Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1March2012) This issue will continue an essay about underwater board shape. The 15 March issue will continue the topic.
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. Then Duckworks sends me an email about the order and then I send the plans right from me to you.
Underwater Board Shape
Aircraft designers have been trying to predict the performance of wings since the very beginning. The Wright brothers not only developed the airplane but made some of the best wind tunnels of their time. And from the very beginning they noticed that long skinny wings usually performed better than short fat ones. And so it is with the underwater board of a sailboat. In its own way it "flies" through the water producing a force that counteracts the side loads produced by a fore-and-aft sail rig.
Designers quickly developed the term "aspect ratio" to measure that feature of a wing. The aspect ratio of an underwater board is AR=2 x B x B / S, where B is the length of the board and S is its area. Figure 1 shows how you measure those elements. (Aero students will note that the above equation is twice the usual given for an aircraft wing which has two tips while an underwater board has but one tip.)
Next, for a board with a symmetrical cross section (so it can function equally on all tacks), the board must go through the water at an "angle of attack" to generate force, as we discussed in the last issue. Figure 2 shows how the situation applies to a close hauled sail boat. The skipper wants to go to on a certain couse but because of the angle of attack required by the board he must head his boat upwind by the angle of attack. So he might think the boat is "slipping" and producing "leeway" equal to the angle of attack. One other important thing to notice here is that any angle of attack of the underwater board must subtract from the angle of attack of the sails. A bad situation. Sort of a double penalty.
Now look at Figure 3. Here is the big message: High aspect ratio boards (deep and skinny) reach their lift coefficients with less angle of attack than do shallow fat boards. The figure is sort of idealized but Marchaj gives examples of tests of different actual boards that agree with the figure.
Low aspect ratio keels can be successful if they have sufficient area to allow them to always operate at a low C. The Micro has I think about 14 square feet of keel, about twice the value I recommended of 4% of the sail area for a deep skinny fin. Large low aspect ratio keels also make for steadier boats. Marchaj points out that they have superior damping which makes for a safer boat in rough going.
As an example, let's use Frolic2 again. Her board has 4.4 square feet of area immersed and it is 3.5 feet deep. So the aspect ratio is 2 x 3.5 x 3.5/ 4.4 = 5.6. Now let's say we wanted to guess at leeway on a close reach in 10 knots wind, and we'll assume she is going 4 knots at the time. Frolic2 has 114 square feet of sail and 10 knots produces about .5 psf on a typical good sail, so that is 57 pounds of sail force. We'll assume that all of that force is counteracted by the board. So the overall pressure on the board is 57/4.4 = 13 psf. Now we need to calculate the value of C for the fin. Remember that the psf on the fin is = 2.86 x V x V x C. Using V = 4 knots, we can solve for C and get C = .28. Now get into the aspect ratio figure, guess where AR = 5.6 might be and it appears that the leeway angle would be about 4 degrees.
Here's an interesting "what if". What if the Frolic2 narrow board were instead mounted like a shallow keel such that it still has 4.4 square feet of area but now has a length of 3.5 feet and a depth of 1.25 feet? Now its aspect ratio = 2 x 1.25 x1.25 / 4.4 = .71. Going into the aspect ratio chart you would see the leeway for this fin would be about 15 degrees under the same situation.
Let's get one step fancier and say we have the original board set up as a centerboard that folds into a similar sized shallow stub keel as shown in Figure 4. This is a pretty common arrangement. Now the two areas will share the side load, but in what proportion? One thing we can say is that they will operate at the same angle. Let's guess that the angle of attack is 4 degrees. Then the deep skinny board is operating at a C of about .3 while the shallow fat board is operating at a C of about .1. They have the same area so the deep skinny board is producing three times the lift of the stub keel! It's doing nearly all the work. And it will continue to do so until the angle of attack reaches about 15 degrees at which point the deep skinny board reaches C max of about 1.2. Then the stub keel is working at a C of about .3 and will continue to be effective until it reaches an angle of maybe 50 degrees. You wouldn't want to sail at a 50 degree leeway angle although there might be times when it would be advantageous. I can't really think of such a situation.
