Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1JUL98) This issue will start a discussion about sails. To tell you the truth as I'm starting this I have no idea of where it will lead or how long it will take. So the next issue, about 15 july, will continue the topic..
WEB SITE ALERT...Click on over to SAN DIEGO PLASTICS if you have questions about plastics. This is the only site I ever seen that supplies technical info on all sorts of plastics such as strength and weight. There is also info on working with the materials and email techs waiting by to help you chose the best material. They sell the stuff of course so they even quote prices! (Accept no substitutes. Insist on Genuine Plastic!)
MESSABOUT ALERT....Craig O'Donnell (who is sort of responsible for the startup of my own web page) has a messabout scheduled for July 24 through July 26 at the Dublin Dock Restaurant and Pub in Betterton, Maryland.. Click on over to Cheap Pages to get the latest details.
ED HEIN'S TWEEN
A few issues ago I described how I like to size the sail of a boat by balancing the sail's ability against the stability of the hull. At that time I made no effort to give any details of the sail other than size and sail area center because those were really the only factors involved. "Efficiency" was of no concern because anything you put up there has the ability to produce the maximum force when hit broad side by a gust. That's when most sailboats capsize, I think,
But now I'll try to get efficiency into the effort. In particular we'll try to go to windward as best we can with a sail and rig that are simple to make. We're not going racing around the world for our corporate sponsor, we're going to keep it all in the "made it myself for next to nothing" catagory that I always design to. I think in the end the direction the essays will take will be to seek simple ways to improve present low tech rigs for sailing to windward.
Why the emphasis on sailing to windward? For one thing, most sail boats will go downwind at about the same speed. You can't outrun the wind without tacking downwind (on some boats) or firing up a motor. But the differences in sailing to windward can be enormous. For example, a boat tacking 60 degrees off the wind at 3 knots will progress to windward 1.5 miles in an hour. That's actually a fair "to windward" pace for a low tech boat in most wind conditions. But anyone could easily walk faster than that or row twice as fast in an ordinary row boat. If the boat will point 10 degrees closer and maintain 3 knots, you get 1.9 knots towards the wind, an improvement of almost 30 percent.
THE PERFECT SAIL....
Here, in a very general way, is what I might use as a starting point as the best sail I could get with low tech materials.
Let me explain. Most of what I'm basing this sail on is tech data from old aero books I have plus some stuff from Marchaj books which I got on library loan because I can't afford them. Using airplane data isn't always appropriate but the aero people from early on did a lot of basic research that applies. I remember seeing a long time ago at the Air Force Musem in Dayton, Ohio, a wind tunnel built and operated by the Wright brothers. As I recall, in their day the Wrights were on the cutting edge of wind tunnel technology in addition to making the first flying machines that really worked. I also recall that a lot of early aiplane designers were also sailboat designers and they actually did a lot of weird testing back in the first decades of this century, most of which didn't pan out. Indeed, when I see a modern racing yacht (I confess I don't follow yacht racing) it seems that the new ones differ from the old ones mostly in detail and materials. In the same way, modern airplanes still have the same basic layout as those of WW1 although they go through fads which die away each decade as with the T tail and the winglet.
So let's review some thoughts that went into the ideal sail.
First of all it has the normal soft sail. The lines I drew that look like battens really aren't. They are supposed to give you an idea of the cross sectional shape of the soft sail once it is "inflated" by the wind. Really extreme boats have used hard surface sails like the wing of an airplane. I can't see that for a boat that is supposed to be easy to use in a variety of conditions. I know the America's Cup catamaran from a while back had a very successful wing. But the rest of us have to haul our boats on trailers and a full wing is a big and delicate thing, conditions flyers have learned to accept. Then there is the problem of reefing the hard sail. Don't see how it can be done without making the whole project a Rube Goldberg project.
Also, I really don't like battens having been taught by Bolger not to like them. I used to have a Hobie 16 with battens and we got by OK as far as the battens go. The real problem with them was the wear and tear, and the problem in light winds where you can' t look at the sail and see what the wind is doing.
So my idea of the perfect sail is a soft surface sewn to a shape that delivers a good untwisted airfoil from top to bottom once inflated by the wind. That cloth fits inside a structure of mast and booms and maybe yards or gaffs. Ideally the structure is rigid to the point where the sail won't lose its ideal shape because deflections in the structure or stretching of the cloth under the press of the wind. I haven't shown any structure on my ideal sail because there wouldn't be any.
As for the overall proportions of the sail, the aero boys say the taller the better. Why? Because the aero boys have shown that a major portion of the drag of a wing or sail results from vortices resulting from the very production of useful force ("lift" for an airplane) and that one way to reduce the vortices is with a long narrow wing. It's the same argument that we used for selecting long narrow leeboards a few issues ago. It's the reason why high performance sailplanes and a lot of transport aiplanes have long narrow wings. True, really fast airplanes with power to burn can get by with short stubby wings. But even for a hard winged airplane the long narrow wings are a structural bother. For the simple low tech sailboat, we will see later that real structural considerations can make a low sail better than a tall one.
As for the actual planform shape of the wing, early airplanes actually were much more elaborate than their modern children. Lot's of the first ones tried to copy the the birds, with everything beautifully rounded. Aero theory came to speculate that an ellipical shape was the ideal and thus we have the beautiful elliptical wing of the Supermarine Spitfire. Was that the last serious plane to use an elliptical wing? Testing showed the actual shaping of the wing planform was a minor detail. Almost any plane designed since WW2 has had straight leading and trailing edges. The fancy ones have a straight taper with the tip about half as big as the root. Simpler ones use a basic rectangle with lots of benefits possible with regards to assembly cost. I've never seen an airplane wing that was triangular like a jib sail except for delta wings of old fighter designs or hang gliders.
