Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1AUG98) This issue will continue the sail discussion with the workings of sailcloth. Next issue, about 15 August, I hope to take a break from the sail thoughts to present some capsize testing Tim Webber has done with Scram Pram..
WEB SITE ALERT...If you've been wondering where your ol' buddies Crawdaddy and Jerkbait have been hanging out, click on over to the Walleye Central Message Board. Don't remember how I stumbled on to this one but it is the best discussion area I've seen for outboards, old or new. Lots of discussions of smaller 4 stroke engines. Lots of public head scratching over engine and boat problems. One fellow had an old outboard with a key that turned to make it a 5, 8 or 12 hp engine on demand. One man said to junk it. The next apparently had been watching Antiques Road Show and said a collector might pay a fortune for it. Lots of fun.
RAY SCHAEFFER'S STRETCHED ROAR
TAKE YOUR HANKY FROM YOUR POCKET.....
And hold it as in Figure 1:
Now pull on it's corners from A to B. Mine stretches from 14" to 14.5" with a medium tug, about 3.5%. Now tug from A to C. Again mine stretches 3.5% with a medium tug. Now tug across the hanky diagonally from B to C. Mine stretches from 20" to 26" or 30%! I know that a hanky is pretty flimsy stuff but the real problem here is that the weave runs from A to B and from A to C and nothing is there to handle a diagonal load. Simple woven cloth (a lot heavier than the hanky) for sails for a long time because there was nothing else around. Sailmakers became wise to its stretchy ways and learned to deal with the problems of diagonal stretch. In particular when triangular sails became common (the squarer sails of olden days weren't as demanding) various ways of strenthening and shaping cloth became important. Sometimes you will see sails which are sewn in a radial pattern such that none of the cloth crosses the luff, leach, or foot at an angle.
I think all that ended when synthetics, especially Dacron, came into use for sailcloth. I recall reading a comment by someone who said that, as far as sailmaking books are concerned, you can throw out anything that was based on cotton material because it just doesn't apply to modern Dacron.
Dacron has three really big advantages over cotton that I can see. First it is a lot stronger. I don't have figures, but I've never seen a Dacron sail tear except perhaps where it was chaffed. For smaller boats like I design, I can't imagine any strength failures in ordinary 4 ounce Dacron cloth. The largest boat I designed that got built was Petesboat, 24' and maybe 2500 pounds max. I specified 5 ounce cloth on the plans to be cautious. But when Pete had the sail made by SailRite, they had no 5 ounce in the colors he wanted and made the sail in 4 ounce saying that was plenty strong for his boat. And so it has proven to be.
Second, Dacron is very stiff compared to most other cloths that might be used for sails. As we saw in the essay on ropes, Dacron is by nature about 3 times as stiff as nylon.
Third, and best of all, real Dacron sailcloth is quite stiff in all directions, not just along the directions of the threads. It has a resin pressed into it that makes it so. I suppose eventually the resin wears out and the cloth loses some stiffness. But for a daysailor that is hardly a problem and a well stored Dacron sail might outlast its wooden hull. (Racers sometimes discard their Dacron sails when the resin starts to lose its stiffness, I've heard, and very good buys for the fun boater can be had by buying up the partially worn sails at rock bottom prices.)
Dacron has some drawbacks. It's a bit tough to work with because of its stiffness, the very quality that makes it such a good wind machine. Essentially it is like sewing heavy slick paper, not like cloth. In the same way it can be hard to handle on a boat in the finished sail. After it has been used a lot it becomes easier to handle.
And Dacron sailcloth can be expensive and hard to get. I know of only one retail supplier - SailRite. If there are other retail sources I'd like to know about them. I think standard white 4 ounce sailcloth sells for about $10 a yard now (36" wide) and you should really buy about 40% more than your sail area to allow for waste. So a 100 square foot sail might eat up about $150 of sailcloth (plus other odds and ends).
OTHER POSSIBLE SAIL CLOTH MATERIALS....
I've made quite a few sails from ordinary polytarp. My favorite is the grey colored "Coleman" brand 12' x 24' tarp from Walmart for $20, about 7 cents a square foot. It is no where as stiff or as strong as Dacorn sailcloth but it has sufficed for boats up to the size of AF3. I don't know what happens beyond that size. In fact many of my customers have build sails of polytarp with no thought of changing over to Dacron.
Polytarp as you know is a mixture of threads in a film of the same material. I'm not certain how it is all bonded together. I suppose it is the film that gives it the quality of being somewhat stiff across the diagonals.
