Leeboard question

[Phil Locker]

I agree completely with all that Seadrive said. BUT...

perhaps I'm being difficult, but I think it would be interesting to see some discussion of what does happen on the lift side of system when, say, switching to a cambered leeboard.

Its all well and good to say that lift of the foils is in a fixed linear relationship to lift of the sails (leading to the discussion of gaining performance through reduced drag) but I think there's a bit more going on than that. The performance curves of the different sections are different, after all...

Cheers
Phil

[bfrowe]

[quote=Skippy]That's true SeaDrive, but you can still optimize for "average" conditions. The point is that the demands placed on each leeboard are asymmetric, i.e. one tack only.

The point of my question derived from the memory that my old Prindle catamaran had asymmetrical hulls with the flats to the inside. LORSAIL says that if I use asymmetrical leeboards, that the flat side will be on the outside of each board. Is this the consensus of the thread?

[bfrowe]

[quote=Skippy]That's true SeaDrive, but you can still optimize for "average" conditions. The point is that the demands placed on each leeboard are asymmetric, i.e. one tack only.

The point of my question derived from the memory that my old Prindle catamaran had asymmetrical hulls with the flats to the inside. LORSAIL says that if I use asymmetrical leeboards, that the flat side will be on the outside of each board. I assume outside to be away from the hull and inside to be against the hull. Is that correct?

[Skippy]

Originally Posted by bfrowe

I assume outside to be away from the hull and inside to be against the hull. Is that correct?

Yes, I'm sure that's what he meant.

Originally Posted by bfrowe

The point of my question derived from the memory that my old Prindle catamaran had asymmetrical hulls with the flats to the inside.

As far as I know, there are two reasons for the asymmetric hulls:
1. The bottleneck effect: with symmetric hulls, the water passing between them is constricted at the point of max beam, more or less depending on how rounded and how far apart the hulls are.
2. Stability: when the boat heels leeward, the leeward asymmetric hull will tend to head the boat into the wind like a monohull does.

But as for why face the leeboards outward, the leeboards are foils, and you only have one of them down at any given time. So LORSAIL is assuming you have the leeward board down, and you want the flat side facing leeward (out) so the lift is windward, just like the flat side of an airplane wing faces down so the lift is up.

[bfrowe]

Thank you for the clarification

Yeah, thats what I meant.When the LEE leeboard is down if it's using an asy section the flatter side should be outboard.
Interestingly, I just saw an old Prindle today-flat on the outside as is the Hobie 14 and 16. They're flat on the outside so that when the boat is flying a hull the lee hull acts like a (very inefficient) foil.
Another example is seen with the twin asy daggerboards used on Open 60 mono's...
Have you definitely decided to pursue changing your boards?

[bfrowe]

Well, I got the asymmetrical part right, but not the right side. Sorry. Yes, I think I'd like to try to make or have made an asy board without a lead shoe.

[m_doles]

I wonder if a leeboard heels a boat differently then a keel? I am thinking of pivot points and fulcrums here. A keel and the mast line up and their forces would be in a single plane. With the leeboard being to one side of the hull it seems the board would try to lift the entire weight of the hull and that the pivot point would lie somewhere between the leeboard and the mast. Any ideas here.

[Skippy]

Quote m_doles: A keel and the mast line up and their forces would be in a single plane.

md, it's not the plane that matters. The thing that affects rotational moments is what's called the "line of action". In normal english, that's a line in the same direction as the force, passing perpendicularly through the center of the sail, keel, or leeboard, i.e. the spot the force is effectively acting on. Both the keel and the leeboard act along roughly the same line, athwartship and slightly below the hull (i.e. far below the sail), which is why they produce a large heeling moment. The keel and the leeboard both lift the boat whenever it's heeled over, and the sail likewise pushes it down. The center of rotation will be determined by the hull, since that's the thing holding the boat up in the water. The biggest difference, in addition to some hydrodynamic inefficiency of the leeboard, is that the leeboard sinks deeper into the water as the boat heels, except for single-leeboard setups, where on the opposite tack, the leeboard (now on the weather side) pulls out of the water. That affects the amount of leeway the board allows. The only other significant functional difference I can think of, is that the keel is mounted lower (on the bottom of the hull), so it might produce a little more heeling moment.

[m_doles]

Thanks for the explanation.