Angle of Vanishing Stability - SailNet Community
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post #1 of 19 Old 12-06-2011 Thread Starter
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Angle of Vanishing Stability

I am reading the book "Adlard Coles' Heavy Weather Sailing" by Peter Bruce. In the chapter "The stability of yachts in large breaking waves", it tells about an experiment done on boat models in a water tank, that measures the AVS of various boat shapes. One of the outcomes of this experiment is that adding a coachroof to a boat without one may move the AVS further towards the 180 degree, or alternately, two boats of the same height above water line, one with high freeboard and no coachroof and the other with coachroof and lower freeboard, the later shows much less inverted stability.
The reason for this phenomenon is that the coachroof, when submerged in the water, adds buoyancy and raises the center of buoyancy to higher point with regard to the vessel.
This may raise some thoughts:
1) An inflatable boat stowed on deck may cause the same effect and thus contribute to the stability of the boat.
2) We can take this idea a step further and install a balloon on deck. Such a balloon can be empty in normal sailing and automatically inflated when the boat is inverted. If this idea is right, I suppose I would have not been the first one to raise it. So why don't we see such systems on boats ?

Btw, I have a picture of the said page from the book but I don't know if I am allowed to display it here.
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post #2 of 19 Old 12-06-2011
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Probably because it is cheaper to built it with a roof, vs with out per say and an inflatable balloon/pfd if you will. The other you would have to deal with on the balloon, is where the lines etc are at the time of the balloon explosion! you may do more damage to the rigging etc with this option than designing a boat to roll easily when upside down.

Along with, most of us will probably never be in a rolled situation where and how we sail, so the balloon is a REALLY expensive solution. now maybe as an option for some one crossing and ocean, where the waves are big enough to roll a boat.........maybe......

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post #3 of 19 Old 12-06-2011
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Off topic but I know that Donald Crowhurst was trying to develop a balloon at the top of the mast that was supposed to right his trimaran.

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post #4 of 19 Old 12-06-2011
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post #5 of 19 Old 12-06-2011
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It seems to me (this is worth less than 2 cents btw) that few boats will stay turtled in the kinds of conditions that caused them to turn over. The question may be how watertight is the boat, how much water gets in before the next breaking wave flips you back upright.
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post #6 of 19 Old 12-06-2011
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Quote:
Originally Posted by peterchech View Post
The question may be how watertight is the boat, how much water gets in before the next breaking wave flips you back upright.
I think Peter hit on a big issue: How watertight is the boat? Any water allowed to enter the cabin/hull will quickly change the AVS and righting ability. Properly secured hatchboards, strong ports, sealed deck hatches, watertight lazerettes, etc. are likely just as critical to surviving turtling/capsizing.

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post #7 of 19 Old 12-06-2011
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The current breed of beamy ocean racers often do sit happily upside down, most especially if they happen to lose the keel bulb... There are some images around, Tony Bullimore for one, where he's clinging to the keel fin on a seemingly very stable upside down boat. Multis, too, are happiest that way.

But then again not many of us sail those types of boats....
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post #8 of 19 Old 12-06-2011
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Airbags for keelboats ?
BTW I've seen tri's with some kinda giant floatation device at the masthead, usually pretty "unique" looking older tri's.

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Last edited by capttb; 12-06-2011 at 01:02 PM.
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post #9 of 19 Old 12-06-2011
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Hobie Cat makes a "baby bob" and a "momma bob" to be attached to the top of their masts specifically to prevent turtling. They won't bring the boats back upright but that's multi-hill bit of physics anyway.

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post #10 of 19 Old 12-06-2011
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Quote:
Originally Posted by Faster View Post
The current breed of beamy ocean racers often do sit happily upside down, most especially if they happen to lose the keel bulb... There are some images around, Tony Bullimore for one, where he's clinging to the keel fin on a seemingly very stable upside down boat. Multis, too, are happiest that way.

But then again not many of us sail those types of boats....
Rambler is a good example of this from the recent Fastnet.

If the keel falls off I think you have bigger problems than just remaining turtled. However, the types of racing boats which are most likely to have keel issues, flush decks and super-wide planing arses are also built very light, usually foam cored. So without their keels they are likely to remain afloat like a liferaft. Kind of like a multi.

If something like a canting keeled mini 650 turtles, if the keel is canted this may help get the boat back on its side at least. However the flush deck, and relatively low ballast to displacement ratio might make recovery from turtle much less likely, even with wave action, than recovery on your standard cruiser.

I often wonder about what you do if you are in the cockpit when the boat rolls. If you are tied in (you should be), you may be held under for some time before the boat comes back up especially if sheets are not released in time or if wave/wind action coincidentally holds the boat inverted longer than would be expected. If you cut away from the tether to get to the other side of the boat, you risk being separated and lost at sea. Webb Chiles talks about being rolled twice in the southern ocean on one of the "furled sails" podcasts, but he was in the cabin both times and the boat came right back up. I think I would be tempted to throw out a sea anchor and hide below if the going got really rough, but that isn't always an option and often it is a rogue wave that breaks unexpectedly which rolls the boat.
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