Polytarp comes in really big sizes which is both good and bad. It's good in that a sail for a small boat can be built from a single sheet, although it still must be tailored to have a sail's shape. It's bad in that the large size can be difficult to handle. Also the new tarps are usually folded and crinkly with lots of ridges after unfolding, making accurage measurements difficult. (Unlike true Dacron sailcloth which comes off the roll in a smooth straight sheet flat and smooth as glass.)
Polytarp's softness makes it easy to work with. It sews very nicely. (I don't trust adhesives here. Many have used duct tape, contact cement, hot melt glue, and claimed success. I doubt if they are boating a lot - initial results can be misleading. Reed Smith, who has made more polytarp sails than anyone I know, has tried adhesives and says they will slip and fail eventually.) Polytarp can be sewn into three dimensional shapes just like real sailcloth. The corners and places where the tarp attaches to the rig structure need to be heavily reinforced but that can be done easily.
I've never tried Tyvek, which is a paper used mostly to "wrap" houses. So it comes in huge rolls. It is also used to make clothing and envelopes. If you want a free sample go to your post office and ask for a free "priority mail" envelope which is Tyvek. It's quite strong. Sometimes you can tear it with your hands but usually not. But being paper, it has no weave direction. It is equally strong and stiff in all directions, the ideal quality for sail cloth.
Tyvek has a couple of problems. It usually comes in huge rolls, big enough to wrap a house, and you often must buy the whole roll. Expensive even though the price per square foot is low. Also it is marked boldly with the maker's name everywhere across it.
Most experimental Tyvek sails are taped together, something I don't like as mentioned above. I don't see why a permanent job of sewing can't be done as with Tyvek clothes. As with polytarp I would think the attachment points would need to be heavily reinforced.
Lastly Tyvek is degraded by weather. The lumberyard man told me a house wrapped in Tyvek must be sided within a few weeks or be re wrapped. For most weekend sailors I doubt if that is really a factor. After all, any material, including Dacron and the rest, is degraded by the sun. Keep your sails stored out of the sun and they will last a lot longer.
Assuming you have a rigid cloth, the sailmaker can shape the sail in three dimensions reliably in the same way a tailor can make a shirt fit. Look at Figure 2:
Here is a sample "sail" of 150 square feet. The width is 10' and it has 1' of draft which I have shown having its maximum in the center of the sail. The head, luff and foot are supposed to be secured to rigid spars. The leech is unsupported. In a way it is a gaff sail.
It's a three dimensional thing to be made from a flat material. How to do that?
You won't get the desired shape without doing some seaming to turn it from a flat panel into a three dimensional sail. The seams might be something as in Figure 3. Imagine the wedged gaps sewn shut and you can imagine the flat pattern taking on a 3D shape. In general that extra material that forms the draft in the center of the sail must be removed around the edges. Because the sail has a 1' draft at its center, the actual distance across the cloth (arc length) in the middle must be about 10.2'. The difference amounts to about 2.4" - Not a big number but if not allowed for, you won't get 1' draft in the center with a stiff cloth and rigid structure. Or with sailcloth in a roll the sailmaker would vary the widths of the seams (broadseaming) to produce the same effect. There are programs available on the internet which calculate the seams in cases like that. The crosscut sail is considered to be the best. Usually the broadseaming is almost all done on the luff, none on the leech. By doing that the sail will take on an airfoil shape with the maximum draft perhaps 30% aft of the leading edge like an airplane's wing.
And the shape of the sail pattern may not have straight edges. True the edges hook to spars which are shown here as straight. But in real life, the spars do bend and they may bend quite a bit. In fact the allowance the sailmakers makes for draft in the sail can actually be a lot smaller that the allowances made for the bending of the mast, boom or gaff. Quite a guessing game. On the other hand if you can control the bending of the spars you can go a long way to control the shape of the sail.
STRESSES IN SAILS....
Figure 4 shows how something like a rope or wire or cloth will handle a side load. As with the rope discussion last issue, the deflection is the key. In this case we might assume no stretch of the fabric and a desired draft or deflection of 1' in the center of the 10' wide sail. If the pressure on the sail were 1 psf, as we showed it might be in a 14 knot wind a few issues ago, the end loading to support the wind load would be about 12 pounds per foot. It's hard to say it would be exactly that because in fact the stress is being distributed to all four sides of the sail, not just the luff and leech as this quicky check assumes. Essentially all of the tension in the sail is pulling inward to the center of the sail as far as the spars are concerned.
