Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1JUN98) This issue will show the relationship between leeboard (centerboard, daggerboard, keel, etc) shape and performance. In the 15Jun issue I'll describe how I design the pivoting leeboards I like to use.
MIDWEST MESSABOUT NOTICE...
The 9th annual Midwest Homebuiilt Boat Messabout will take place at the Gun Creek Recreation Area at Rend Lake in Southern Illinois on June 13 and 14. Take exit 77W off I-57, head towards the golf course, and you'll see the signs. Lots of folks arrive on Friday and leave early Sunday. Electric hookups at most camp sites and the fee is $8 a night, which includes the ramp fee at this Corps of Engineers facility. There is also a Super Eight motel at nearby Benton.
There is no schedule of events. We usually have a pitch-in dinner on Saturday evening.
If you plan to attend, drop me a line by the end of May and I'll send you an update. (As I'm writing this several folks have responded. We should have at the meet my AF4 and Toto, Kilburn's Dory, a Woobo and a Piragua, a new Swamp Yankee canoe, a Peep Hen and a Micro, the prototype Fusebox, and at least one Brick. Haven't heard from all of the St Louis crowd (they never call, they just show up).
ROGER'S SKAT UNDER CONSTRUCTION
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 a year now, no completed photos or 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 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.)
Now is a good time to explain the why's and how's of pivoting leeboards.
ROW/SAIL CUDDY BEACHBOAT, 13' X 4'', 180 POUNDS EMPTY
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue.
This little jonboat sailer was the smallest "cabin" boat I could think up. The prototype was built to inexpensive perfection by Robert Smith of Pleasanton, California.
Cubit has a slot top cuddy cabin that I originally tried on a revamped Bolger Jinni and which has found its way into a lot of my designs because it has so many good features. This cuddy cabin is about as large as the one on my old Jinni. It's the minimum for one person to spend the night in, with the slot covered over with a bug net or a tarp or even fancy segmented hard covers. With the covers off you can walk around as you would in an open boat. In a knockdown the cabin provides buoyancy up high for self rescue. The cockpit will flood some. The mast steps offcenter and out of the path of the walkway slot.
The open cockpit is short - just enough for two adults. But for better balance it might be preferred that the second adult sit in the cabin. The use of the pivoting leeboard makes Cubit all people space. She has a balanced lug rig which economical, well behaved and surprisingly efficient. She spreads 74 square feet on 11 foot sticks and a 13 foot mast.
The entire boat is built of lumberyard stuff which includes six sheets of 1/4' plywood. If you've been reading slick boating magazines you may have no idea of how inexpensive a boat like this can be. With imagination and normal lumberyard and discount store materials, Cubit might be a $300 project including a polytarp sail (which works fine). This is simple nail and glue construction. Harold Payson wrote the bible on this type of construction in his INSTANT BOATS. Written in 1979, it's all still valid although most of us have gone to latex paints since then.
Plans are three blueprint sheets with basic keyed instructions for $25. No lofting or jigs required.
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. (If you order a catalog from an internet page you might state that in your letter so I can get an idea of how effective this medium is.) Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currancy that I can admire but not spend.) I'm way too small for credit cards.
Anyway..... Anytime a design from the Catalog of Prototypes starts getting built I pull it and replace it with another prototype. So that boat goes into limbo until the builder finishes and sends a test report and a photo. Here are some boats in that catagory (although I been leaving most in the prototypes catalog).
Sportdory was not in the prototype catalog very long but there are two well underway. Both seem to have their hulls taped and getting their gunwales in place. These boats have no interior fitting out to do. But the painting comes next and that takes time. Check out John Bell's progress by clicking here .
Skat - the prototype Skat builder is Roger Palaski. Up until now he was a mystery man know to me only as "Roger", his Skat plans bought for him by someone else. Then I find out he has his own web page with the Skat construction photos on it! Skat is a small somewhat traditional 12' cat boat with a gaff rig. Even has a centerboard, the only boat I've ever designed with one! See Roger's progress by clicking here.
The Way Up North AF3 project by Herb McLeod has been launched. Says he built it in 70 hours although he had a sail and mast ready to go. I got this letter from him-
Launched the AF3 today and it should be on no suprize that it floats, does not leak and better yet sails beautifully. we had only light wind (5mph) but there was always a slight weather helm (a few ounces of pull on the tiller). The boat comes about well and I never got in irons (not even close). On a beam reach I was able to keep up to a Flying Junior that had one sailer and we had two and our dog in our boat. I did not have my compass so I could not check how close winded the AF3 is but the tacks seem to be about 90 degrees. I did not have to row much so I cannot comment other than the oars that I have (Barkley Sound cheapies) are klunky. The lee board works great stays down when in use and at the slightest touch of the bottom pops back up. When the boat is heeled to a comfortable angle the lower leeboard bracket just clears the water (nice design touch).
We took lots of photos so I hope that there is at least one good one in the bunch. I will send you a scan in a week or so as I do not have a scanner and have to send my photos by mail to my son in Victoria BC. He does the scans and then emails them back to me.
I am very very pleased with the boat and will write more as I get to know it better. We named this boat Shorebird as the shallow draft lets us explore the shores of our local lakes.
Get updates on Herb's progress by clicking here.
Concerning the Texas IMB that is under construction. Click here to visit Tim Webber's page and see some photos of the IMB as of a few weeks ago. Then poke around Tim's web page a bit.
Harmonica (ex Fusebox) is still being fitted out and painted. Plans to have it at the Midwest Messabout. By the way, Tim Webber, who has been sailing the Scram, says the roof and maybe the interior of a boat like this must be painted white to be cool in the summer. All of my boats were done that way and I've had no problem. But my own choice is offwhite. Pure white is just too hard on the old eyes in the bright sun.
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