What would you do with 40 sqm/430 sq' of sail?

So has short and fat. Designing sailboats is the second oldest profession, there's not much hasn't been tried before.

 

I know you've seen JPZ's 18' foiler, Doug. For anyone who hasn't, check out http://boatdesign.net/forums/showthread.php?t=13513&page=2

It seems to me the boats to beat, in the near-shore contest, will be sailboards & Ketterman hydrofoils (a.k.a. Hobie Trifoilers). But that shouldn't stop others from trying.... sounds fun!

I'd like to nominate Bob Esmer to create the trophy: www.bobemser.com

 

Metacentric height varies with Beam^2 / Fairbody Draft.
As long as that ratio remains constant, you should be able to make your boat as long as weight/structural considerations allow without having a big effect on stability.
Each time the beam is halved, the draft needs to be quartered, meaning you will only have 1/8 the midsection area. Therefore you only halve the beam if you are increasing length by a factor of 8 and maintaining the same displacement. I want to say beam would scale with 1 / the cube root of the length scale factor, while draft would scale with 1 / the square root -- but I'm not sure that's exactly right. Anyone want to take a shot at writing the function correctly?

 

trophies and hydrofoils

Good on the trophy if it didn't cost about the same as one of the boats.
The Ketterman or Bradfield three foil foiler would have a good chance inshore in winds over 15; the monofoiler or "bi-foil tri" in lighter and probably rougher conditions. The Ketterman and Bradfield boats don't come close to the pointing ability of already existing monofoilers but their top end speed may be higher. I've seen a Rave up close in 15 knots and video of a Moth in similar conditions-I think the Moth would beat a Rave in 5-20knots with a 9m² sail particularly around a course where the monofoiler's superior upwind ability would be a major player. I think a 24-28' super Moth(either bi-foil tri or canting keel foiler -see sportboat thread) that had been debugged and refined in a two boat test program would be the fastest boat with 430 sq.ft.in inshore conditions and quite possibly in a relatively peacefull transpac as well. If the conditions were less than 50% foiling conditions Robs boat wins hands down in the ocean.
I urge anyone- who hasn't already- to read Rob Denney's technical description of his 50' proa -link in his last post.

 

Stephen, could you please repeat that slowly for me? I am cranium-challenged.

 

I agree with what you are saying, but the trouble is that maintaining a constant B^2/T whilst increasing length results in a lighter displacement (for a constant block coefficient). You therefore have to carry less lead, assuming the boat is being built as light as possible anyway - so there is no chance of using lighter hull materials. With only 40sqm of sail, you don't need that much ballast, but obviously there comes a point where you will have too little. Without having done any calcs, I guess this will be when L = 35-40ft.

 

Sorry, SuperP, I didn't see yuor post when I put mine in. What Stephen is saying is that you don't halve the beam when you double the length, to maintain a constant displacement or initial stability. You only halve the beam when the boat is eight times longer. The problem (that's too strongh a word, but I can't think of a better one of the top of my head) is that stability and displacement don't scale the same, so you'll need to change the block coefficient (and hence hull form) to keep the two balanced.

Is that right Stephen?

 

I was assuming a constant block coefficient, a constant displacement, a constant VCG, and that we're talking about a monohull sailboat. The main point is the first, about maintaining a constant B^2 / T to maintain comparable form stability. The implication, as I see it, is that if you're thinking about increasing length and decreasing midsection area, fairbody draft (T) should diminish faster than beam (B).

When I say fairbody draft, I mean hull draft, not counting the keel. And what I'm really referring to when I speak of its diminishing is all vertical offsets (waterline to outer surface of hull) diminishing by some factor.

I've made an edit to my earlier post because I'm not sure I had the math exactly right. Not your fault you were confused .

 

I did, on your advice Doug, and I approve. I'm liking those Harryproas quite a bit.

As the site itself notes in discussing the most radical of the proas <http://www.harryproa.com/Texel/texel.htm>, though, the achilles heel is lack of ability to move weight aft, resulting in the bow being pressed if pushed hard.

But perhaps this is an opportuinty for a design breakthrough, esp. where Rob is in Australia. Is there an active Moth fleet where you are, Rob? Sensor wands and T-foils on proas might be the answer! Another possibility might be a kite sail or a canted sail to leeward like http://www.boatdesign.net/forums/showpost.php?p=114875&postcount=406... the idea being that the sail pulls & lifts rather than pushing the bow down (of course).

Quite a group of innovators down under.... Lindsay Cunningham (Yellow Pages Endeavour), John Ilett (Fastacraft), Andrew McDougall (KA Sails), Rohan Veal and the other Moth sailors, Leo Lazauskas on theory, and Rob Denney with his proas. Get all those guys in a room and put a challenge like this in front of them...

Hey wait a minute, Rob, you're the guy who started this thread! Sometimes it takes me a while.

No relation to the proas, but the general direction of this caused me to think of the boats in these photos:

 

Limiting sail area drives the class to very high aspect ratios and does not allow for adding sail area for downwind legs. Basically you're talking about an A - D class catamaran without any constraints on the hull. The American open-class landyacht classes are of this type. The tall rig leads to a large chassis to counter the heeling moment, and you end up with a large, spindly, lightweight craft.

I think an even more interesting class would be one that only had a limit on the height of the mast. This would allow you to vary the sail area as appropriate. The limit on mast height also limits the height of the center of effort, and that naturally limits the beam because there's no point in making it too wide.

The interesting trade here would be efficiency vs area. There may be an optimum area to drive long, fine hulls. More sail area would add parasite drag and weight, cutting into the L/D and limiting speed. For example, my landyacht is faster with its small sail than with its big one in all but the lightest winds.

 

G'day,

How large, spindly and lightweight the craft become is what I am interested in. Racing with crew mid summer on a short course in San Francisco and sailing solo across the Pacific will ensure that the boat has some robustness, and may penalize ultra high aspect ratio rigs.

However, thinking about what I would design for these two races with a mast height limitation, I don't think I could do much better than a very long, light proa with a single sail. My mast is 11m/37' off the deck. F 27 tri, Stilletto 27 cat, Thompson 770 sportsboat, Hobie 33 keel boat are all a little higher than this and I will be disappointed if I don't beat all of them on all points of sail in most, if not all, breeze strengths.

I have a lot of respect for your opinion and knowledge, I would be very interested to hear your thoughts on what would be fastest under both rules, for both or either, courses.

Curious about your land yacht. Is the smnall rig advantage due to a stability limit, or a sail shape control limitation? Is there anything you can change to make it faster with the big rig? If nothing, does it follow that small land yachts beat big ones? If so, what is the optimum smallness? These are genuine questions, I know next to nothing about land yachts.


regards,

Rob