Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15 May 2019) We finish shaping polytarp sails. The 1 June issue will review capsize recovery.
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.
 | Left:
Christian Oliver's Piccup Pram in its natural habitat. |
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Sail Shaping 2
BACKGROUND...Remember in the last issue I tried to come up with some "rules of thumb" for the darts used to shape the draft in a three sided sail. Here is the resulting chart:
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 "gap" you are looking for. The "gap" refers to the amount of material to be removed from the sail's tack to give it the proper shape. In a polytarp sail that might be removed with a shaping dart that is half the size of the calculated gap.
The gap shown in the chart was calculated assuming a 10% draft, which is sometimes thought to be the optimum. I ran some numbers to figure the effect of draft on the gap. The example used in the numbers had a foot of 10' and a luff of 15'. The chart shows a sail with a 10' foot would need a gap of about 6.4". The same sail with a 5% draft would need a gap of just 1.5" and with a 7.5% draft would need a 3.7" gap. So there is a large effect. Showing that as a chart in percentage, it would look like this:
For example, if you had a sail with a 12' foot and wanted a 7% draft, what would the size of the gap at the tack? The first chart shows the gap needed for 10% draft to be 7.6" for the 12' foot. The second chart shows that for 7% draft the gap would be 50% of that, for a tack gap of 3.8" which would mean a shaping dart 1.9" wide.
NEXT...
You may remember that when I did the calculations for the first chart that I put the point of maximum draft 1/3rd of the way aft from the luff and 1/3rd of the way up from the foot. What if the point of maximum draft were lower? I ran an example using the 10' x 15' triangular sail where I lowered the point of maximum draft from 60" up from the foot to just 20". The effect was to increase the tack gap size a tiny amount from 6.4" to 6.8". So I don't think that is really a factor.
FOUR SIDED SAILS...
Next I looked at the darts needed to shape a four sided sail like a lug sail or a gaff sail. I have been using two darts in these sails, one radiating out from the tack and another from the throat. Here is a photo of Jeff Blunk's prototype Frolic2, now owned by Richard Harris in Illinois. The photo was taken at Rend Lake years ago and you can clearly see the two shaping darts.
From figuring the shape of lug sails in the past I had the opinion that the peaking of the yard has the effect of reducing the size of the dart needed at the throat. In fact I suppose the extreme case would be a Solent lug where the yard is peaked all the way up as an extension of the mast - a triangular sail needing only one shaping dart. I looked at four sided sails that look like this:
So the overall size was the same as the triangular sails, 120" foot and 180" height.
All of these sails needed a gap at the tack of about 5.5" compared to about 6.5" for the triangular sail, 85% of the triangular sail. So the peak angle really has no affect on the tack dart.
The throat dart changes with the angle of the peak. Sail A needs 5.5" gap at the peak dart. Sail B needs 5" gap at the peak dart. Sail C needs 2.6" gap at the peak dart, and sail D needs 3.4" gap at the peak dart.
Plotting it out looks like this:
I expected a smooth curve! But no, there is a bump. I double checked my numbers and it was still there. If there is an error in there I think it is in the 50 degree example. I would expect that sail would need a gap of about 2" instead of 3.4".
Let's think of a real life example. Let's try the large Piccup pram sail which has a foot of 105", and a peak angle of about 45 degrees. Let's assume 10% draft.
Let's figure the tack gap first. If you look at the triangular sail data for 8.75 ft foot, the gap for the triangular sail would be about 5.7". If we use an 85% factor to adjust that to a four sided sail then I would expect the tack gap for the large Piccup sail to be about 4.8".
Now for the throat gap. Looking at the above chart I would expect it to be about 50% of the tack gap, or 2.4".
Back in the 15oct98 issue I figured the large Piccup sail out the long way. The results then were a tack gap of 5.2" and a throat gap of 1". So the tack gap using the above system compares quite well with the complex calculations I did back then. The throat gap is off by a ways, isn't it? I noticed on the Piccup article that I used a flatter draft in the top of the sail then which might account for some of it. I can't explain the rest other than to say the gross approximations I used above give reasonable answers and will give very reasonable sails.
Here is a photo of Rob Rhode-Szudy's Piccup at the Rend Lake messabout with his Piccup with a large polytarp sail made with the twin dart method. It sailed quite well indeed!
Laguna
Laguna, Sailboat, 23' X 5.5', 450 POUNDS EMPTY
This boat was designed for Duckworks to run the Texas200, 200 miles of brisk downwind running they say in the somewhat protected waters of the Texas intercoastal waterway. The boat was supposed to be cheap and quick and hold two crew with floorspace for them to sleep on board. The basic idea I had was to stretch the Mayfly16 with an 8' plug in the middle. In the end I kept the basic cross section and stretched it out with a simple flat iron skiff shape. The bottom is the width of a sheet of plywood which simplifies things even more. The first doodle looked like this:
I drew it up in detail getting this:
And a card model from the detailed drawings looked like this:
My cross seats are supposed to be big and comfortable and removable to give sleeping room on the floor. Each crewman has his own stateroom.
The sail rig was to be a split rig with identical lugsails fore and aft with the option to sail with just one of the sails mounted in the center, like this:
I think the single sail option would be preferred if sailing solo since that fore mast is way up yonder and one may not be able to handle it and the tiller at one time.
The prototype was quickly built by Gordo Barcom in Texas and here he is blasting along with family and polytarp sails straining. It was clearly a fast roomy boat:
Then another, the Blue Laguna, was built and run in the 2010 Everglades Challange, 300 miles from Tampa to Key Largo, by Andrew Linn and Michael Monies. This boat had very little testing time as far as I know before the race. But they finished in 5 days and a bit but I think they slept in motels for two of the nights while the "press on regardless" guys were daring the night waters. Here they are launching off the beach (a requirement for this race):
And then pushing off in shallow water (there is a Sea Pearl behind them for comparison):
Finally a really nice photo of them approaching one of the three checkpoints in the race:
I think both Gordo's boat and the Blue Laguna have been modified in details from the blueprint but the rigs and basic hull features all look correct. I've noticed that no one takes sleeping floor space as seriously as I do but that is OK.
Well, I was thrilled. My main caution would be that this probably is not a good solo boat due to its size and split rig. I think it might be a good family boat in that there is plenty of room for everyone and a rope for all idle hands to play with. The other caution might be that, even though the boat has big buoyance boxes, it also has large cockpits to swamp in a knockdown and it will roll upright with a lot of water to bail. So you note on Gordo's boat he has inflated fender tied to the mast head, an idea to reduce the carnage of a knockdown.
Plans for Laguna are $45. It is all simple nail and glue construction needing six sheets of 1/4" plywood and five sheets of 1/2" plywood.
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
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
1apr19, Polytarp Sails 1, River Runner
Table of Contents
