Agree with your comments except for the mast angle. I suspect it will be pretty close to 90 as the lee hull will be partially in the water. As long as it stays at 90 until the boat swings around with the mast to windward, then the wind and waves will be working to right it, so it will be less than 10 degrees. Important to have the weight low down in the windward hull, as Robert pointed out. I think it would be very difficult to capsize anything bigger than Harry as the mast would be bending so much.
That's about as good a masthead flotation system as one could imagine. Nice design, pretty aerodynamic, and it doesn't look like the hobie "training wheel" bob at the top. The renderings are nicer than the sketches I was anticipating.
Two thoughts:
a) It looks like it would be a better fit for a rigid stayed mast where it wouldn't whip around up top.
b) It would need to be able to rotate to allow for sail twist. The higher you go, the further aft the apparent wind tends to move, so a twisted sail becomes more efficient. This means the sail top will likely not be fully aligned with the mast at the masthead
It seems like it could be a great idea, but I'm not sure if I'd want to be the one to test it. I'd probably take the lazy approach and wait for someone else to go through the trial and error.
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The big question is how much buoyancy you'll need.
Let's say you take a 12m Harry that's on it's side, assume that the windward hull weighs 1000kg with gear, water, and people, and assume that the beams have slanted 10 degrees past vertical because the masts aren't located directly on the leeward edge (or due to swells). With 6m of beam above the water, that puts the center of the windward hull 1m horizontally past vertical, yielding 1000 kg-m of heeling moment.
Assuming a 15m mast, and going with Robert's calculations of 160kg of flotation at a center of effort 5m up the mast, that's 800kg-m of righting moment, leaving another 200kg-m to be dealt with. That means 13.33 kg of flotation at the mast head. Let's round up and call that the equivalent of ten two-liter bottles of foam at the masthead.
I do think it's possible to come up with that, given your design. A bit bulky, but probably not that bad if distributed the way you suggest. It wouldn't be the same, though, if much of that volume is on the sail top and the sails are reefed.
Four to six square meters of 5mm foam would also probably be enough, but only if the sails were all the way up.
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On a separate note, you'd likely need a lot more flotation for a catamaran or trimaran because both would have to tilt further before the masthead float would become effective.
- Mike
Chris Ostlind wrote:
OK, Mike... and everybody else who may have an opinion on this.
Located in the Files section is a folder titled End Plate Sail where I have put a collection of renderings of a concept device for preventing turtling, principally for multihulls. The idea here is pretty straight forward.
I'd been seeing a bunch of folks talking about adding foam panels up high on the sail to assist the sealed mast when capsized. It occurred to me that you could go all the way with this idea and just make the foam a specific shape to take advantage of an end plate effect at the head of the sail. If it were slotted to fit the sail chord up high on the head, it would help to contain the airflow that gets lost up there and contribute additional drive to the sail. I was going for the same, wingtip thing that you see on most modern airliners to enhance wing lift.
A matching form would be fit to the mast head as well so that the flow would be smooth and not abrupt. This piece, too, could be made to provide buoyancy at the far end of the mast righting lever if tipped over. The two pieces would not be attached, so the sail can be hoisted and/or reefed like normal and the endplate float would move with the sail.
I haven't done any aerodynamic calcs, so I don't know if the design would make more drag than it could save. It will, hopefully, provide two tasks at once and might be worth it. I know for sure that other floaty stuff up on the mast like Hobie Bob's are not making any additional power for their drag, so it might be a cool idea, especially for cruising boats that don't have big worries about total performance like a racer would. They cold pick-up some lost power from their sails and gain some peace of mind at the same time.
If anyone is an aeronautical engineer and can contribute to the design of the product, I'd be interested in hearing from you. There's bunch of prior art established and I have dropped the dime on the USPTO for the provisional patent thing, so I'd like to see where this could go... if anywhere.
Did some rough calculations on the Harryproas stability at 90 degrees. Empty, a Harry is stable on its side but not by a large margin. Probably the resistance...
Hi Rob I do not have experience with the bouy at the top mast but I wonder how much a cleated sail in the water acts as a fulcrum to force the mast to go under...
Hi Carlos, My own experience with lots of capsizes in small boats and windsurfers is that the sail damps the initial rotation and then if you don't uncleat you...
... Brian Phipps does it successfully with Darts, AFAIK. But a standard Dart will not normally turtle immediately anyway. ... Don't forget that in strong winds...
I agree. Beyond the lower righting moment when reefed, buoyant sails would also be less useful once you uncleat the sheet in order to right the boat, allowing...
So, Mike, what if a mast head flotation system could be built that actually enhanced the anti-capsize function while having a positive gain on aerodynamics?...
What about preventing the capsize with a device attached to the ama? Let's say a parachute hung on the side of the ama in a bottomless square box in such a...
<<what if a mast head flotation system could be built that actually enhanced the anti-capsize function while having a positive gain on aerodynamics? Would that...
OK, Mike... and everybody else who may have an opinion on this. Located in the Files section is a folder titled End Plate Sail where I have put a collection of...
-In -http://groups.yahoo.com/group/kssboat/ there are a series of well referenced posts on the problems of capsize after a post by Derek Kelsal about a...
... He doesn't seem to have deployed anywhere near enough parachute with anything like the right amount of tether line. It sounds like some pretty skilled...
That's about as good a masthead flotation system as one could imagine. Nice design, pretty aerodynamic, and it doesn't look like the hobie "training wheel"...
Note , if you are careful with the weight distribution in the ww hull with batteries and water storage you can just bring it back the other way, but it is...
Oh, I forgot... The full treatment to the sail also includes a boom like you would see on the really spendy, mega-yachts where there is a decided wingish,...
An interesting idea, alltogether. Bouyant panels and endpieces. I will discuss this with a sailmaker. I´ll relay his opiinion. Thanks, Peter ... like you...
G'day, Agree with your comments except for the mast angle. I suspect it will be pretty close to 90 as the lee hull will be partially in the water. As long...
-my calculations are in the order of 3-5 degrees and have tried to imagine the circumstances where the wind wave would work together. It would be coming off...
... degrees. ... stop ... situation ... big ... the ... I just don't see the point. The HP is the most stable multihull design in existence relative to the...
One of the big reasons I started looking at passive, functional flotation at the mast head was the complexity factor of anything with moving parts such as an...
Very solid arguments for fixed masthead buoyancy. The potential failure of an active system, and the problems of re-arming it in nasty seas, are tough to...
... My guess is that mast buoyancy or leepod are only good to deal with errors of judgement in moderate conditions. If you flip a cruising multihull by...
<<My guess is that mast buoyancy or leepod are only good to deal with errors of judgment in moderate conditions.>> I agree. I'm not sure that anyone here...
A fully flooded Harry lw hull will have in the order of 300-450kg bouyancy and you would need to move ~9 tonnes of water to fill the hull. It would be easier...
<<It would be easier to lift the 350kg weight of the hull... This is only 700-900kg off a 3m gantry.>> Good point. Not unimagineable once the weather has...
-If I had the drogue set up I would be wanting to avoid being side on, lying ahull. I feel surface area is too great to withstand such a hit of white water....
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