Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(15oct07) this issue will rerun the old leeboard geometry essay. Hard to believe I wrote it almost ten years ago, but a lot of the newer readers have perhaps never seen it. The 1 November issue will continue the topic.



is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the....


...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.


Chris Feller's Philsboat has a full house at the last Chicago area messabout.




Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Sizing Underwater Boards

This issue will tell you how to figure the size for the "underwater board" that is used to balance the side loads produced by the sail.

By "underwater board" I mean a leeboard or daggerboard or centerboard or fixed fin or keel. I don't see how the exact type makes much difference as far a abililty to counteract side load goes. There are other practical differences of course.

As in previous discussions I'll assume the underwater board will counteract all the lateral force of the sail. "But," you might say, "the immersed hull itself provides some lateral resistance." True, but that is usually quite small in proportion to that of a good fin because of aspect ratio considerations that we'll go over in the next issue. You might also ask, "Hey, if you make all the sail's force side force, what's to drive the boat forward?" Correct again, but when close hauled a sail produces mostly lateral force with only a small of forward force left to push the boat forward. It's a vector thing and vectors don't add up like regular numbers. For example if you 100 pounds of sail force directed 60 degrees off centerline, you would have 50 pounds of force pushing the boat forward and 86 pounds pushing to the side. Assuming all of the sail's force is side force doesn't introduce a huge error.

I'm going to use Frolic2 as an example again. Remember in previous issues we worked up a righting moment curve for the boat using the Hullforms6S program we got as freeware at Blue Peter Marine's web page. From there we found that the maximum righting moment for the boat with two big men sitting to windward was 1550 ft pounds. The sail (114 square feet) and leeboard areas are 12 feet apart on Frolic2 so the maximum sail force we can stand without capsize is 129 pounds (which happens in about 15 knots of wind).

The force on the leeboard is assumed to be the same as the lateral force of the sail at that time, so the maximum force on the board would be 129 pounds. If it exceeds that the boat will capsize with that crew configuration. How do we size the leeboard such that we can be pretty sure it will produce that 129 pounds of lateral force?


The leeboard, or any underwater board, "flies" through the water in the same way that an airplane wing flies through the air. If the board is pointed dead ahead into the flow of water, it produces no lateral force, only drag. If a small "angle of attack" is introduced, a large amount of lateral force can be produced. To produce that angle of attack , skipper need only point the boat slightly upwind of his desired course. So the boat points one way and goes slightly downwind of where it is pointed. You might view this as "leeway".

The equation for the lift, or lateral force, developed by the board is F=2.86 x S x C x V x V. It's a lot like the equation used for figuring sail force in the wind. In fact it's exactly the same equation with an allowance that the density of water is about 900 times the desity of air. But there are some other differences.

The S in this equation is the area of the underwater board in square feet. Remember it is only the area that is immersed in the water flow.

In the case of the underwater board, the value of C is, I would think, less that the C of 1.5 that I recommended for sails. The reason is that the underwater boards cannot have camber, as a soft sail can have, and function through tacking left and right. It must be symmetrical. (OK, boats with two leeboards can have cambered boards.) Thin airfoils with no camber seldom have maximum lift coeffecients much greater than 1. So I recommend the value used for C in this equation be 1. Then the equation reduces to F=2.86 x S x V x V.

(Below is shown a chart of lift coefficients for various fins of various "aspect ratios". I'll explain what aspect ratio is next issue. This chart was gleaned from info from Marchaj. Looks like his test data indicated the C maximum was 1.2 but the chart seems highly idealized, which is OK. Besides the note of C max, note that the general trend is for each fin to gain C directly proportional to the angle of attack until the maximum is reached. Then it levels off.)

V in this case is the boat's speed through the water, not the wind speed. How fast is your boat going to go? Boy, is that a tough question. The speed you want to use in the equation in not the "hull speed", the usual assumed maximum speed of the boat The hull speed (in knots) of a displacement boat is often shown as about 1.3 times the square root of the waterline length (in feet). So the Frolic2, with a waterline length of about 18' would have expect to have a top speed of about 5.5 knots. But the speed we want to use in the equation is about half of that maximum speed. Why? Because for the underwater board, the worst condition is when beating to windward at low speed. At that time the load on the sail, and thus the load on the leeboard, is maximum, but the boat's speed is well below maximum as it beats through the waves. Let's call this something like "beating speed". So for Frolic2 we might take the beating speed to be 2.75 knots. (It's still pretty fast for a sailboat going to windward.) So now for Frolic2 the equation becomes F=2.86 x S x 2.75 x 2.75. which is equal to F = S x 21.6. If we think the maximum force on the board is going to be 129 pounds as per our stability analysis, we can solve for S = 129/21.6 =6 square feet of leeboard area.

