stability and trim for ships, boats, yachts and barges – part i

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Understanding Ship and Boat Stability

(Stability & Trim - Part1)

By: Brian Trenhaile, P. E., Naval Architect & Marine Engineer, Hawaii Marine Company, revised Sept.2005.

This article was originally published in the April/May 1998 issue ofHawaii Ocean Industry & ShippingNews.

For the sake of just one human life, it is worth understanding how stability works. Sadlymultitudes of precious lives are lost due to a lack of awareness and understanding regardingthis most crucial topic.

In 2002 an oceangoing ferry named the La Joola capsized off the West Coast of Africabetween 1,034 to 1,600 crew and passengers perished in this unprecedented peace-timemaritime disaster. Another recent example is the September 1997 capsizing of the Pride of

Gonave in Haiti. Roughly 200 souls perished in this unfortunate debacle. In fact, stabilityrelated ferry disasters are rather common, claiming some 400 lives in Lake Victoria in May1996; 338 lives off Sumatra in January 1996; and 852 lives in September 1994 when the ferryEstonia sank in a Baltic storm.

The ferry industry is not the only segment of the marine industry that needs to be concernedabout stability. Statistically, a higher percentage of people die in commercial fishing than anyother occupation in the United States. Inadequate stability is one of the most commonsources of deaths in commercial fishing. Yet this industry often vigorously opposes any typeof regulation regarding its vessels.

Then there are sailboats. Many of the newer boats are constructed lighter and with wider andflatter midsections to get better race performance and lower ratings. Unfortunately, thesetrends have proved to be deadly for the 15 sailors who perished in the 1979 Fastnet Race.Investigators were called in to find out why this race was so disastrous. They concluded thatthe vessels should have been narrower and heavier than they generally were. Still, these race

performance enhancing trends continue. Racing sailors still go out in some of the worldsmost perilous bodies of water in boats that are often even more extreme than those of theFastnet Race.

Naturally, all segments of the marine industry need to be concerned about stability. A towboat can trip over its tow wire, a boat lifting a load can flip itself over, and too much freesurface in tanks can make a stable boat unstable. A good way to avoid being a victim of asensational stability incident is to understand stabilitys basic concepts. Paying attention tolittle details, however mundane, often saves lives.

This article touches some of the more crucial aspects of this important topic, in particularsome key principles regarding "static intact stability." Static intact stability involves heelangles that are restricted to less than 10 degrees and makes extensive use of the metacentricheight concept.

United States Coast Guard regulations and American Bureau of Shipping rules will oftenrequire that a vessel have a certain amount of metacentric height. The concepts described

here are applicable to boats, yachts, ships, and basically anything that floats.

The Weight Story
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Static intact stability involves two stories. This simply means that the weights side of thestory must match the waters side of the story. First, each side of the story will be described,and then the two sides will be combined.

The weight story is described first. In nature each component of weight contributesautomatically to the weight of the whole. Each weight has its own center and all the weightscombined have a combined center. Determining a vessels total weight (W) and the verticalcenter of gravity (KG) of all weights present is basically the crux of the weights side of thestory.

Here is a simple example. Aboat has a hull that weighs200 pounds. Attached to itstransom is a 100 poundoutboard. The hulls weightcenter is 1 foot above thekeel. The engines weightcenter is 2 foot above thesame spot on the vessels keel.The calculations required areas follows: Total Weight = W= 200 + 100 = 300 pounds.The sum of vertical moments= VM = 200# x 1 foot + 100#x 2 foot = 400 foot pounds.Vertical Center of Gravity =KG = Vertical Moments

divided by Total Weight =VM / W = 400 / 300 = 1.333feet (or 1 4) above the keelreference point. Normally avessel consists of hundredsand sometimes thousands ofitems. Weights and Momentstables are often utilized todetermine a vessels totalweight and combined centerof gravity.

The Water Story

Now lets go to the waters side of the story. The waters story is based on a rocking point thatnaturally occurs in nature. This rocking point is called the transverse metacenter. This

metacenter is labeled as M in Figure A. To obtain the distance KM two items need to be firstcomputed. The first is KB and the second is BM. Both will be explained in followingparagraphs.

The first thing that needs to be determined is the distance from the vessels center buoyancyto the keel. The symbol for this term is defined as KB. (T in the following formularepresents the draft at the Longitudinal Center of Floatation, but the draft at amidships ormean draft should be close enough for estimating purposes. Note also that T is the canoedraft of the hull only, it does not include the effects of keels, skegs or other appendages.) Fornormal shaped vessels KB can be estimated with Formula A as follows:

Formula A) KB = 0.55T

The transverse moment of


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The formulas provided in this article are for teaching purposes. They also can be used forrough estimating. However, this type of estimating is normally not acceptable to Coast Guardfor official stability submittals. Naval architectural computer programs are normally used inconjunction with official submittals. These programs very accurately calculate values of KB,BM, and KM. They also generate other important and often required stability data like LinesDrawings, Curves of Form, Cross Curves of Stability, and Curves of Statical Stability.

Static stability was primarily discussed in this article. This term static is applied as opposedto dynamic stability. Dynamic stability involves righting arms over a large range of heelangles. This type of analysis involves quantification of volumetric centers of heeleddisplacement volumes. Dynamic stability is also important because it is a measure of a

vessels ability to withstand the effects of wind and waves. The Coast Guard also has manyregulations that deal with dynamic stability.

Other important concepts like floodable length, damaged stability and free surface effects arenot covered here. Even some of the finer points of static intact stability were not covered.The reader is advised to do a further study of these concepts in order to obtain a fullerunderstanding of this topic.

Note: The formulas provided in this article are for teaching purposes. They are also usefulfor rough estimating. However, this type of estimating is normally not acceptable to theUSCG for stability submittals. (Added to article: Use of templates in this website should beacceptable.)

Recent News: On September 27, 2002 the ocean going ferry Le Joola capsized off the coast ofWest Africa. The death toll ranges from 1,034 to 1,600 passengers and crew. Uncertainty inthe count stems from the fact that many passengers did not have tickets. Tragically many ofthe passengers were children. This is another painful reminder that stability must continue to

remain a high priority in vessel design and operation. From: Article entitled "Major MaritimeDisasters Mark the Year," on page 6 of Pacific Maritime magazine, January 2003 issue.

Note: This article describes how to find GM, metacentric height, of a vessel based on weightsand moments data and hydrostatics data. This is the method normally employed by designersfor a vessel that is "not yet built." But if a vessel already "exists," the metacentric height canbe estimated from it's roll period or it can be determined accurately by conducting aninclining experiment (stability test and deadweight survey).

Update: Another article in this website, "Understanding Ship and Boat Trim (Stability &Trim - Part 2)" adapts some of the theory, discussed in this article, to longitudinal stabilityand how it affects trim. Whereas this article apples to the more critical stability that is in thetransverse direction.

Application: The concepts described in this article are utilized in the following templates:

Trim and Stability Sheet, English Units

Trim and Stability Sheet, Metric Units

Home | Background | References | Stability Article | Trim Article | Composite EAM | Example EAM | Usage Terms | Privacy Policy | Tech Support

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