This sort of analysis flies in the face of some designs. As mentioned earlier, the shallow keel can work very well if given enough area. But I've seen designs with "keel runners" or "chine runners" which I think are just small projections off the bottom. Given what we have seen here I don't see how they can work effectively. I've sailed boats with skids and external chines which approach those things but my boats always had leeboards or centerboards too. If you think the skids and chine runners are effective, you need only take one of my boats out, get close hauled, and raise the leeboard or centerboard to see what happens. You'll slide right off! I don't think those shallow keel and chine runners are effective. (I'd really like to try or see a good test on Matt Leyden's Paradox. That boat has chine projections and nothing else. MAIB ran an excellent series about making a Paradox but, after years now, no test results. My own feeling is that everything about Paradox is optomized for downwind sailing, something the designer is expert at. I'm not making fun of him - few of us have the patience, experience, smarts, and knowledge to do what Matt has done. But I'm not sure if the builders are aware of what Paradox's limitations might be as far as sailing to windward goes. At any rate, it seems to me that Paradox could be given a more weatherly rig pretty easily with an effective pivoting leeboard and a more balanced lug rig. This essay was originally written in 1998 and now in 2012 I've still not seen a chine runner sailing boat in action.)
Next Time...
We will delve even deeper into the subject.
Paddleplank
PADDLEPLANK, PADDLE BOARD, 14' X 32", 70 POUNDS EMPTY
Can you walk this plank? Actually the request was for a stand up paddle board, sort of like a surf board that is paddled while you stand up like on a real surf board shooting the big waves in Hawaii.
I had all sorts of reasons for not designing it, the first being that I could never stand up on such a thing myself for more than a second or two. As a starting point I decided to take the bottom of a light dory, which is usually about this length and about 2' wide. It is generally held that you should never stand up in a light dory and my experiences confirm that. But it looks like the surfers are well ahead of us mortals in that respect and besides they expect to get dumped regularly and maybe the long double paddle offers a bit of stability, sort of like the poles the tightrope walkers use.
So I started with that bottom and added a little top, maybe 4" of freeboard which will not keep you very dry. I put just enough rocker to support 470 pounds before the bow and stern start dragging, enough for two people plus the weight of the hull usually. I can't see two people standing up on this at once but if they both sit it might be a fun wet boat for warm waters. The deck is supposed to be totally watertight, of course, so it will shed water like a duck and be low enough in the water for a swimmer to reboard.
I knew from the start it would not be light, at least not light enough to carry under one arm as they did in "The Endless Summer". Bolger had warned all of us long ago that decks weigh a lot, more than the bottom he said because they must be strong enough to take your weight with no water underneath pushing upward. So Paddleplank will indeed weigh about the same as a light dory. I figure 75 pounds if made with the usual 1/4" plywood, maybe 15 pounds lighter if the topsides are made with 1/8" ply. You can't really go thinner than 1/4" on the bottom and you need some thicker area where you stand, so it is hard to make it much lighter.
I wondered a bit about how Bruce Brown carried his surfboard around under one arm. Sure, he's a big hunk but even so there were expensive tricks involved. Let's say you need to float a 180 pound man on a board. Light wood weighs over 20 pounds per cubic foot, so if you totally immerse a cubic foot of that wood it will have about 40 pounds extra buoyancy, given that water weighs about 60 something pounds per cubic foot depending on where it is from. So you would need about 5 cubic feet of wood to float the man, or at least 100 pounds of solid wood. But then he would be just awash, so to give an inch or two of freeboard to go surfing would require even more. I suppose that is the situation with old time historic surfboards.
I googled around some surfboard history. The first hollow boards were made about 90 years ago and they first broke the 100 pound barrier, so my guess was pretty close. Then they went to balsa cored boards (Hey! Balsa is really a hardwood!) and got down to 60 pounds or so. Then post WW2 they went to fiberglass and foam, etc.
Anyway, after the google, I stopped feeling guilty about the weight of Paddleplank.
I'm thinking Paddleplank is too wide for a real surfer dude but would be a good plaything for the rest of us.
Three sheets of 1/4" plywood with the chines done with taped seams. I have it drawn for jigless construction but you might not get away with that since the sides will be so thin and limber. Thus I have also shown some dimensions to a "base line" so you could set up the initial construction on a ladder frame to hold it all true until the bottom is on.
Prototype Paddleplank plans are $20.
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 out West.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.
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:
The prototype Twister gets a test sail with three grown men, a big dog and and big motor with its lower unit down. Hmmmmm.....
And the first D'arcy Bryn is ready for taping. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
And the first Brucesboat is in the water for testing. A full report soon.
OK, so he found a major league goof in my plans on fitting the bilge panels. He did some cut and fit and did a great job of salvaging the work, but I have corrected the drawing for the aft end of the bilge panel (I drew it in upside down!!)
And a Hapscut goes together in Texas. He has scarfed some material on the stern to finish the boat with a built in motor well like Laguna. Good idea:
AN INDEX OF PAST ISSUES
Hullforms Download (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