(Here is a chart that you can use to figure the pressure that water exerts on an underwater board at various boat speeds. I've shown two values of the force coefficient, C=1 that I recommend, and C=1.5 for you optomists.)

That is actually a pretty large board for Frolic2. As designed it has about 4.4 square feet of leeboard immersed. What does that mean? That means that with two big boys sailing on the rail in 15 knot winds, they need to maintain a speed that will produce about 129/4.4 = 29 psf on the leeboard. If you look at figure 2, you will see that is 3.2 knots. If they don't do that here is what happens. The crew will slide off to leeward. Rudder movements intended to increase the board's angle of attack and thus its lift will do no good. About the only things they can do to regain complete control would be to head downwind a bit (to increase the boat's speed), or let out the sheet a bit to feather the sail somewhat and thus reduce the sail load to something the board can handle. If they try to "pinch" the boat closer to windward, their hull speed will diminish more, they will be stuck, bouncing up and down in the waves, probably going one foot sideways for each foot forward. I've been there many times! How about you? (Another solution is to reef to reduce the sail force.)

Now if you don't like doing math, I'll tell you where I probably got the value of 4.4 square feet when I designed Frolic2. It turns out that if you have a pretty normal boat and make the board are 4% of the sail area, you will probably have something that works quite well. Frolic2 has 114 square feet of sail, so 114 x .04 = 4.6 square feet. I guess that's how I figured it.

I do think it is possible to make the fin too big. Certainly you can add a lot of drag by making it huge. The reason I say this is that when I made my Bolger Jinni a long time ago, I used a leeboard that was about 5 feet long. The next season I chopped about 12" off its bottom. Easier to handle. And I can't say I ever really noticed any difference in its performance going to windward. As a contrast to that, I enlarged the leeboard on my Piccup Pram after a couple of seasons and noticed an enjoyable improvement in its ability to sail to windward, especially with the larger sails had I started to use.

One interesting thing about the way the board behaves is that if you have two boats with the same rig in the same wind, and one is faster than the other, the faster boat can get by with a smaller board. Why? Because its faster speed through the water produces more pressure on the board than the slow boat. But I don't think you can say something like, "I'm going to increase sail area. That will increase my speed and then I can reduce my board area." It might happen that way if you are lucky. But when you increased the sail area you increased the sail force. And you will need increased board force to balance it. Whether the boat speed increase will sufficicently increase the board force is hard to say.


We'll see that not all board areas are created equal. Deep skinny ones can get to a certain value of C a lot sooner (at less angle of attack) than shallow fat ones.




Imresboat is a slimmed up version of Larsboat which was designed for Lars Hasselgren to replace a Folboat that had finally met its end. Lars wanted capacity for two, plus decking, as with his old Folboat. But Imre wanted something more slender and lower in the same length. The result is 26" wide instead of 30" wide. And the result of that is that Imresboat is a single seater because the capacity has been reduced with the width. Imre wanted it even narrower but I hoped to stay within the tippyness bounds of most folks. Not sure yet how narrow something like this can get before the normal guy can't sit upright in it. I do know that you apparently can get used to about anything in that respect as experienced kayakers do.

The deck opening is smaller to sit just one man but I think it is longer than the usual kayak because us older folks have trouble fitting our legs through the hole in a performance kayak. Something like putting on a pair of pants without standing up.

(Here is a photo of Paul Moffitt's Larsboat with two aboard.)

Another change I made with Imresboat is to go to thinner plywood to save weight. In this case I suggest 1/8" ply for everything except the bottom which is still 1/4". Not sure where the 1/8" plywood comes from but it used to be what they skinned doors with. Be sure to boil a sample and hope it is waterproof. Anyway, Chuck Leinweber brought a Toto to our messabout with 1/8" ply sides and 1/4" bottom and it struck me as being fine for stiffness and strength. Like Toto and Larsboat, Imresboat is taped seam construction. I'm guessing at the 50 pound weight but for example Larsboats built of 1/4" plywood have weighed between 60 and 80 pounds depending on who built them.

Taped seam construction from four sheets of 1/8" plywood and one sheet of 1/4" plywood.

Imresboat plans are $20 until one is built and tested.


Prototype News

Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.

I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.

Here is a Musicbox2 I heard about through the grapevine.

This is Ted Arkey's Jukebox2 down in Sydney. Has a temporary rig and he is working on the blueprint rig.

And the Vole in New York is Garth Batista's of www.breakawaybooks.com, printer of my book and Max's book and many other fine sports books. Boat is done, shown here off Cape Cod with mothership Cormorant in background, Garth's girls are one year older. Beautiful job!





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