SEAKEEPING AND MANOEUVERING

Seakeeping is an aspect of the overall performance of a ship concerned

with its behavior in rough weather. Seaworthiness reflects the capability

of the ship to survive all dangers at sea, such as collision, grounding,

fire, as well as heavy weather effects related to the environment in

general and waves in particular.

Most ships are designed to operate in an environment which can be

hostile due to winds and waves. Regardless of their type or size, the

operational effectiveness of ships will be degraded by the adverse effects

of the environment. Therefore, the technological success of a ship

design hinges upon a good seakeeping design.

It should be the objective of the designer to minimize this degradation

and ensure that the safety of personnel and equipment on board is

achieved.

Consequently, in today's competitive environment it is becoming

increasingly important for ship designers to demonstrate that their

proposed design has good seakeeping performance characteristics.

Today, it is widely accepted that seakeeping is a consideration that can

affect the final decision because it can affect the system’s cost (or profit)

through voluntary/involuntary speed reduction.

The way the various vessels meet and respond to those waves depends

strongly on their dimensions. The sea that is giving large motions to a

small tug boat or fishing vessel may go all but unnoticed by a passing

supertanker. Conversely, the structure of a Supertanker may be stressed

by a swell that, while awesome to view, may have little effect on the

operations of the smaller vessels.

In general the operability is increased by larger principal dimensions.

Obviously economic considerations impose limitations on the principal

dimensions, particularly length.

A ship in waves experiences motions in all six degrees of freedom, three

translational; heave (vertical motion), sway (transverse motion) and

surge (longitudinal motion superimposed on the steady propulsive

motion) and three rotational; roll (rotation about a longitudinal axis),

pitch (rotation about a transverse axis), yaw (rotation about a vertical

axis). All six are unwanted except in the special circumstance where yaw

is necessary in changing course.

Ship Motions and Related Ship Responses

Rotational motions Translational motions

Heave, pitch and roll require particular mention because they are the

three motions with hydrostatic restoring forces (and moments), and

therefore possess natural response periods and the potential for

resonant behaviour. Heave tends, in almost all cases, to be well

damped, so resonant heave is usually not a problem. However, for a

surface effect ship (SES) natural heave oscillations can be excited due

to high encounter frequency between the vessel and waves. The

compressed air in the cushion causes heave resonance. Heave motion

is also important for offshore drilling vessels. Pitch likewise is usually

well damped. Pitch damping is maximized by full waterplane

development in the ends (high CWP) and relatively high beam-to-

draught ratios overall. Pitch motion is one of the main concerns in

head and bow seas. A strong pitch motion is usually followed by deck

wetness or bow slamming which are generally of greater importance.

Heave, Pitch and Roll

Roll creates problems in a number ways. It can cause structural

damage to fittings and equipment. It can cause people to thrown into

bulkheads and equipment, resulting in injuries. Roll also affects the

ability of the crew to perform their duties efficiently. In a warship,

most weapon and sensor systems have roll limitations which severely

hamper the capability of the warship to carry out its prescribed

mission. The problem of rolling is usually regarded as one of

inadequate damping. As the only underdamped motion, rolling

exhibits true resonant behavior. Traditional remedies have included

bilge keels and antiroll tanks, both of which perform well at slow

speeds, and passive and active antiroll stabilizer fins, which are most

effective when moving at speed.

Most large ships are fitted with what are called bilge keels. When a

vessel rolls, the keels move large masses of water and also create

turbulence. This acts to resist rolling and so reduces the amplitude.

Bilge Keel

When properly designed, this system produces an oscillating transverse

flow of water so timed as to generate loads that are opposite to the

perturbing force. Simply stated, they add weight to the high side of the

ship when it rolls. This will not stop rolling, but can markedly reduce

its amplitude. Their disadvantage lies in the considerable weight of

water, and corresponding loss in useful deadweight.

