# Leeboard question

Discussion in 'Multihulls' started by bfrowe, Dec 16, 2004.

1. Joined: Oct 2004
Posts: 95
Likes: 4, Points: 0, Legacy Rep: 49

### Phil LockerJunior Member

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

2. Joined: Oct 2004
Posts: 23
Likes: 0, Points: 0, Legacy Rep: 10
Location: Wellington, Ks

### bfroweJunior Member

3. Joined: Oct 2004
Posts: 23
Likes: 0, Points: 0, Legacy Rep: 10
Location: Wellington, Ks

### bfroweJunior Member

4. Joined: Nov 2004
Posts: 568
Likes: 1, Points: 0, Legacy Rep: 12
Location: cornfields

### SkippySenior Member

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

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.

5. Joined: Oct 2004
Posts: 23
Likes: 0, Points: 0, Legacy Rep: 10
Location: Wellington, Ks

### bfroweJunior Member

Thank you for the clarification

6. ### Doug LordGuest

flat on outside

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?

7. Joined: Oct 2004
Posts: 23
Likes: 0, Points: 0, Legacy Rep: 10
Location: Wellington, Ks

### bfroweJunior Member

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.

8. Joined: Sep 2004
Posts: 2
Likes: 0, Points: 0, Legacy Rep: 10
Location: Montana

### m_dolesNew Member

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.

9. Joined: Nov 2004
Posts: 568
Likes: 1, Points: 0, Legacy Rep: 12
Location: cornfields

### SkippySenior Member

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.

Last edited: Feb 6, 2005

10. Joined: Sep 2004
Posts: 2
Likes: 0, Points: 0, Legacy Rep: 10
Location: Montana

### m_dolesNew Member

Thanks for the explanation.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.