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Old 29-01-2010, 14:46   #16
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Some years ago a 40ft IOR being delivered here to Bermuda to a local owner did a 180, and stayed there Wide beam on deck, narrow on W/L. Crew members felt that when she righted, thru the 360, and riding up the curl of the wave, a lifting force on the sheeted-in main helped her roll upright. The thought was that the C/G was well within the triangle of g/wale, g/wale and bulb,== and no wave had pushed her the rest of the way over. If I remember correctly, they were inverted for about 8 minutes.
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Old 29-01-2010, 15:34   #17
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Originally Posted by LtBrett View Post
I don't think I've done as good a job as the book at explaining negative stability. It's not practical to make a sailboat with no negative stability, if it's even possible.

They would if the mast fills with water. Plus all the unsecured crap in your cabin, your water/fuel tanks, even your "new" underwater hull profile have shifted your center of gravity substantially and changed its relationship with your center of buoyancy (google metacentric height).

Bottom line, if you are turtled, a wave, whether the initial wave or a subsequent one, has to overcome your negative stability to right you.
I know what metacentric height is. I haven't dug out my copy of Derrett's, but I doubt he covers small yachts. A small ballasted mono has a big chunk of lead/iron/concrete in its keel - you would have to have an awful lot of stuff come loose to change the CG enough to significantly affect the stability. The water in the mast is neutrally buoyant, so it doesn't change the CG when immersed.

In Blue Stocking's example, an exceedingly wide vessel essentially became a catamaran when inverted. It would have a lot of negative stability as it would need to be tipped over enough to move M down past the CG. My theory is that when a large enough wave came along it started sliding down the face of the wave, the sail's drag caused the boat to start tripping and when the keelbulb was far enough over it created a positive righting moment.

This is the sort of example I'm looking for. Thanks.
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Old 29-01-2010, 22:12   #18
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BRRRRR....Scary Stuff! Since there is nothing I can physically do at that point of inversion then I reinforce my confidence by saying the heavy, 3/4 keel of my boat prevents the sails from taking a bath in the first place. Actually, the book mentioned, 'Desireable and Undesireable Characteristics of Offshore yachts' is quite good at explaining this whole scary event.

Would not the mast and sail considerable slow the intial roll once it hits the water and thereby prevent a capsize to 180?

Or...

Would the mast and sail act as a 'fixed point', much like a pole-vaulters planted pole, to cause the hull to flip over and exceed 180 in one motion?
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Old 02-02-2010, 08:44   #19
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Once the boat is rolling, you have to factor in angular momentum. If the boat rolls to 180 and by some miracle stops exactly there, you are correct. Otherwise, the momentum of the boat makes it easier to keep rolling in the same direction.

I don't think I've done as good a job as the book at explaining negative stability. It's not practical to make a sailboat with no negative stability, if it's even possible.

Bottom line, if you are turtled, a wave, whether the initial wave or a subsequent one, has to overcome your negative stability to right you.

Brett
A number of boats built to the old Universal rule have "essentially" no negative stability.(AVS= 180 deg). So any disturbance off of 180 deg. would flip the boat. The "flip side" of that is that the max righting moment for these boats is around 90 deg (they sail on their ear).

The slightly beamier CCA boats (keel) were usually around 150-160 deg. so it doesnt take much to right these.
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Old 03-02-2010, 13:12   #20
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It depends: stiff or tender, continuous momentum, number of times etc...etc... etc...
Design rules: No boat can withstand a broach wave higher than 55% of it's waterline length.
Here are 2 sites with calcs:
Capsize Formula
Sail Calculator Pro v3.53 - 2000+ boats
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Old 03-02-2010, 15:31   #21
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Boats with large deck salons, and lost of motor boats actually can have no negative stabilty , ie like the new Moody 45DS, providing they stay watertight. Thats how RNLI lifeboats can self right, its the buoyancy in the cabin structure.

Mo/bo's tend to sink because of downflooding rather then stability issues
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