Anti-roll tank

Another approach is to fit antiroll fins port and starboard at the turn of the bilge

as shown in Fig. These fins are on pivots and are controlled by sensors that can

tell which way the ship is rolling. These sensors cause the fins to pivot in such a

way as to counteract the roll. The fins are turned so that, as they move ahead

through the water, the one on the low side provides an upward thrust and the

one on the high side provides a downward thrust. To prevent damage while

docking, most fins can be retracted into the hull. Obviously, this system is at its

best only when the ship is moving ahead; it is of little use when at anchor.

Fin stabilizers

A boat's period of roll is directly proportional to its radius of gyration,

which is a measure of the boat's distribution of mass about its axis of

rotation. The greater the radius, the greater the resistance to change

in rotational motion. In practical terms this means that you can slow

your boat's rolling period by moving weights symmetrically

outboard, away from the center line.

A key factor affecting the ability of crew to properly function is the

level of vertical accelerations to which they are subjected. Also the

passenger comfort is closely related to the vertical acceleration

levels. It is known that vertical accelerations together with roll

motion are the main cause of seasickness.

Transverse accelerations on the ship, combined with sway and roll

motions can cause a shift of cargoes like ore or grain. Sea-fastenings

of containers at deck can collapse by too large accelerations and

vulnerable cargoes like fruits can be damaged.

Vertical and Transverse Accelerations

Deck Wetness

Deck wetness is defined as occurring when the bow of a vessel

plunges into a wave and the water rises above the edge of the deck at

the bow and washes aboard. Deck wetness is an important

seakeeping phenomenon, which frequently causes voluntary

reductions in ship speed, and threatens ship stability and structural

integrity. In extreme conditions, the frequent shipping of water may

lead to the capsize of the vessel; in more moderate conditions the loss

of the vessel is unlikely, but frequent deck wetness may still cause

damage to exposed fittings and deck cargo and make the upper deck

untendable for the crew.

When the forefoot of the ship emerges, an impact can occur at the

instant of re-entry. At certain level of intensity the hydrodynamic

impact of the forebottom can be noticed by the crew as a bang and a

deceleration in the foreship, followed by a shudder through the hull

girder. Slamming under the forebody has long been recognized as

imposing a limit on the performance of many classes of ships. Cases

are reported of ships experiencing bow structural damage due to

severe bottom slamming. Moreover, the psychological effect on a ship

operator of severe slamming is such as to demand a reduction of

speed or a change of course and this subjective reaction will differ

from individual to individual.

Slamming

Slamming and Deck wetness

Deck wetness

The figure illustrates how attainable speed varies with increasing sea state

number. It shows both the involuntary speed reduction, caused by added

resistance and reduced propulsive efficiency, and the voluntary speed

reduction caused by extreme effects such as deck wetness.

Speed reduction in a seaway

Manoeuvering Performance

All ships must be able to control their speed and follow an intended

course when in transit. Additionally, when entering congested waterways

or harbours, they must be able to position themselves accurately. To

achieve this, ships must have the means of producing ahead and astern

thrust, turning moments and lateral thrust. The last two are provided by

rudders of various types assisted, in some cases; by lateral thrust units at

the bow and/or stern. Ahead and astern thrust is usually provided by the

main propulsion system. Because rudders are usually sited close to the

propulsors there will exist an interaction between the two. Where more

than one shaft is fitted, a turning moment can be produced by going

ahead on one shaft and astern on the other.

The IMO Standards for ship manoeuvrability identify significant

qualities for the evaluation of ship manoeuvring characteristics:

Inherent dynamic stability: A ship is dynamically stable on a straight

course if it, after a small disturbance, soon will settle on a new

straight course without any corrective rudder. The resultant deviation

from the original heading will depend on the degree of inherent

stability and on the magnitude and duration of the disturbance.

Course-keeping ability: The course-keeping quality is a measure of the

ability of the steered ship to maintain a straight path in a

predetermined course direction without excessive oscillations of

rudder or heading. In most cases, reasonable course control is still

possible where there exists an inherent dynamic instability of limited

magnitude.

