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-   -   Capsize and Stability (http://www.sailnet.com/forums/seamanship-navigation/76879-capsize-stability.html)

casey1999 07-27-2011 06:35 PM

Capsize and Stability
 
Sailboats seem to have good capsize and stability with the keel. What about power boats, once they roll over will they right themselves? What about say an cruise ship or and aircrafty carrier? The new super size cruise ships look very top heavy. What keeps these things upright? What do they use for balast? Could an aircraft carrier be capsized by a large enough rouge wave or are they designed to handle say a 150 foot breaking wave?

Boasun 07-27-2011 07:42 PM

There are not to many self righting power boats. Most power boats may heel as far as 70 ~90 degrees before they capsize totally. But if you can get a USCG surf boat, then you will have a self righting boat.

The stability data for my Hardin 45 is 162 degrees... Which is much much better than any old power boat.

casey1999 07-27-2011 07:54 PM

Quote:

Originally Posted by Boasun (Post 755306)
There are not to many self righting power boats. Most power boats may heel as far as 70 ~90 degrees before they capsize totally. But if you can get a USCG surf boat, then you will have a self righting boat.

The stability data for my Hardin 45 is 162 degrees... Which is much much better than any old power boat.

Found a USCG surf boat for sale- quite a boat!
FOR SALE: Monterey Bays Historic US Coast Guard MLB-44346 rescue boat

bobbylockes 07-27-2011 07:58 PM

I'm no expert but here is my 5 cents..

every boat can capsize. the question is how easily they pop back up.

stability is objectively demonstrated by to GZ curves and GM (see wikipedia). subjectivey it depends on what you consider stable ie A very wide boat shallow boat (like a cruise ship) may appear stable as it rolls less than a narrow-beamed, deep keeled sailboat but should the wide boat capsize it is far less likely to right itself than the narrow-beamed, deep-keeled boat. The stability curve of a wide shallow boat looks great up to the point of the capsize. In general the centre of gravity of a wide boat is not sufficiently low enough to get the boat to right itself. A sail boat can take knockdowns and pop back up many times (catamarans can't though) as mass of a sailboat is as low as possible and alot is held in the keel.

cruise ships look very unstable to me. They are shallow in order to visit tourist ports like venice and amsterdam) and have a huge air draught. But I wouldn't know what their stability curve looks like. I can't imagine a cruise ship popping back up from a 180 degree knock down simply because the air draught, massive superstructure , and their width is so enormous.

military vessels probably have fantastic stability curves. They are narrow, prbably deep, lots of tonnage in the keel and their speed lends alot to stability.

The stability curves on the internet I've seen don't show the full picture. at some angle of heel GZ must be less than 0 in order for a boat to capsize. The longer GZ remains under 0 wrt angle of heel the less likely it is that the boat will pop back up. The rescue services invest alot of money in boats that capsize and right themselves at the slightest moment. Those boats don't look like cruise ships or aircraft carriers.

acunningham 07-27-2011 09:07 PM

Quote:

Originally Posted by bobbylockes (Post 755318)
cruise ships look very unstable to me. They are shallow in order to visit tourist ports like venice and amsterdam) and have a huge air draught. But I wouldn't know what their stability curve looks like. I can't imagine a cruise ship popping back up from a 180 degree knock down simply because the air draught, massive superstructure , and their width is so enormous.

Bear in mind that the stability curve will depend on the distribution of weight within the vessel. Cruise ships will have enormous weight low down, in the form of engines, fuel and water tanks, and the myriad systems needed to run the ship, and relatively little weight high up (passenger cabins are mostly air). They'll therefore get pretty good stability, though of course they won't recover from a 180 degree capsize.

acunningham 07-27-2011 09:09 PM

A good explanation of stability curves can be found here:

GZ Curves

In particular, note the last stability curve for an RNLI lifeboat. As long as it's kept watertight it can recover from any position.

nolatom 07-28-2011 11:56 AM

And stability is designed in, to suit the boat/ship's purpose. A navy ship will have lots of righting arm, and be "stiff", meaning she snaps back quickly from a roll to level. Why? So the gun deck will be level most of the time, for firing. This is harder physically on the crew than a slower rolling period.

A cargo ship will have shorter GM and a slower rolling period, since a slower roll is gentler on the cargo, lashings, and crew. It's 'tender', but meant to be that way.

GeorgeB 07-28-2011 01:10 PM

From the “what it’s worth department”: Dad was on a Fletcher Class destroyer (2,500 tons) when it rode out the 1945 typhoon. The captain had ordered everybody who was not part of the bridge watch and engine crew down to lowest parts of the ship in the attempt to lower the CG and to keep the ship from capsizing. A ship in another division did turtle and sink. Said it was his scariest time in the Navy – worse than picket duty off Okinawa. Naval guns have gyros in their gun directors and firing circuits to cancel out the effects of roll, pitch speed and heading. I was part of the APECS III program for the Mako class and our antenna platform had to be able to tilt 50 degrees in both directions in order to accommodate high speed turns and combat in deteriorating sea conditions. Needless to say, the hydraulic actuation system was a major headache.

casey1999 07-28-2011 01:20 PM

Info on Fletcher Class- In design section talks about ballast issues.

Fletcher class destroyer - Wikipedia, the free encyclopedia

GeorgeB 07-28-2011 08:26 PM

The “ballast issue” referred to in the Wiki article was on the destroyer classes (Sims class was the last) preceding the Fletcher. The Fletchers and Gearings had flush decks and wider beams, eliminating the roll issues of the older cans. The problem with flush decks is they didn’t have portholes which made for lousy ventilation in the tropics. The design was so good that they lasted well into the seventies whereas the Sims and her ilk were all retired by 1946. Gearing design was so successful that they underwent FRAM I and II modernization, the last being the addition of a helo deck and hanger!


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