# effects of waves on ship stability calculation

Discussion in 'Stability' started by janelove, Sep 28, 2007.

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### janeloveNew Member

hello,

I recently started learning about ship stability due to my studies. I understand that in ship stability calculations (assume good internal CG arrangements) , the main basis is that the righting arm moment need to be greater than the heeling arm moment (caused by the wind).... maybe I have not got to it but why the whole calculations does not consider the effects of waves?

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### GuillermoIngeniero Naval

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### jehardimanSenior Member

While, as Guillermo points out, the whole righting moment calculation needs to include water (wave) surface, the calculation itself can only be done as a static one. I.e. an instant in time. As it would be impossible to do all possible surface conditions for every roll position, certification societies and marine regulations usually specify a required Righting Energy (i.e. the area under the righting curve) be available. This takes care of the "three sisters" and other transient conditions (water on deck, etc.) of stability. See Chapter II of PNA, or Basic Ship Theory by Rawson and Tupper for a long discussion.

For where this concept breaks down see the SNAME paper "Predicting Complicated Dynamics Leading to Vessel Capsizing", a UNO paper on fishing vessel loss of waterplane capsizes I can't recall right now, and the MV Rocknes loss report.

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### gadflyJunior Member

jehardiman

What does "three sisters" mean ?

Thanks.

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### yipsterdesigner

"effects of waves on ship stability calculation"
here a page from Marchaj's book seaworthiness

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### charmcSenior Member

"Three Sisters" is a term describing a series of waves much higher than the average for the prevailing conditions. It's not precise, as waves much larger than the average can be alone, or in groups of as many as five or six, but a group of three is probably most common. The effect of three or more large waves in a row would be an extreme transient condition, but one which must be considered in stability calcs for a vessel to be suitable for operations in the open ocean.

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### jehardimanSenior Member

Charile is pretty much correct here. They are formed when the maximum amplitude of two wave trains of slightly different nominal periods are in sync. Similiar to the harmonic wow-WOW-wow-WOW sound heard when two engines are slightly out of sync, but as wave trains are comprised of a much broader spectrum, they appear infrequently.

They are so named for the three sister fates (the Moirae) of greek myth.

The first wave heels the ship (Clotho-who spins the thread of your life).
Before it can recover, the second wave knocks it on it's beam ends (Lachesis-who measures the length of your life).
And finaly, while knocked down, the third wave causes a capsize. (Atropos-who cuts off your thread of life).

Last edited: Apr 12, 2008
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### GuillermoIngeniero Naval

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### GuillermoIngeniero Naval

A different "Three Sisters"

Seahouses relief lifeboat "Peggy and Alex Caird" launches to escort fishing vessel "Three Sisters" into Seahouses in heavy seas 10/03/06.

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### gadflyJunior Member

Now I know what the "Three Sisters" mean.
Thank you all.

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### Guest-3-12-09-9-21Senior Member

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### TeddyDiverGollywobbler

When those containers get overboard it's time of Atropos for a fin keeler..

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