Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(1October 2021) We size underwater boards. The 15 October issue will be about choosing a design to build.
Armed with a fresh vaccination I ventured forth again into the world and quickly found an engineering print shop that did very good scans (alas, a 40 minute drive). Chuck at Duckworks has been updating the catalog so they are available for sale only as instant downloads, about 60 designs available.
ALSO...In addition to the Duckworks downloads I also now have access to a large format inkjet printer which is making very nice full sized prints on paper. So I can return to what I started 30 years ago, you order direct from me by snail mail using the address above only with cash or check in US funds with the prices shown on this website, and I mail you full sized 2'x 3' paper prints. The price includes first class mail to US and Canada.
THE BOOK IS OUT!
BOATBUILDING FOR BEGINNERS (AND BEYOND)
... 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....
ON LINE CATALOG OF MY PLANS...
...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Only downloads right now.
Neil Shipley's Larsboat in the early stages of construction. He has the main panels cut out and lashed together with plastic wire ties.
1024 Merrill St,
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 another 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 have 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 FORCE OF THE WATER ON THE LEEBOARD....
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 optimists.)
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 area 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 I had 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.
LARSBOAT, DOUBLE PADDLE CANOE, 15.5' X 30", 65 POUNDS EMPTY
Larsboat was built by Lars Hasselgren to replace a Folboat that had finally met its end. Lars wanted capacity for two, plus decking, as with his old boat.
I took Toto and lengthened it with a 30" plug in the middle to gain capacity. But lengthening a hull with a straight plug like this usually improves a boat in almost every way and Larsboat should be faster than Toto in good conditions. In this case the plug meant I didn't have to refigure the shape of the twisted bow panels as I would if I'd lengthened Toto with an overall stretch. (I can figure twisted panels pretty reliably now, but not back when Toto and Larsboat were drawn.)
The decking was quite simple because even the original Toto could take a forward deck of flat sheets with a center peak. I should add that I feel the decking is very optional. This prototype weighs 61 pounds and deleting the deck might cut another 10 pounds or so. The undecked boat also would have a better cartopping shape. I'd keep the stern chamber. It will ease your mind about taking a big wave over the stern.
This would be a preferred project for someonw who intends to do a lot of cruising and camping. In the Toto camping I've done the sleeping room has been OK, but the storage is limited. Larsboat would be better both because of increased capacity and because there is dry storage under the bow deck.
The basic hull is taped seam construction needing four sheets of 1/4" plywood for the decked version and three sheets for the undecked version. No jigs or lofting required. Plans are two blueprints with keyed instructions for $20.
The photo above is of Bob Smithson's Larsboat. He customized the decking a bit. I think he also built the boat of 1/8" ply to save weight. I've forgotten what his boat weighed but he did say it was sufficiently rigid for him.
Bob Hoyle built this one without a deck down in Florida:
Paul Moffitt built this one. You can see this is a much better two person boat than the shorter Toto:
And remember Garth Battista's vertical Larsboat?
And the old outboard motor guru Max Wawrzniak often goes for a paddle in his Larsboat:
Larsboat plans are $20.
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.
We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...
And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's old outboard book and many other fine sports books. Beautiful job! Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.
And a Deansbox seen in Texas:
Another prototype Twister is well along:
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
15oct20, Choosing A Design, Jonsboat
1nov20, Balanced Lug Jiffy Reef, Mayfly14
15nov20, Weighing OliveOyl, Piccup Pram
1dec20, New OliveOyl Junk Rig, Ladybug
1jan21, Rowboat Setup, Normsboat
15jan21, Sail Area Math, Robote
1feb21, Bulkhead Bevels, Toto
15feb21, Trailering, IMB
1mar21, Small Boat Rudders, AF4Breve
15mar21, Sink Weights, Scram Pram
1apr21, Sail Rig Spars, RiverRunner
15apr21, Water Ballast, Mayfly16
1may21, AF3 Capsize, Blobster
15may21, Mast Tabernacles, Laguna
1jun21, Underwater Boards, QT Skiff
15jun21, Capsize Lessons, Mixer
1jul21, Scarfing Lumber, Vireo14
15jul21, Rigging Lugsails, Frolic2
1aug21, What Is Horsepower, Oracle
15aug21, Sharpie Sprit Sails, Cormorant
1sep21, Measuring Prop Thrust, OliveOyl
15sep21, Leeboard Issues, Philsboat
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Barry Builds Toto
Table of Contents