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Old 08-06-2008, 17:29   #76
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Terapy - diferent boat diferent weather diferent results - I was over as stated.
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Old 08-06-2008, 17:57   #77
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It seems to me that this boat got flipped by a combination of wind and wave, not just a knockdown from having too much sail up. Reminds me of a time I watched a small monohull going out of sebastian inlet in Florida about 40 years. Big breakers were coming in in sets on a glassy sea. The boat started to run the inlet and as he was crossing the bar about 12-15 footer broke over the bar. The boat went streight up the face of the wave and folloed the curl right over on its back. Fortunately the tide was coming in and I helped pull three people out of the water. They all survived, but the boat had no mast. and was floating upside down with only about a foot of the hull above the surface. As this was a relatively light center board boat with the board up It didn't have much self righting ability. It would have probably done better with a conventional knockdown. I guess no two capsizes are alike and if you took steps to protect yourself from one, another could still get you. Guess what its and ocean, if you want absolute safety stay in bed. Ooops, earthquakes, fires, airplanes falling out of the sky. Guess beds not safe either.
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Old 08-06-2008, 18:06   #78
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I got it.
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Old 08-06-2008, 19:32   #79
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Originally Posted by BigCat View Post
David, the answer can be calculated from information given in my posts, above, on floatation for monohulls and on righting moment for catamarans. The 9000 pounds needed to right BigCat 65 at the masthead equals 140 cubic feet, and that requires 6 standard scuba bottles to fill at sea level.

However, as the masthead is well below sea level, it would take a bit more than 3 times as much air to fill it at 70 feet of depth as it would at the surface--and the bag would explode from the expanding air as it rose, as the air would need to triple in volume as it rose to sea level.

The air pressure / air volume issue is scuba diver knowledge, if you want to get into it--66 feet = 3 atmospheres, with 1 atmosphere being the air pressure at sea level.



So, it would be very tricky to valve such a bag so that it didn't explode before you righted your catamaran. Too much venting, and you lose your righting moment. Too little, and your bag breaks. Not a simple problem. Alternatively, your bag would have to be made from a very stretchy material indeed, if you didn't vent the expanding air from it. (The air would need to expand as the float bag rose.)

So you can add these problems to the somewhat iffy issue of whether the mast would still be there after the catamaran capsized.

So, such a system on BigCat 65 would involve 750 pounds of scuba tanks, and a hose from them to the masthead via the inside of the mast, as well as a solution to the expanding gas problem to prevent breakage of the bag.

Divers use parachute-shaped bags open on the bottom to prevent this explosion when using air bags to salvage relatively small things, but they are down there to control the bag and keep it from spilling air out the bottom.
BigCat,
Floats used by divers have openings on the bottom to prevent them from exploding as they approach the surface...at least all the floats that I have seen them use for recovering sunken objects.

Don't typical recreational aluminum dive tanks hold around 80 cubic feet of air? If you apply Boyle's law, eighty cubic feet of volume divided by three atmospheres would be 26.6 cu-ft. Multiplying 26.6 cubic feet times the weight of salt water (64 lbs/cu-ft) would work out to be 1702 pounds of buoyant force per bottle at 68 feet (three atmospheres). That's not enough?

What if the bottles were put below deck with the hydrostatic release at the mast head? Having 6 or so scuba tanks below deck is not unfeasible.

How do you come up with 9000 pounds of buoyancy at the masthead for a catamaran in order to bring the masthead to the surface? Is it not more a factor of righting moment (force times distance) than force alone?

David
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Old 08-06-2008, 19:51   #80
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Hi, David - We covered the issue of the amount of air in a standard scuba bottle above. I also mentioned that divers use bags open on the bottom. One atmosphere = sea level, 2 atmospheres = 33 feet, 3 atmospheres = 66 feet. If you want zero atmosphere, try outer space, or the inside of a resin infusion bag.

Didn't I explain how I got 9000 pounds? It is the boat's righting moment (my boat) divided by 70' (my boat's mast height above water.) I assumed that the righting moment upside down is about the same as right side up.
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Old 08-06-2008, 20:13   #81
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Hi, David - We covered the issue of the amount of air in a standard scuba bottle above. I also mentioned that divers use bags open on the bottom. One atmosphere = sea level, 2 atmospheres = 33 feet, 3 atmospheres = 66 feet. If you want zero atmosphere, try outer space, or the inside of a resin infusion bag.

