Leeboard Curvature

I have recently purchased a Bolger Black Skimmer. Like many of his designs it uses leeboards. Elsewhere I have read that making the inner sides of these convex will substantially improve pointing and make tacking easier. (I have read that tacking is an issue with this design.)

With my CNC router the creation of any convex surface is quite trivial. But I don't have any idea of the curvature I should be aiming for.

Can anyone supply a formula or rule of thumb? The boat is 25 feet long.

Thank you.

 

My limited experience with Black Skimmer did not show any hesitation or any less ability to tack smartly. The one I sailed for a while had a sprit boomed main and small sprit boomed mizzen.

Not too sure about its suitability in BC but it was an exceptionally capable boat in the Bay of Florida where the water is almost always very shallow. Not to say that the shallow water does not get lumpy, it does. I have sailed the black Skimmer in a squall, and it held its own rather well. As for the leeboards, I reckon the flat ones are entirely adequate and cambered lee sides are not likely to be worth the trouble. In fact that mod might be problematic because the board might not lay perfectly flat against the inner support runner.

That boat has some of the most sensible design features, such as the flooded forward compartment and the flooded outboard well. Sounds wrong but it jolly well works and causes no apparent problems. Bolger boats are sometime derided as floating boxes. Those marina smart asses who do so have not lived with a thoroughly practical floating box like the Skimmer. What other boat of that size can actually sail in twelve inches of water?

 

Maybe you want to go with a NACA profile like this one (from Building a custom wood sailboat, the 25' Black Skimmer https://www.nexusmarine.com/skimmer_construction.html)

If you look closely you'll notice that the board is shaped convex on both sides, same as a daggerboard.

Edit: You could make use of assymetrical leeboards to provide more lift to windward.

 

I don't have a scientific answer but it is fun to think about, maybe NACA0009 through 0020? I might try to pull the thickness more towards the bottom (top on picture). Program is finFoil, which is now online as finFoil.io

 

If built to plan, the chine log is on the outside. That has to create a whole mess of turbulence and probably some leeway resistance too. The lee board is a surface piercing item that is also not the best for lateral resistance or lift. It is inclined to ventilate which reduces the effectiveness. It is a big board that is adequate for the job in most cases.

The leeboard gives the boat ability to operate with reasonable safety in very skinny waters. It also relieves the cabin or cockpit of the clutter of a centerboard trunk. Good trade off if ultimate performance is not the sole aim.

For what it is worth the boat is very fast when overpowered, in that case it has near planeing performance. I do not recommend that kind of fun but the flat bottom and external logs and stringers are both a measurable disadvantage at low speeds and a clear advantage at higher speeds.

If your Skimmer refuses to point acceptably, get some new sails. It ain't ever gonna point as high as a megabuck 12 meter. Don't mess with the lee boards except to make them smooth and reliably in place. The rope top constraint, as Bolger contrived, is a real convenience for lazy sailors. If you have a need to race the ballasted boats, wait for brisk winds and then show 'em your transom. The Skimmer is not a racing boat but it is not a dog either.

 

I had expected that by now we might have seen some suggestions from a Dutchman because there are lots of leeboards in their waters and centuries of experience of them.My own fairly cursory observations have noticed that they are asymmetric foils with distinctly greater curvature in the inboard face.I expect that the water in the Pacific North-West will behave in the same manner and may best be sailed on with something very similar.Which means that the challenge is determining the best shape and I wonder if Xfoil or one of the similar programs would lead to a satisfactory solution.It would be easier than turning a lot of good wood into shavings as well as less expensive.

 

1/4 Dutchman here, not a NA but i have some clue about leeboards. I've also overheard old salts endlessly discussing the curvature in their boards.

First picture is how a classic leeboard is constructed, the rightmost drawing gives an indication about the profile. The curve is on the "inside" but flat where it rests against the "chine log, don't know name in english" or "inner support runner, as messabout called it in his first post". The log actually is quite a bit above the waterline and the profile is flat at that level as to not wobble against the log. The angle of the "chine log" relative to the centreline in top view is also heavily debated and experimented with. (Edit, called an old salt, 2 instead of 3 degrees of angle seems to be a good start to experiment from. On factors that influence less or more fractions of degrees from there he mentioned so many, it's hard to replicate his story. Underwater shape of the specific boat also plays a big role). The leeboard in this picture is short and has a long chord, optimized for shallow water. It also is adjustable in the longitudinal direction although those details are not very clear in the drawing.

