Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(1 May 2019) We continue a series on polytarp sails. I just found yet another one so the May 15 issue will finish 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.
REND LAKE 2019...
...will take place on June 7 and 8, always on the weekend before Father's Day weekend. I promised to remind all of us to try to get the good campsites on the North Sandusky loop. They are sites 24 through 30 and especially 26 through 29 if possible, this at the North Sandusky campground at Rend Lake. Let's give it a try. This is not really organized but if you nail one down, let me know. I will be trying too. UPDATE: AS OF DEC 6 WE HAVE ALREADY NAILED SITES 26 THROUGH 28. IF YOU DON'T GET A SITE NOW, DON'T FORGET THAT SEVERAL TENTS ARE ALLOWED AT EACH SITE AND YOU CAN DOUBLE OR TRIPLE UP ON A SITE. THERE WILL BE ROOM FOR ALL.
Sail Shaping
BACKGROUND...I have presented in past issues articles on making sails from common polytarp. Sails made from those articles have worked quite well. In general the sails are made with the one piece tarp which is cut to a predetermined shape, shaped with one or two "darts", which are tapered folds sewn in place, and then rimmed with a sewn in hem with stiffening elements like fiberglass tape and then given sewn in patches in places where the sail will be tied to the spars.
Although the method of making the sail is pretty straight forward, the calculation of the predetermined shape is not. In particular, the size of the shaping darts is critical but not easily calculated. I was hoping to come up with some "rules of thumb" to simplify the process.
WHY SHAPING DARTS....
Sails should be three dimensional surfaces which will get the most out of making the wind pull the boat forward. A good sail will have a surface shape like the wing of an airplane (indeed, some boats have wings for sails) with a "camber", which is a smooth outward curve. Aero scientists have argued for 100 years about the best camber shape - the optimum shape varies with the speed of the wind. I'm not getting into that except to say that Marchaj, the sailor's aero scientist, stated somewhere that the optimum camber for almost all sailing is 10%. So a sail with a width of 10' would curve outward by 1'.
But, you say, many homemade sails are made from flat panels simply cut to shape and hemmed, with no 3D shaping. True. I think the reason they have had some success is that homemade sails are often made from stretchy materials that give and bulge out on the leeward side, making a camber of sorts.
All sails made by sailmakers have this shape sewn right in. Those sails will not lie flat on the floor. This is where most of the witchcraft of sailmaking is applied and sailmakers don't talk much about how it is done. Usually the sails are sewn together from 3' wide cloth. By varying the width of the seams and by cutting and adding seams where needed, they can shape their sails in 3D. If you are interested in real sailmaking, you might try getting info from Sail Rite Kits or the book The Sailmaker's Apprentice.
When I made my first polytarp sail, that is how I did it. I cut the big tarp into 3' wide strips and sewed it all back together like I would a Darcron sail. I worked fine but It was clear that the idea of cutting the thing apart and sewing it back together was not efficient.
My second effort was to use the full sheet of polytarp and slit it where the shaping seams would be and then sew it back together with a gusset over the slit. Then one builder had his wife make a polytarp sail to those instructions but she knew a lot more about sewing than I do. She used regular shaping darts, as are common in tailored clothes, to simulate the varying seams of the sailmaker. I tried it and found it superior!
Eventually I found a way to put all the shaping in one or two darts instead of placing a dart every 3' as the sailmaker does with his material. Four sided sails like the lug and gaff sails I often use need two. Triangular sails need only one! Here is an example, my own Piccup Pram, using a triangular sail:
The Piccup sail went very well. You can barely make out the shaping dart in the photo running from the tack to a dark shaded point in the center of the sail.
The full details of making this Piccup sail in polytarp were presented in the 15jul99 issue of this webpage. You can still look at it by going to the past issues links at the bottom of this page and clicking on "the way back issue archives". That takes you to a large site where the old stuff is stored with an index.
To review, the sail is drawn in "3D", with the perimeter as shown on the boat drawing and with a point in the center of the sail bulging outward. Like this for the sail shown on the Piccup:
Next the lengths of all the lines shown above are calculated and then the sail is redrawn as a flat pattern with one edge slit like this:
Then this sail flat pattern is drawn out full sized on the polytarp and cut out (with allowances for hems, etc.). When it is sewn up the gap is sewn shut with a "dart" like this:
So the final sail is not flat but has a 3D shape to the size and draft specified. It really works!
IF YOU DON'T LIKE DOING MATH...
I thought there might be a way to simplify things a lot by coming up with some rule of thumb for the size of the gap at the tack shown above. When I made the sail shown in the photo I didn't really measure out all the triangular elements. Instead I figured with math that the gap at the tack would be 4.4" and then I drew the pattern of the sail on the tarp with allowance for a 2.2" "dart" at the tack. (A 2.2" dart would remove 4.4" of material from the surface of the tarp which is what is desired.)
But after making a few of these it was clear that the size of the dart is the main thing you needed to know and that the size of that dart was determined, in a triangular sail, mostly by the size of the foot and by the amount of camber. Since I use Marchaj's advice to stick with 10% camber, it all comes down to the length of the foot. Or so I guessed. If true then the instructions for making this sail in polytarp might be simply to "lay it out on the polytarp and sew in a 2" dart at the tack".
I figured a bunch of examples with math to see if my guess was pretty close.
THE FIRST EXAMPLE...
The model I used in the calculations looks like this:
Then I varied the height h and the width b and calculated the flat pattern that would result in the desired sail shape. Then I calculated the the gap at A. Hopefully a pattern would result such that the gap would be more easily calculated.
I ran through several combinations and here is a table of the results:
Let's first look at sail 1. It has a luff of 12' and a foot of 8' and a draft of 10% of the foot dimension for 9.6". The flat pattern gap at A needed would be 4.6".