Didn't I explain how I got 9000 pounds? It is the boat's righting moment (my boat) divided by 70' (my boat's mast height above water.) I assumed that the righting moment upside down is about the same as right side up.
Please read my post again...I made some edits while you were writing your response.
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Old 08-06-2008, 20:28   #82
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Foot pounds

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Please read my post again...I made some edits while you were writing your response.
I'm afraid I'm not following you, David. Righting moment is, in the U.S., calculated in foot pounds, which is a measure of force and distance (leverage.) 10 pounds times 20 feet (distance) = 200 foot pounds.
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Old 08-06-2008, 21:29   #83
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Righting moment is what will bring a cat from an inverted (turtled) position to a horizontal position in the water. The mast is the lever arm and the force is the flotation in pounds from the float bag. A 66 foot mast times 9000 pounds of force is 596000 foot-pounds of moment (torque). Is this large amount of righting moment really necessary to right a cat? Yeah I realize that the effective length of the arm is zero (x-axis component of the lever arm) when the boat is completely turtles but what if wave action or another boat was pushing the boat sideways so that there is at least 10 feet (x-axis) of arm? The moment would only increase as the masts goes from inverted to horizontal. See what I mean? That would imagine that would have to work.
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Old 08-06-2008, 23:13   #84
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Righting Moment

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Righting moment is what will bring a cat from an inverted (turtled) position to a horizontal position in the water. The mast is the lever arm and the force is the flotation in pounds from the float bag. A 66 foot mast times 9000 pounds of force is 596000 foot-pounds of moment (torque). Is this large amount of righting moment really necessary to right a cat? Yeah I realize that the effective length of the arm is zero (x-axis component of the lever arm) when the boat is completely turtles but what if wave action or another boat was pushing the boat sideways so that there is at least 10 feet (x-axis) of arm? The moment would only increase as the masts goes from inverted to horizontal. See what I mean? That would imagine that would have to work.
It is necessary to right my cat, shown on my website. This is a big cat weighing 45,000 pounds in cruising trim, with a 35.5' beam-hence the moniker 'BigCat.' Narrower, lighter cats will have a smaller righting moment. Paragon, a very light 47' cat discussed above, has 1/5 the righting moment with 10 people on board.
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Old 08-06-2008, 23:36   #85
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Why try?

"Why try to right a turtled multi with on-board equipment?"

Don't get me wrong, I am a multi owner and a fan, but I have to ask the question.

It would seem to me that you have the best lifeboat available. Why not design an emergency beacon, radio and/or transponder to call for big help if the boat is ever turtled? After all, the main saloon is under water and presumably so are the radios and antennae. So IMHO a solution that could be deployed on a turtled multi would be far more interesting. Could be the old Boy Scout in me.....
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Old 09-06-2008, 03:28   #86
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Righting??

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"Why try to right a turtled multi with on-board equipment?"

Don't get me wrong, I am a multi owner and a fan, but I have to ask the question.

It would seem to me that you have the best lifeboat available. Why not design an emergency beacon, radio and/or transponder to call for big help if the boat is ever turtled? After all, the main saloon is under water and presumably so are the radios and antennae. So IMHO a solution that could be deployed on a turtled multi would be far more interesting. Could be the old Boy Scout in me.....
I think the question should be why sail a boat that has a high chance of requiring outside assistance. With cruising multi-hulls getting lighter and lighter with bigger and bigger rigs, one has to reach the conclusion that this arms race for speed is going to end in tears. Screaming for help and expecting Joe tax payer to save our arses is not the solution as this will surely end in more regulation.

John Gross (fastback catamarans)did some work on self assistance for capsized multi a few years back with Blackbird (I think the name was). Surely this is the best solution.?
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Old 09-06-2008, 04:05   #87
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I think the question should be why sail a boat that has a high chance of requiring outside assistance...
Of course, “high chance” is a highly subjective and relative term.

As has been noted, capsize inversions are certainly not every day occurrences, and many (if not most) would characterize these events (requiring outside assistance) as “rare and unlikely”.
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Old 09-06-2008, 05:52   #88
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If a cat was righted to the point of having the mast at water level and presumably unless you had a very big flotation bag, what then lifts it that degree further to actually fully right it ? The angle would still be on the wrong side of the gz curve unless the boat was floating so low in the water that the mast was parallel with it.
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Old 09-06-2008, 05:59   #89
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As has been noted, capsize inversions are certainly not every day occurrences, and many (if not most) would characterize these events (requiring outside assistance) as “rare and unlikely”.
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Old 09-06-2008, 06:25   #90
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I must admit to also wondering what the point is here, making a multi self recoverable from capsize is like making a mono self recoverable from sinking.
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