Subject: [harryproa] Re: mast bouyancy
From: "Robert" <>
Date: 2/9/2006, 7:33 AM

-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 the top of a wave. I those cases the wave would
have moved under before the boat went over too far . What happens
before the next wave, don't know , but theoretically would have the
forces to come upright again. I'd still like a baloon to pop on the
end of the mast in case. I agree that the flex ofthe top of the masts
makes the capsize unlikely. The forces on the top of the mast when
the boat is at right angles is not as great as that that is needed to
tip the boat over in the first place and the top of the mast doesn't
provide much bouyancy, so the loss of righting moment from the flex
is not such an issue-- In, "Rob Denney"
<proa@i...> wrote:
> 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 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.
> regards,
> rob
> From: Mike Crawford
>   To:
>   Sent: Thursday, February 09, 2006 9:10 AM
>   Subject: Re: [harryproa] Re: mast bouyancy
>     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
>     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.
>     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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