Initial turning/course-changing ability: The initial turning ability is

defined by the change-of-heading response to a moderate helm, in

terms of heading deviation per unit distance sailed (the P number) or

in terms of the distance covered before realizing a certain heading

deviation (such as the "time to second execute" demonstrated when

entering the zig-zag manoeuvre).

Yaw checking ability: The yaw checking ability of the ship is a measure

of the response to counter-rudder applied in a certain state of

turning, such as the heading overshoot reached before the yawing

tendency has been cancelled by the counter-rudder in a standard zig-

zag manoeuvre.

Turning ability: Turning ability is the measure of the ability to turn

the ship using hard-over rudder. The result being a minimum

"advance at 90° change of heading" and "tactical diameter"

defined by the "transfer at 180° change of heading". Analysis of

the final turning diameter is of additional interest.

Stopping ability: Stopping ability is measured by the "track reach"

and "time to dead in water" realized in a stop engine-full astern

manoeuvre performed after a steady approach at full test speed.

Lateral deviations are also of interest, but they are very sensitive to

initial conditions and wind disturbances.

A turning circle manoeuvre is to be performed to both starboard and port with

35° rudder angle or the maximum design rudder angle permissible at the test

speed. The rudder angle is executed following a steady approach with zero yaw

rate. The essential information to be obtained from this manoeuvre is tactical

diameter, advance, and transfer.

Turning tests

A zig-zag test should be initiated to both starboard and port and begins by

applying a specified amount of rudder angle to an initially straight approach ("first

execute"). The rudder angle is then alternately shifted to either side after a

specified deviation from the ship’s original heading is reached ("second execute"

and following). The essential information to be obtained from these tests is the

overshoot angles, initial turning time to second execute and the time to check

yaw.

Zig-Zag tests

This is a manoeuvre aimed at giving a feel for a ship's directional stability. From an initial

straight course and steady speed the rudder is put over say 15° to starboard. After a while the

ship settles to a steady rate of turn and this is noted. The rudder angle is then reduced to 10°

starboard and the new steady turn rate noted. This is repeated for angles of 5°S, 5°P, 10°P, 15T,

10T and so on. The resulting steady rates of turn are plotted against rudder angle.

The Spiral Manoeuvre

If the ship is stable there will be a unique rate of turn for each rudder angle. If the ship is unstable the plot

has two 'arms' for the smaller rudder angles, depending upon whether the rudder angle is approached from

above or below the value.

A full astern stopping test is used to determine the track reach of a ship

from the time an order for full astern is given until the ship is stopped

dead in the water. The "crash-stop" or "crash-astern" manoeuvre is mainly

a test of engine functioning and propeller reversal. The stopping distance

is essentially a function of the ratio of astern power to ship displacement.

A test for the stopping distance from full speed has been included in the

Standards in order to allow a comparison with hard-over turning results in

terms of initial speed drop and lateral deviations.

Stopping Tests

Modifying the Manoeuvering Performance

The effect of design changes on a ship's manoeuvring qualities can be expected that:

(1) Stern trim improves directional stability and increases turning diameter.

(2) A larger rudder can improve directional stability and give better turning.

(3) Decrease in draught can increase turning rate and improve directional stability. This

is perhaps due to the rudder becoming more dominant relative to the immersed hull.

(4) Higher length to beam ratios lead to a more stable ship and greater directional

stability.

(5) Quite marked changes in metacentric height, whilst affecting the heel during a turn,

have litde effect on turning rate or directional stability.

(6) For surface ships at a given rudder angle the turning circle increases in diameter with

increasing speed but rate of turn can increase. For submarines turning diameters are

little affected by speed.

(7) A large skeg aft will increase directional stability and turning circle diameter.

(8) Cutting away the below water profile forward can increase directional stability.

By and large the hull design of both a surface ship and a submarine is dictated by

considerations other than manoeuvring. If model tests show a need to change the

manoeuvring performance this would normally be achieved by modifying the areas and

positions of the control surfaces and skegs