Jump to content

Leeboard aspect ratio


JeffM

Recommended Posts

Does anyone know any simple formula that converts a high aspect leeboard/cb to a lower aspect one of the same lateral plane effectiveness? I am looking into shortening the design of the leeboard in my Michalak "Piccup pram" to enable it to sail in shallower water, but I expect I cannot simply widen it by the same number of square inches I will lose by shortening it: the loss of pressure over the tip of the foil will be greater with a wider board. I'm hoping someone can tell me how many inches of the end of a cb/leeboard are basically useless, so I can build that area back into the shorter, wider leeboard design.

Link to comment
Share on other sites

  • 8 months later...

Howdy Jeff,

A fabulously difficult question to answer!!!

How fast do you sail your boat!?

Generally the depth is the single most important determinant.

If you reduce the depth, you will have to increase the width by a greater percentage. 

By the way, if the foil is poorly shaped now you may actually gain performance with a shorter, wider one with a more perfect shape.

Best wishes

Michael

Link to comment
Share on other sites

Michael, your picture is a pretty little double balanced lug, which made me curious to check out your web page.  Unfortunately my browser can't find it.

I finished the boat (a Michalak Piccup Pram) in the spring, giving it the designed leeboard, but I moved it forward a bit and swept it aft to bring the center of lateral plane back to its original location (right below the center of effort).  My idea was to be able to raise the board further in shoal waters without moving the clp much farther aft and goofing up the boat's balance.  I can't say I've given the idea any rigorous test yet, since I mainly sailed my bigger boat this past season.

Link to comment
Share on other sites

There is no "simple formula" or easy conversion Jeff, unfortunately. Foils are a bit of black magic in this business.

To be frank, most of us use shapes that we've had success with previously. This includes aspect and sectional arrangements.

Leeboards permit the shallowest gunk holing possible. Unlike a dagger or centerboard, the lateral area of a leeboard is effective from the LWL down. More convention "cased" board's lateral area begins at the bottom of the boat.

A leeboard retracted to the bottom of the hull depth will still provide lateral plane. Of course, it will not be nearly as effective as if it was fully deployed, but it's better then nothing. Since the Piccup pram draws only inches of water when underway you can "bounce" the board along until the bottom of the boat, finds itself firmly aground.

Call Jim and see what he has to offer. How much do you want to shave off the board? Any decrease in aspect will lessen your windward abilities. Personally, I don't think you need to add area to a lower aspect board, you'll just lose efficiency and no additional lateral area will make up for it. If you feel you must, then add 10% more area, but focus on the sectional shape, to try to enhance the limited lift you are generating with the new board.

Link to comment
Share on other sites

There is no "simple formula" or easy conversion Jeff, unfortunately. Foils are a bit of black magic in this business.

I don't think you need to add area to a lower aspect board, you'll just lose efficiency and no additional lateral area will make up for it. If you feel you must, then add 10% more area, but focus on the sectional shape, to try to enhance the limited lift you are generating with the new board.

Quite right ... The end result will probably be a performance decrease.

But area can help compensate - probably not so much in smooth water, moderate wind conditions but when the water gets rough and the boat starts to bounce around.  A low area high aspect foil relies on speed to get lift - so if you hit a slowing wave there is suddenly not enough lift and the boat slips sideways and the foil will stall.

Because it is stalled you can't get up the speed to unstall it - so you have bear away a fraction to ease sheets a little to get the speed up and then point again.

A boat with a less critical, larger-area keel might have a bit more drag but it will be much more tolerant in such conditions.

I would be making the board a bit wider to try and make sure that this area effect does not come into play.  Perhaps many leeboards reflect this - being not particularly deep but very wide indeed.

Best wishes

Michael Storer

The Balance lug yawl/ketch thing is here

http://www.storerboatplans.com/Beth/beth.html

Link to comment
Share on other sites

  • 3 weeks later...

Jim Michalak's designs typically use one leeboard, generally on the starboard side. That leaves the other side for you to try lower aspect foils you might build. You might even try something like a side mounted daggerboard trunk that will allow you to mess about with profile, width, depth and for and aft positioning. It would require a little more reinforcing than a Michalak leeboard since you don't have the pivot pin to support it on the port tack.

I also saw a clever "clothespin" style leeboard in Woodenboat's supplement on building a cheap,small, sailing skiff.  That might work even better on a Piccup than it did on their design since Piccup's sides are quite vertical. The more I think about it the more I like it. I'll try it myself!

Chad

Link to comment
Share on other sites

I thought of this earlier but realized it would not work very well for a leeboard because it does not work when rotated but if you were going to try a side mounted dagger board slot then you might try going with a board that has runners on the bottom. It would be as easy as attaching a 3-4" wide board to to the bottom of the vertical board. (So it would make an upside down "T" shape in the cross section). By increasing the width of the bottom surface you reduce the effects of tip losses in the foil as the water on the leeward side tries to move under the board to the windward side and creates a vortex.

It would make the board more effective at reducing leeway maybe to the point where a lot of extra width in the board to offset the lost length would not be as much of an issue. And it does not increase you draft.

aV23bJNi.jpg

Sure the foil shape could be a little more refined but you get the idea. Anyway, it's a pram, not a racing boat. And I know thats not a piccup pram. 

More ideas the better.

Link to comment
Share on other sites

Alan, I toyed for a while with the idea of borrowing the T-section idea that Bolger uses in his shallow draft rudders.  Since the leeboard would need to be useful at different angles, I thought I'd make a free-swinging thing out of two pieces of right-angle aluminum stock, pivoting on a bolt through the lower forward corner of the leeboard, and kept level by the flow of water over the leeboard.  (Someday I'll have to learn a drawing program.)  I decided against it partly because it was hard to imagine smooth water flow around such a monstrosity, and even harder to imagine it working when banged up, clogged with seaweed, etc.

Link to comment
Share on other sites

I thought of this earlier but realized it would not work very well for a leeboard because it does not work when rotated ...

I was thinking about using the clothespin leeboard idea just as a way of testing alternate boards. With the snap-on on one side and the original on the other, you could compare and contrast. You could easily mess with boards of different sizes and shapes and when you found one you liked, you could attach it in a more permanent manner. Or, you could use a low-aspect, snap-on board for the shallow stuff and switch to the higher aspect board in the deeper spots.

Anyway, it sounds like fun, building and trying trial boards. A Piccup is so straightforward it really wouldn't be much work.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

Supporting Members

Supporting Members can create Clubs, photo Galleries, don't see ads and make messing-about.com possible! Become a Supporting Member - only $12 for the next year. Pay by PayPal or credit card.




×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.