# Foil Ratio

Discussion in 'Multihulls' started by caiman, Dec 26, 2010.

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### Tim BSenior Member

"Hence the CE of the sail is AFT of the CLR, providing a head-to-wind moment, if not countered by the rudder."

Apologies, this is a (relatively rare) case of me thinking something through correctly, then trying to simplify it and using completely the wrong terminology.

I should have said that the sum of all moments acting about a vertical axis through the yacht (typically the CG) should resolve to give a head-to-wind moment in the absence of the rudder. The rudder then counters this moment (weather helm), giving a vessel which is safe. The forces acting in the transverse direction (ie. sail, keel and rudder sideforce) must summate to 0.

However, these forces are really quite hard to calculate. Therefore some massive simplifications are employed. Firstly the centre of effort from the sails is assumed to act at the centre of area (for a sloop rig), and the effect of the hull, keel and rudder are assumed to act at the center of profile area of the underwater body (some sources ignore the hull, others ignore the rudder). The centre of area of the sail is then set a "bit" forward of the centre of area of the underwater profile, and this "bit" is termed "lead".

Personally, I think the latter simplification causes far too much confusion, so I would suggest that more research is required on the actual force/moment balance outlined in the former statement.

Hope this is less confusing,

Tim B.

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### RHoughRetro Dude

I agree 100%

Trying to solve this using the aerodynamics that I know has been an ongoing part time effort for a few years.

R

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### brucebSenior Member

tri yaw?

RH- I don't pretend to be an engineer here, but from experiments with my tri, in light air/low speeds there is some yaw from the floats- I use it to help during a tack. However, as speed picks up, at least on several tris I have sailed, the lift from the leeward float bow overcomes most all of the yawing tendency. The profile of most tris' floats are biased forward and balance the yaw. Good designers get it right and it is not an issue. Monos don't have a choice, and I have sailed lots of short fat race boats that develop lots of drag/yaw when they are not kept flat. The board issue is somewhat different on small tris also. On most monos, the hull works as an effective end plate, but certainly on faster moving tris, the top of the board is almost always ventilated as the hull is too skinny to keep the air out. The first two feet of water from the surface is dynamic anyway if the waves are up at all, so a tri with a 1.5' hull draft and a 5' draft overall only has about a 2.5-3' effective board. Not a lot. I guess the 60' multis don't have to worry about the surface much, but on my small boat 2' of foam on top of a moving wave amounts to a lot, and the lift/drag formulas don't mean much. I have watched the separation from my windward float many times- tris give you a great view down through the tramps Next season I am going to tape some yarns to my board and try to get some pictures- it should be interesting. B

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### Gary BaigentSenior Member

At one stage on my foil trimaran, I had 45 degree angled foils set forward of the beam connection in the small floats, the rudder was an inverted T, quite deep for the boat's size; there was NO main hull dagger, the boat relied purely on the angled leeward foil for lateral resistance plus being the lifting foil simultaneously. Now, the rig was/is a wing mast una, no headsails to shift the CoE forward (see jpeg), so upwind sailing the CoE was way aft of the CLR ... so the boat was supposedly way out of balance with theoretically so much weather helm it would be unsailable ... but it was not. The helm felt sweetly in balance. However and admittedly, it was easy to get in irons halfway through a tack if you didn't execute it correctly; but this is an inherent problem with all square and lightweight platform craft. But two dimensionally, this boat should be an utter pig ... which it wasn't/isn't.
So what I'm saying to Caiman, it is better to have an unbalanced weather helm craft than going the other way and suffering lee helm. A reasonable quick fix for you is to build and place a balancing dagger board further aft, say somewhere in the cockpit; that will shift the CLR back and get rid of your lee helm problems. A plus with such a setup is that you can lower or lift the balancing board so the boat will sail hands off on many points of sail. Ron Given's famous 40 foot cat Split Enz was originally setup something like this, two daggers, one forward, one aft, plus a rudder in each hull.

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### RHoughRetro Dude

I'm going to have first hand experience in a year ... I'm starting to build the work space for the W-17 next month ...

I'm sure you have read Betwaite's book, he did some interesting foil experiments using dye or ink to get a visual feel for the extent of laminar flow. The same sort of thing might be easy to video from the windward ama?

R

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### oldsailor7Senior Member

Gary.
You said:-[Quote}place a balancing dagger board further aft, say somewhere in the cockpit; that will shift the CLR back and get rid of your lee helm problems.

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### oldsailor7Senior Member

Caiman.
You said

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I think there has been some really good advice given here so I am a little leary of introducing more information than necessary-but here goes.
Bill Roberts concept of "shared lift" was used on his Arc 21 catamaran. Basically, he moved the daggerboard forward and made it smaller while at the same time making the rudder a bit bigger. He didn't change the position of the center of effort or the center of lateral pressure-he just changed the foils a bit. For purposes of illustration ONLY I'm including picture of the Mirabaud foiler that uses this concept as well-note the position of the daggerboard.

Pix: Arc 21 with daggerboard forward of the crossarm and Mirabaud with daggerboard forward of the mast:

(click on image for better detail)

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### oldsailor7Senior Member

Doug.
That's an excellent illustration of how the board and rudder produce and share the windward force necessary to resist leeway.

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### brucebSenior Member

Yes to several things here. I hope all the theory can help Caiman. B

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=====
Thanks-I hoped it might illustrate some of the factors being discussed and how they can be used in modern design.

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### RHoughRetro Dude

The CBTF boats also use this. The goal is to minimize drag. If you can maximize the L/D of the foils in the water, you have minimum leeway. If you can trim both foils so that the hull is aligned with course sailed you get better windward performance. Asymmetric daggers on cats and canting or flapped daggers on tri's make great sense. The very last thing you want to do is drag the hull sideways. The hull L/D sucks compared to the foils.

R

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### Gary BaigentSenior Member

Oldsailor7 said.
"With respect---- Don't you mean Further Forward.
After all, The board Lifts to windward at the same angle of attack as the leeway of the hull, therefore moving the board aft would increase the lee helm."
Absolutely correct Oldsailor7 - I was thinking of Flash Harry's theoretical out of balance weather (but not problematic) helm where a balancing board aft would have rectified it - and mistakenly conveyed it to Caiman's lee helm problem. Of course, for Caiman, the balancing board would have to be placed Foreward, apologies everyone.

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### Gary BaigentSenior Member

Further to forward boards and balancing boards aft; Ron Given's Split Enz had the forward dagger well forward of the main beam/mast position and the balancing board near or in the cockpit. He did this setup in the early 1980's but the crew found the shifting the four boards to be too much of a problem and changed to a more conventional setup (see jpeg). But Ron was proud of his four boards/two rudders approach because by careful positioning, Split Enz would sail herself. That was a catamaran, but on a trimaran you would only have two boards/one rudder - and that would not be difficult to handle and trim.

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### oldsailor7Senior Member

Gary.
I wonder if the people who hold the patent for CBTF could hold out for a licence payment if two boards and a rudder are used in this instance.

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