.

Double hull, with a gap between them, the gap may be air or filled with water ballast. If the outer hull is punctured the inner will hopefully not be.

Flat hull shape with a rounded rather than pointed bow. This allows the front of the ship to drive forwards, rise above the ice and then let the weight of the ship break the ice.

Specially formulated hull polymer paints for strength and also low friction when in contact with ice.

Special engine cooling arrangements so that the inlet for water taken on board to cool the engine doesn't get blocked with ice - likewise the water outlet.

No stabilizers or any other kind of hull protuberance that might get ripped off by ice

Helicopter, for scientific work, but also for spotting leads and open water in the ice to guide the ship.

Rudder and propeller protected by the shape of the hull, so that ice moving backwards is less likely to cause damage.

Thicker than normal steel, particularly at the bow and at the level of the water-line

Reinforced "ice belt" that typically extends about 1m above and below the water line. This is where the hull has thicker steel and also has extra internal ribs to help the stiffening. These are usually twice as many of these ribs than in a comparable "normal" ship.

Powerful bow and stern thrusters to help maneuvering in tight spaces such as pack ice.