What about the trailing edge which has no spar? The sail material on the sail's leech will also act as in Figure 4. It will deflect inward some amount until the tugging load on the sail cloth is in balance. You might imagine a stout rope down the leech, as was done in the days of cotton sails. If the loading in the sail were 12 pounds per foot as mentioned above, and the 15' length of the leech deflected inward say 6", then the tension delivered by the rope in the leech to the rig structure would be about 675#! So you see a little air pressure can make for some high internal loads. (I really doubt if it would be that high.) Boats that are stable enough to stand up in higher winds can need really strong sail reinforcements, especially at the corners where these loads are delivered to the spars.
One thing I've noticed about the above leech situation: if you cut the leech of the sail straight, a "stress line" as I might call it will form inside the cut of the leech which will roughly take the shape of the deflected rope. The above mention tension loads are not going straight down the leech, but rather inside a bit. The stronger the wind the greater the stress line will run inside the actual trailing edge of the sail. The material between the stress line and the trailing edge will go slack and flutter. It can flutter quite loudly and with bad effects to airflow. One of the best fixes is to "hollow" out the leech to an anticipated stress line. I usually use maybe 3" in a 20' leech. Another common solution is to use battens along the trailing edge to support it.
When we return to this discussion in a few issues we'll ponder the working of spars.
LIGHT ROWBOAT, 12' X 45", 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.
A few week ago I got the wonderful photo of Vireo shown above from Frank Kahr of Providence, RI, along with this equally wonderful letter:
"Yesterday I accomplished a long-time goal of rowing from Providence, RI to Newport, RI in a Vireo rowboat I built from your plans. This was a trip of approximately 26 statute miles which took 6 hrs, 12 min. I had a little help from a fair tide, but this was still a good time for a long trip made without great physical effort.
"I think Vireo is a good balance of light weight, seaworthiness, and speed. A longer, narrower boat would be faster, but would also be less stable and would have less spread of the oarlocks. I built mine from 6mm okoume plywood with a permanent center rowing thwart, it weighs only 57 pounds and is very easily managed out of the water. I have built several other small boats, and this one went together easily in about 30 hrs. The plans are very clear.
"Thanks for designing a boat which is ideal for my intended use of exploring and day cruising under oars.
A letter like that will make your day! I would suspect that Frank is a very experienced builder, boater and oarsman. Very few people would have the patience to row 26 miles (true marathon distance). I think the farthest I've ever rowed in one session was 16 miles, still a good distance, which took a bit of getting in shape and a bit of good weather. I say this because if you are a new boater and think you can finish up a good rowing boat and charge off into the wild blue for 26 miles, chances are you won't get anywhere close to your goal the first time. With practice and planning you will get there. Frank's story also reminds me of Phil Bolger's comment somewhere that one major trick to designing a successful boat is to see that it gets to the right boater!
Anyway, here are some more Vireo details from my catalog:
The prototype Vireo was built by Charles McMahan of New Matamoras, Oh. He used common Lauan underlayment for plywood and said all the lumber in his boat was salvaged from motorcycle crates or planks found floating in the Ohio River.
Vireo is an expansion of my WeeVee dink. Vireo is 50% longer and 50% shallower than WeeVee. She won't be 50% faster but I'll bet she'll be good for 5 mph and will track and handle superbly. Charles reports the boat is very fast, rowing it on the Ohio River. I'll bet she'll be quite good in rough water. I'd expect her to beach a lot easier than WeeVee because she has less draft and less angle to her bottom V.
As for seating in one of these, you can't use a ditty box/rowing seat because there is no flat place to put it. In WeeVee I use a flat board which spans the V and wedges itself in place when I sit on it. Once I found the ideal spot I glued cleats to locate the boat and cleats to brace my feet.
Vireo plans are $15. Taped seam construction from three sheets of 1/4" plywood. No jigs or lofting 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).
Sneakerbox: Here is a boat that was in the prototypes catalog only a short while so most folks won't know about it. I'll try to feature it soon. It's an "instant" version of the 12' garvey box that appears on page 65 of Chapelle's American Small Sailing Craft, a book you should have in your collection. A builder in Pennsylvania has one just about completed. With luck we'll have a photo in a few weeks.
Sportdory: Word from Dave Burdecki in California was that he had his completed but no word of the launching. 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.
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)
Table of Contents