https://friesscheepvaartmuseum.nl/collectie/detail/fsm-col1-dat1000016324

Second picture gives a more detailed example about how some classic leeboards were constructed in relation to the rest of the boat:

https://friesscheepvaartmuseum.nl/collectie/detail/fsm-col1-dat1000016474

Third picture is a long, deeper board for deeper waters. It will be more efficient than the board in pic. #1&2 It also shows a somewhat more sophisticated profile than the first. Note the board will be held together with a steel, profiled, band around the rounded bottom and the leading and trailing edge of the profile. That steel will be rounded at the front and sharp at the trailing edge, completing the profile of the leeboard.

https://thumbs-eu-west-1.myalbum.io/photo/360/c1f2f0c1-03a6-463a-a7dc-5fd975dbe8e7.jpg

The difference between a flat board (even if it's thin) and a profiled one will be huge. Any profile resembling the one in pic. #1&3 will be reasonably efficient with diminishing returns from putting effort in a more optimized profile. Perhaps a real NA can chime in on optimum board thickness and profile.

Of course there is nothing withholding you from constructing boards from ply/cloth/epoxy.

Edit: picture of completed boards, modern insights were implemented in these with restrictions to keep the classic looks. The metal plate on bottom is for protection when grounding. Unfortunately the profile is not visible but it gives a good indication of their construction.

https://kroesbootbouwers.nl/wp-content/uploads/2016/01/werkzaamheden_overig_zwaarden_lemsteraak.jpg

 

The most comprehensive set of articles on leeboards i could find online. In Dutch but pulling them through Google translate should be possible. Pictures speak for themselves.

https://www.ssrp.nl/publicaties/spi...z-1991-nr03-april-over-zijzwaarden-1-kleinpdf

https://www.ssrp.nl/publicaties/spi...sdz-1991-nr04-mei-over-zijzwaarden-2-kleinpdf

https://www.ssrp.nl/publicaties/spi...dz-1991-nr05-juni-over-zijzwaarden-3-kleinpdf

https://www.ssrp.nl/publicaties/spi...-zijzwaarden-voor-platbodemschepen-4-kleinpdf

I'll include the same articles as a file upload on this site.

Edit: google translate seems to have trouble with these, i'll put some effort in getting parts relevant to the question translated into English tomorrow.

 

I don't think a foiled lee board would be hard to design and fabricate, especially using some sort of CAD and CNC as the original poster is intending. It also seems like a person could make one and install it, then compare it to a 'stock' lee board on the other side of the boat for a comparison. On my small boat I foiled the rudder and leeboard with an electric planer as shown in the third picture from Floatation's first post except I did not use templates, I just watched the grain in the plywood as a guide.

@David Ford , are you getting more ideas? Are you comfortable designing objects in 3D to machine?

 

Thank you for this.

Gentlemen: I am overwhelmed by the number and helpfulness of your replies! I was only notified about one of them, missed replying, so I have some catching up to do.

 

This boat will be based at our place on Thetis Island, in the heart of what some consider to be the best cruising ground in the world. This is particularly true if you have a boat that can get into the small spaces, and this one has been carefully reinforced to do just that! I was also attracted by the possibility of keeping it on the hard, and thus avoiding the anxiety and expense of a marina.

The builder needed it gone, but after the hours and hours he put into it couldn't bear to part with it (I know this one). I paid an embarrassingly low price for it because I promised it would get a good home.

You should know that the CNC router is my latest tool (or "toy" as some here insist on calling it) and I am anxious to see what it can do.

 

I looked at that picture many times and didn't notice that. Thanks!

 

Wow! I will forward this link to my propeller head son.

 

The builder did not want to put the chine logs on the outside, but in a series of exchanges Bolger convinced him that this was the right thing. Seems he was concerned about the quality of the interior accommodation, strange considering he made the cockpit 10 feel long and the cuddy only 4.

This boat is really for my son and grand children. I plan to make sure they can reef quickly! Not that wind is much of a problem around here in summer.

The sails are from North, so I am hoping they are adequate. The boat is in surprisingly good basic shape but I will certainly have some work to do before it can go back on the water. Wish I knew someone who enjoys sanding.

 

Another one to send to my son! No idea these foil design programs were out there.