I've shown the cross section as a smooth arc with its peak about 1/3 back from the leading edge. I think the position of the peak is somewhat arbitrary but what I show is pretty typical of good low speed aero designs. You will see some "laminar" airfoils sometimes with the peak more aft of the leading edge, say up to 50% aft. The idea was to get lower drag from such a section but I've heard the laminar foils worked better in theory than in practice. Also, the "thickness" of the airfoil causes a lot of debate. For the soft sail that would equate to the "draft" of the sail, or how much it bellys out when inflated. I think Marchaj makes the case that the draft should be no more than 10% of the chord, or width, of the sail. For example a sail 100 inches wide should belly out no more than 10" when inflated. The sailmaker can control the draft of the sail fairly easily if the rig's structure is rigid enough. (Indeed, the real challange to the sailmaker is to predict the effects of the rig's flexings.)
(One other thought about the shape of the cross section. In Phil Bolger's 100 Rigs book I think he says that the thin plate airfoil is ideal and that the thicker streamlined shapes of the usual airplane wing ribs are not as good as the thin curved plate. Not sure where he got that info. My books all show the usual thicker streamlined shapes to be better in most ways. Indeed, I think some high performance (but low speed) planes have wing thicknesses well over 10% of their chord length and you will find good ones right up to 20% thickness with lots of curve. But the thick foils don't really apply to a soft sail that must flop over to function on both tacks. Phil thinks double luff sails are also no good. A double luff sail wraps around the mast and reattaches well aft of the mast. It forms an elongated sleeve to make the sail approach a streamlined teardrop around the mast. I don't follow his theory there either except I can see that the double luff makes for difficulties in hoisting and reefing a sail. Some very successful racing dinghies have simple double luff sails.)
The "foot" of the sail is of interest. The aero guy would call it the "root chord" and he would go to great efforts to make sure the transition to wing and fuselage was smooth and leak proof. How different for the sailboat! We need about 3' of clearance between hull and boom to keep from getting bopped unconsoius! As I recall Marchej quoted sail tests which showed the gap between hull and sail must be quite small to prevent the air on the high pressure windward side of the sail from leaking to the other side and thus reducing the effective pressure difference on the sail while producing a robbing vortex. Only a deck sweeping sail would work in that respect.
As for the shape of the upper tip, I don't know if the aero boys ever agreed to anything. I've seen endplates, rounded tips, winglets, etc., etc, etc. And to tell you the truth the aero boys have formulas and theories to fit every occasion just as the physicists have all their different particles. A long time ago I worked on a missile that needed thumb sized studs added to make it fit a certain launcher. The aero boys pointed to the added parasitic drag and wetted surface and turbulence. "Of course," said the aero boys, " range would be reduced." The program manager agonized but went through with the studs. Testing showed range was increased! "Of courser!" said the aero boys. "The studs act as vortex generators that energize the boundary layer, etc., etc., etc." So always question technical stuff.
Next time we'll venture away from the ideal and get back to earth. We'll face the realities of strings and things.
ROW/SAIL DINK, 7-1/2' X 4', 60 POUNDS EMPTY
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue.
Tween is another one of those "in between" boats. It has a flat bottom for the most part like my Moby Dink, but that bottom twists into a V bottom entry like that of my WeeVee dink. The original Moby Dink, which can be built without thinking, is fairly stable and roomy for such a small boat. But in rough conditions she is easily stopped. The v bottomed WeeVee dink, on the other hand, hops over th chop like duck but would be too tippy for use as a tender where you need to step down into the thing from another boat. I'd expect Tween to be between them and be a good stable pram which can cut a bit of chop due to its twisted V entry.
Ed Heins of St Johnsbury, Vt., built the prototype to perfection. Ed reported the V entry cuts through the motor boat wakes common on his lake. (I should add that my own boat Twixt, very similar to Tween but 11 feet long, is also quite good that way. Pete James who built Petesboat with the same hull shape but 24' long, said he felt the twisted bow was probably not worth the effort. I get the impression that the smaller the boat, the more effective the V entry bow.) This little boat has plenty of capacity for the 200+ pounds of Ed plus the weight of his son. Also you can see the lateen sail Ed and wife sewed up from the instructions given in the plans.
I drew a lateen rig for Tween, unlike the gaffer I put on the other dinks I mentioned. I think it will be easier to set up. (That's the only disadvantage I see with the gaffer.) For storage of the long lateen yard, I've shown an idea where the yard is folded switchblade like with the sail still tied to it. Ed says it all works great!
As for hull construction, two sheets of 1/4" or 5MM plywood will do it. Because of the twisted bottom panel, the chine angle rolls and this boat is best built as a taped seam job.
Blueprints for Tween 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. (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: No new word from either Sportdory prototype builder. You can check out John Bell's Sportdory 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 is not in the prototypes catalog anymore. It's in the "done" catalog, plans priced now at $30. I've added Herb's web page to the permanent links at the end of this page.
Nothing new on the Texas IMB. 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.
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
Messing About In Boats
Shantyboats (archived copy)
The Boatbuilding Community
Kilburn's Sturdee Dory
Bruce Builds Roar
Herb builds AF3 (archived copy)
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