AVS of ships and others

[extravert]

Seeing a ship on my drive to work this morning near Manchester made me wonder what the AVS of other sea going vessels is, like ships, trawlers, lifeboats and even motor cruisers and power boats. Any naval architects out there or anyone else know?

This is just idle curiosity. I'm not intending taking any boat/ship to anywhere where AVS might be an issue. Than man who tried to take a canal boat to the Isle of Man and disappeared without trace has put me off any idea like that.

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[Happy1]

I asked the very same question on the motor boat forum a week or so ago. Apart from people thinking I was daft to even worry about it, I was just as interested as you. There were some people who gave helpful info, do a search on that site, it may help you

Cheers

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[Jacket]

I read somewhere that a typical WWII destroyer had an AVS of around 35 degrees. And apparently in bad North Atlantic weather they'd often be rolling to 32 - 33 degrees, which must have caused a few worrying moments!

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[qsiv]

Recalling from my dim and distant past reading Naval Architecture at Southampton, I dont actually recall the AVS as ever being discussed as hugely significant. We were concerned mightily with GZ curves, GM heights etc.

A ships propensity to roll is much higher than a yachts (because the centre of bouyancy is so close to the centre of gravity, and a relatively rounded hull form), and dynamic stability was perhaps perceived as being more real and more important.

With certain ships downflooding is likely to be more of a problem than with yachts, and as soon as this happens, unless great care is taken with free surface effects the ship will capsize quickly. It was this free surface effect which sank the Herald of Free Enterprise.

Our project was a RoRo car ferry - and the margin of stability was frighteningly low. I seem to recall about 5mm of water on the cardeck was enough to remove all positive stability.

I do recall some work was being done on the hydrostatics of Noahs Ark - and it turned out to be significantly more stable than most modern ships.

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[extravert]

Thanks, an interesting subject. No doubt much more complicated than a single AVS value. Are ships that have not suffered down flooding ever capsized by waves, or are they designed so that a wave to do this would be so rare that we never hear of any incidents?

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[vyv_cox]

<A target="_blank" HREF=http://barry-miles.tripod.com/>http://barry-miles.tripod.com/</A> shows a photo of HMS Forester rolling to 48 degrees. My uncle was aboard her on the WWII Russian convoys, possibly when this photo was taken.

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[qsiv]

I'm honestly not sure. I can say that (as of 2000 at any rate) HM fleet warships are designed to operate in winds of upto 90knots and commensurate sea conditions. Inshore warships are designed to a 70knot spec, and lesser categories assume that shelter will be sought if forecast winds are greater than F8. In reality this latter category are harbour launches, small tugs and so on, that are unlikely to be at sea in any case.

As for ships rolling - I'm sure there probably are cases - what has definatively happened (and is more likely from structural angle) is that ships have sunk due to fore and aft (pitch) wave action. A ship is essentially a flawed box girder (flawed due to hatches, accomodation and other necessary structures). The bending stresses in head seas and perhaps more importantly the wracking stresses of quartering seas have undoubtedly caused stress failures which have sunk ships. It was this sort of stress which was always assumed to have sunk the Derbyshire - I cant quite remember if the Board of Enquiry backed this up. This also why all merchant ships (well both ships in the UK merchant fleet!) have a 'stabilty' book, which defines how the ship must be loaded (which tanks/holds in which order) - both to ensure the ship is stable whilst loaded (stability can be quite marginal when fully empty) and also to minimise the stress on the structure. It also defines how much ballast must be carried for differing expected sea states.

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[davidhand]

May I quote from "Fastnet, Force 10" "The Overijessel ( a Dutch Frigate/Destroyer) began to roll badly...until at one frightening moment a wave swept over deck and into her engine room and short circuited her generators. Rolling to within a few degrees of the angle of heal at which she would loose all stability and capsize, "The Overijssel was rolling he gunwales under,........"

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