Sail 2 is the same except now it has a foot of 12' instead of 8'. The draft stays at 10% of the foot for 14.4". The gap at A required to build this sail figures to be 9.0". That is an increase of 96% when the foot has been increased by 50%. So it is not in proportion to the foot alone.
Sail 3 is the same as sail 2 except the luff has increased from 12' to 16'. The gap required at A figures to be 8.2", only slightly down from sail2's 9.0".
Sail4 is the same as sail2 except the luff is all the way to 20'. The gap required at A figures to be 7.6". So the length of the luff does not have a huge effect - and increase of 100% in the luff reduces the figured gap from 9" to 7.6", about 15%.
After looking at the effect of the luff dimension on the gap at A, I tried one more in sail5. Sail 5 is the same as sail1 except luff is increased from 12' to 16'. The gap needed at A figures to be 4.1" to give a draft of 10% of the foot, down from 5.2" of sail1.
CONCLUSIONS???
If you were building a normally proportioned sail with an 8' width or foot, I would say the gap at point A would need to be "about 5 inches". A taller sail needs a bit less gap and the lower sail needs a bit more.
If you were building a normally proportioned sail with a 12' width or foot, I would say the gap at A would need to be "about 8 inches". A taller sail needs less and a lower sail needs more.
What if your sail is in between? I cranked through a few more examples to get a feel for it. Sail1 was the basic pattern with a luff of 150% times the foot and a draft of 10% of the foot. Here us what came up:
I would use this chart for triangular sails of normal proportions. It starts with a 48 sq foot sail which is about the smallest sail useful. And progresses to a 147 square foot sail, which is about the largest sail I would try with normal polytarp technology. Somewhere in between should be the A gap you are looking for.
I guess the way I would use make a sail this way would be to lay out the sail according to the basic dimensions shown in the plans, and then draw a line from the tack upward where I want the dart to run, about 1/3 to the top of the sail and 1/3 back from luff. Then at the tack I would draw the "gap" that is predicted by the chart and sew in a dart that would remove that amount of material. The resulting sail will be a few inches smaller than the blueprint sail but that should be of no consequence.
Here is how it might look for the sail I used on the Piccup Pram. The width of the sail is almost 9' and the chart predicts a gap at A of 5.8". I would draw it out like this, fold it on the black line going from the tack to the center of the sail, and then sew in a dart more or less on the red lines shown.
Blobster
BLOBSTER, SAILBOAT, 16' X 6.5', 750 POUNDS EMPTY
Blobster has a lot of features I like in a boat. Lots of volume for its size, sort of like Micro or Scram Pram. The multichine shape is almost exactly like Scram's but this one does not have a Birdwatcher cabin. It has the more traditional cabin with a raised watertight deck behind. Also it has one feature I would love to have in my personal boats - a step-through bow so that when you beach you can go forward through the cabin and out the front without going into the water or climbing over the bow. The cabin also has a slot top roof.
This shape of boat with multichines has proven good in rough water and with fair speed in spite of its blobular proportions. Blobster has about 600 pounds of water ballast in its belly and should be OK to 90 degrees heel although such depends mostly on weight distribution of the crew, something the designer has little control over. On the other hand, if the crew jumps overboard the boat will be almost assured of righting without their help. Then the problem becomes reboarding. Be prepared!
Sail rig is a large but simple 139 square foot balanced lug on an 18' mast. Mast is stepped off center to allow you to walk upright down the slot top and out the front. Should be rigged in an instant with no one going on deck ever. All very low tech built with common materials but effective.
The prototype Blobster was built by Miles Bore in Australia. He had built several boats prior, including a Micro, and did a great job of it using very nice materials.
Outside and on its trailer for the first time, the likeness to Micro is clear. But you can see it was meant to be much easier to trailer and launch.
Then he used it for a while as a low power motor cruiser while he finished the sail rig. You can see how easy the boat was meant to use from the beach with its step through bow transom.
Then for the rig and off for a sail, shown here with ballast tanks full. Miles reports it sails with no vices so far. No, he hasn't capsized it yet.
Miles got this photo of the inside. The living platform you see is 7' long and 6' wide max and about 42" at its deepest from platform to slot top. With the slot top cabin and step through transom it is a pretty airy home. Maybe not the little cruiser that Micro is but clearly much easier to use in general and a good choice for a daysail when you have but a couple of hours at the lake.
Great job, Miles!
Blobster uses taped seam construction. Five sheets of 1/4" plywood, eleven sheets of 3/8" plywood and one sheet of 1/2" plywood.
Plans for Blobster are $45.
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.
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
15may18, Rigging Sharpie Sprit Sails, Laguna
1jun18, Rigging Lug Sails, QT Skiff
15jun18, RendLake 2018, Mixer
1jul18, Horse Power, Vireo14
15jul18, Motors per the Coast Guard, Vamp
1aug18, Propeller Pitch, Oracle
15aug18, Propeller Slip, Cormorant
1sep18, Measuring Prop Thrust, OliveOyl
15sep18, Taped Seams, Philsboat
1oct18, Plywood Butt Joints, Larsboat
15oct18, Small Boat Rudders, Jonsboat
1nov18, Sink Weights, Shanteuse
15nov18, Piccup Spinoffs, Piccup Pram
1dec18, Electric Boats 1, Ladybug
15dec18, Electric Boats 2, Sportdory
1jan19, Sail Area Math, Normsboat
15jan19, AF3Capsize, Robote
1feb19, Bulkhead Bevels, Toto
15feb19, Leeboard Issues, IMB
1mar19, Hollow Spars, AF4 Breve
15mar19, Underwater Board Shape, Harmonica
Table of Contents
