Subject: Re: [harryproa] Re: mast bouyancy
From: Mike Crawford
Date: 2/8/2006, 8:10 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au


  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.

---

  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.

---

  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.
http://au.groups.yahoo.com/group/harryproa/files/End%20Plate%20Sail/
 
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.
 
So, have at it. Would it work?
 
Chris Ostlind
 

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