Leeboard Orientation

Discussion in 'Hydrodynamics and Aerodynamics' started by Justaguy, Nov 27, 2015.

1. Joined: Jul 2010
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latestarterSenior Member

For the sake of simplicity I have assumed that the hull has no windage and lateral resistance to the water.

If the boat is in equilibrium all forces and moments must cancel out.

Vertically the weight of the boat and crew equals the displaced weight of water.

Horizontally the water force on the leeboard equals the wind force on the sail.

The moment due to the crew WxB1 equals (water force on the leeboard)x H1 or (wind force on the sail)xH1. H1 being the vertical separation between the 2 horizontal forces.

The leverage of the crew is their weight times the horizontal distance to the centre of buoyancy, it is not dependant on the horizontal position of the leeboard/centreboard/windwardboard.

I have only considered the case of the boat being sailed flat, it is too early today to think about it heeled .

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tspeerSenior Member

From a hydrodynamic perspective, any vertical keel, centerboard or leeboard will increase the heeling moment. That's because the hydrodynamic function of a keel or leeboard is to oppose the side force from the rig, not the heeling moment. The boat has to be in equilibrium with respect to the forces as well as the moments.

A conventional keel opposes the heeling moment by virtue of its ballast, lowering the center of gravity of the boat and acting like a pendulum when heeled. A leeboard has no ballast, so it does not serve this function at all. Instead, the lateral shift in the hull's center of buoyancy when heeled has to provide all of the righting moment.

Whether the convex side of the board is toward the hull or to leeward, the board will still oppose the side force from the sail rig. The leeway angle will adjust so as to bring the forces into equilibrium. The difference is how efficiently it does so.

If the convex side is to leeward, a larger leeway angle will be required to generate the same side force (equal to the side force from the rig). The drag will also be higher because the local velocity of the water going around the leading edge will be higher, causing more skin friction. And at low speed, such as when coming out of a tack, the board will be more prone to stalling. (Although stalling isn't a big issue with leeboards, because you tack with both boards down, doubling the lateral area.)

If the convex side is to windward, the drag will be lower and the leeway angle will be lower. So put the convex side toward the hull.

3. Joined: Feb 2002
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tspeerSenior Member

I learned to sail by putting leeboards on a canoe. Admittedly it's hard to see, because you don't peer over the lee rail of a canoe under sail unless you want to go swimming, but I never sensed there was a significant problem of interference between the leeboard and the hull. My canoe was able to literally sail circles around the sailboats of the same kind from which I took my sail rig.

The interference drag of the leeboard and hull is small potatoes compared to the other sources of drag, such as the induced drag due to lift on the board. it's much more important to make sure the board is long enough to reduce this source of drag.

4. Joined: Sep 2004
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Paul ScottSenior Member

We had a Grumman 17' canoe with the Gunter kit. I was about 8 or so, and putting in 8,000 yards or so a day on the swim team, a lot of it with a kick board in front of my nose, so the nature of a board going through the water was right there to look at and feel, for hours and hours a week. So we rapidly discovered that the board dragged less when it was planing right after pushing off the wall, but bogged down quite a bit at normal kicking speed. You could get the wake cleaner if the nose of the board was down , but that meant eventually you had to take your arms or elbows off the top of the board and move them to the sides, or move the board underwater, which meant it all was dragging in the water, and that was really slow, and hard to control, although the wake looked and felt better. Using the board vertically was better if you could get your arms out of the water, but the coach was not amused with dowelings stuck through the board. Or 2 boards glued together. Or sanded boards.....

Anyway, all that to set up that when dad added the sailing kit to the Grumman, all that gurgling from the leeboard when the canoe was healed really bothered me. That and she'd slow down. I wanted to see what was going on, so if I got the canoe stable on a course, laid down across the hull with my legs extended out to windward and my head next to the leeboard attachment point and bent and straightened my knees to control the heel, it was pretty apparent that when the leeboard and it's attachment point went into the water, things slowed down. Sailing against other boats, it was about the same as sticking a fist in the water thumb first over the side. So I found a 2 by 4, put that on top of the aluminum athwartship bracket, attached the leeboard hardware to that, and the canoe would swamp before the bracket dragged. And the gurgling and the subsequent slow down went away, at least when the inside of the canoe was dry. In waves, not so good. Sanding (streamlining) the 2 by 4 helped, as did replacing the lever screw attachment with a bolt.

This is (the board attachment point) what I meant when I said lump, above.

How far from the hull would a lee or windward board have to be from the hull to equal the intersection drag of a submerged centerboard root at a hull? Or are air water interface effects always more draggy?

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phillysailorJunior Member

I sanded the leading edge of my Va’a motu leeboard to a knife edge at the air/water interface at the instruction of the designer, Gary Dierking. There is less noise, splashing and drag that way.

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tspeerSenior Member

The purpose of a leeboard is not to counteract the heeling moment. The purpose of a leeboard is to counteract the side force from the sail rig. In this, the leeboard adds to the heeling moment instead of reducing it. You need to use weight placement to counter the heeling moment.

The leeboard section can be symmetrical, like a centerboard, or cambered. Because leeboards are used one at a time, with the windward board retracted, the lift on the board is always in one direction. A cambered section can be designed to have lower drag for the operating condition of the board, and that's why you see the convex side against the hull.

The fact that the hull is on the suction side of the leeboard is unfortunate from a hydrodynamic point of view. But leeboards are used on shallow draft craft so the adverse interaction with the hull is confined to only a small portion of the leeboard's span. And the structural benefits of not having a centerboard trunk, plus the hydrodynamic benefit of not having an exposed centerboard slot, make the leeboard worthwhile.

I learned to sail in a canoe equipped leeboards, and I think they are highly underrated. Count me in as a fan of leeboards. But please don't flip them around so the convex side faces outward.

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