Tuesday, February 2, 2016

Single Lee Board

I'm thinking that since Autarkia is so beamy and will have very low heeling angles there is no reason to have two lee boards.  One should do the trick.

I think that mounting it on a pivot pin - in this case a 1 inch bolt (could be bronze) with lots of bearing surface provided with nylon or other plastic bearing plates, should handle the loads in either direction.  I have some black plastic (not sure of the type) already that I can use.


I can build up a board with wood, aluminum and fiberglass pretty easily in my shop at home.  I figure about 25 s.f. in the water should be enough for around 500 s.f. of sail.


If I bolt in from the outside it is no problem to house the threaded portion inside the boat to make it water proof.  Or, making more sense I think, is to have the bolt coming out through the hull and holding the board on with the nut outside.  It can be cottered there as well.

The nice thing about this idea is that wherever I mount along the hull it will not impede the view out the picture windows - board up or down.

9 comments:

  1. Most of Jim Michalaks boats have a single leeboard but braced: really a strongly built external centerboard. Maybe some posts by Jim regarding this in his archives. From experience with the hull shape I'd imagine Dave Z. would have the best feel for this idea.

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  2. Thanks Robert. I've looked at Jim Michalaks stuff and that is what has inspired me on this. At some point Dave will get around to seeing this and comment I hope. I know that his lee boards - though there are two - are kept from winging out on the lee tack so they work in a similar fashion. I'm just thinking about the saving in work to build just one - lazy me. Also having it fixed in one spot may not be so bad with a highly adjustable rig.

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  3. Another nice feature of one board is a free and open side for folks to raft up with you or throw a few tires over when tying to a gnarly dock :)

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  4. One off-center board works great as out experiences with Loose Moose (a Bolger Jessie Cooper) and Loose Moose 2...

    That said the forces on a leeboard can be a lot stronger than one might suspect and I'd be just a little nervous with the amount of concentrated force on the non-hull braced tack

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  5. Thanks for the comments guys. I just been thinking about those forces. In both cases the pivot pin is under tension. The hull braces the board either below the pin or above it. When the board is pressing against the hull below the pin there is a breaking force on the board at the chine. On the other tack, the breaking force is at the pin and with 18" or so more leverage. I'm certain I have the hull structure beefy enough to handle it. The board on the other hand...need to think more :-)

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    Replies
    1. Hi Alan,

      A few comments:

      First, I gather from your beam/heeling comment that you may be thinking of the boards as one might a ballast keel? If so, this reflects a common misconception... boards have a little 'hiking' effect on the windward side, but otherwise are all about lateral resistance and not stability.

      RE Single board... They work just fine, and we often sail with only one down in lighter conditions or when windward progress is less pressing.

      A few caveats: You need more sq footage in a single than two which don't wing out (see leverage, below). The single is slightly less effective on the windward tack as heel lifts some of it clear of the water, reducing surface area.

      RE leverage... To my mind, the weak point is not the pin or the hull, but rather the board itself.

      On one tack, the fulcrum is a bottom of the bearing plate, closer to mid board with less leverage on the upper board.

      On the other, it's the pin, with much greater leverage on the upper board. Especially if the tip of the board hits bottom on a sloshy day, or slamming down a beam sea, there's a scary lot of force involved.

      This is made worse by increased sq footage in a single board, as the force applied to the lever is greater.

      So... a wider bearing plate with a shorter board with less surface area exposed to water is stronger. These can be overcome, I suppose, with good engineering, but that's what's in play.

      One of the reasons we went for the pivot/preventer style was to decrease the leverage on the boards (same on both tacks).

      RE positioning... One thing that drives me bonkers is covering the windows when stowed, vs low stowage with clunkage from water action. That's why we came up with the traveling boards for SLACKTIDE. Your smaller windows, however, may afford a 'blind spot' where they can stow without spoiling the view.

      Over time, we've broken our ply cored boards... the veneers start breaking two at a time (the long grain ply gives and it walks through cross grain). Lately we've been building timber or ply-sheathed timber boards with no further problems. Will likely try a free-flooding board again to keep weight down.

      Good luck!

      Dave Z

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    2. Thanks Dave! My point in the beam/heeling comment was that with a single board most of it would be in the water either tack given the shallow heeling angles in most cases. I wasn't thinking that the lee-board would do anything other than provide lateral resistance. Sometimes I just don't make myself clear :-) As for square footage: Is 4 percent or so of sail area enough by your experience? Do both of your boards working together exceed this?

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  6. On Luna (AS31) Dave put on really robust metal guards on the bottom. One side was a 1.5X1.5 thick walled steel bar and the leeboard fit between it and the hull. Essentially a off-centerboard. Really stout and well anchored through the 1/2" ply side with backing blocks inside. The other side was a PVC thick walled pipe with a stout wire inside. This was one of Daves early experimental constructions and each board was different too. One was free flooding, the other solid. We ran Luna with only one board for awhile and if it had been a bit larger I think one would have been fine. Jeez, isn't all this design stuff awful? :)

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