Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1jun02) This issue will rerun the ever important Sail Area Math essay . The 15jun02 issue will discuss making sails from polytarp.
ON LINE CATALOG OF MY PLANS...
... 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. The prices there are $6 more than ordering directly from me by mail in order to pay Duckworks and credit charges. The on line catalog has more plans offered, about 65, than what I can put in my paper catalog and the descriptions can be more complete and can have color photos.
THE 13th ANNUAL MIDWEST HOMEBUILT BOAT MESSABOUT will take place on June 8 and 9 at the Gun Creek Recreation Area at Rend Lake in Southern Illinois. Take exit 77W off I57, head towards the golf course and you will see the signs. Lots of people arrive on Friday and leave early on Sunday. The camping fee is now $10 which includes the ramp fee at this Corps of Engineers facility. There is no schedule of events. We have a pitch in dinner on Saturday evening.
Left:
A Toto nears completion built by Barry and Thomas Johnson.




SAIL AREA MATH
BACKGROUND...I have to rerun this article every year because boaters tinker with their sail rigs and some pencil scratching early will save you a lot of head scratching later. In general your sail area has to center on a certain place in order for your boat to handle right. If you are substituting one sail for another you need to do this math work to get it in the ballpark. Even if you have a factory boat you might find it doesn't handle as well as it could. This essay may give you some guidance about what you can do to improve things.
If you look at the picture below of the sail rig of Mayfly12 you will see on the sail some (fuzzy) writing (that didn't scan well) that says "55 square feet" to the left of a small circle that represents the center of that area (honest).
The center of that area is often called a "centroid" and you will see it is placed more or less directly above the center of the leeboard's area. That is very important.
As you might imagine a shallow flat hull like this with a deep narrow leeboard wants to pivot around that leeboard. If the forces of the sail, which in a very general way can be centered at the sail's centroid, push sideways forward of the leeboard, the boat will tend to fall off away from the wind. You should be able to hold the boat on course with the rudder but in that case the rudder will have "lee helm" where you have to use the rudder to push the stern of the boat downwind. The load on the rudder will add to the load of the leeboard. Sort of a "two wrongs make a right" situation and generally very bad for performance and safety in that if you release the tiller as you fall overboard the boat will bear off down wind without you.
If the centroid is aft of the leeboard you will have "weather helm", a much better situation. The rudder must be deflected to push the stern towards the wind and the force on it is subtracted from the load on the leeboard. Not only that, but when you release the tiller as you fall overboard the boat should head up into the wind and stall and wait for you if you are lucky. It's a good deal but if you overdo it you can end up with too much load on the rudder.
This balance problem is actually one of the few things about sail rigs that is not arbitrary. The type of rig and its area are pretty arbitrary depending on how fast you want to go, how much you weigh, etc. But balance is quite important and is one of the areas where backyard boaters get into trouble, sometimes changing the boat or rig with no thought of balance. So before you go doing that you should do a little homework. This essay will tell you how to figure sail area and find the centroid.
One last item: the balance situation shown for Mayfly12 is what I have found to be best for this type of boats. Boats with large fin keels don't balance that way  usually the sail centroid is well forward of the keel centroid. That distance is called the "lead". That type of boat is not within my personal experience and I'm not going to get into that. But you still would have to figure the area and centroid.
THREE SIDED SAILS...
This one is really easy. The area is just the base time the height divided by 2. Any side can be the base and the height is aways at a right angle to the base.
So when you lay out the sail you draw it up on thin paper to the same scale as your hull drawing with the leeboard (or daggerboard or centerboard) lowered. Draw a line through the center of the board straight up. Now we're going to locate the scale sail on the boat such that it's centroid falls very close to that line.
Here's how you find the centroid of a triangular sail.
Find the midpoint of each side and and draw a line from that midpoint to the vertex opposite it. The three lines will intersect at the centroid. Actually you only need to find the intersection of two lines but the third line is a good check.
That's it! Now you can take you scale sail drawing and slide it around your hull drawing until the centroid is on that line drawn up from the hull's board. Move it up and down and tilt it until you like the way it looks. But don't cheat much forward or aft of that line.
FOUR SIDED SAILS...
To find the area of a four sided sail you just divide it into two triangles, find the area of each triangle as above, and add the two together.
Now to find the centroid of the four sider. Start by finding the centroids of the two triangles that make up the four sided sail as shown above. Now draw a line from one triangle centroid to the other. The centroid of the four sider is on that line somewhere.
To find exactly where the centroid is on that line, measure the length of that connecting line. You need not use the same scale as is used on the drawing. I prefer to use a millimeter scale for this measurement. Then get out the calculator and work the formula shown in the Figure 4. Let's say for example the length of the connecting line on the scale drawing measures 120 mm (that is measurement L). Let's say the example sail has a lower triangle area of 50 square feet (that is A1). The upper triangle is 35 square feet (that is A2). So the total sail area is 50 + 35 = 85 square feet. The length L1, which will exactly locate the sail's total centroid, is L1 = 120 x 35/85 = 49.4 mm. So you take that millimeter scale and measure up from A1 centroid on the connecting line 49.4 mm and make a tick mark on the connecting line. That is the centroid of the total sail.
Another way to find the centroid, especially of a really odd shaped sail, is to take the scale drawing of the sail and cut it out. Then balance the cutout on a knife edge and mark the balance line, rotate the cutout on the knife edge about 90 degrees and rebalance and mark the new balance line. The centroid lies at the intersection of the two line.
Another way is to dangle the cutout on a pin stuck through a corner and into a wall marked with a vertical line that passes through the pin point. Mark the line that passes through that pivot corner and a vertical. Then rotate the cutout to hang it from another corner, and mark a second line through the second pivot corner and a vertical. The centroid lies at the intersection of those two lines. Back at the missile factory the designers had a favorite place, complete with pivot pin socket hole and vertical line, to hang these cutouts and that place was known as the "weighing wall". Meanwhile the super computer cranked away next door but its answers weren't to be trusted unless they agreed with the cutout hanging at the weighing wall.
RIGS WITH MANY SAILS...
Figure 5 shows the rig for Viola22. It has a main gaff sail of 177 square feet, and a mizzen sail of 45square feet. Where is the centroid of the assembly?
It's done exactly as with Figure 4. Draw a line connecting the areas of the two sails. Measure the length of the connecting line. Then run through the same equation as in Figure 4. Nothing to it.
One thing I might point out about the Viola22 rig is that the total centroid falls near the aft edge of the leeboard. By my experience the mizzen is not as efficient as its area suggests so it needs to be a bit oversized by normal rules, fudging the total centroid aft. I think in general the aft sails operate in the scrambled flow of the forward sail, causing loss of force back there.
NEXT TIME...
I'll discuss making sails from polytarp.
Contents
Campjon
CAMPJON, CUDDY JONBOAT, 15.5' X 5', 300 POUNDS EMPTY
Campjon is based on my Jonsboat design. I increased the hull depth to 24" from 18" and again to 30" in the region of the 6' long cuddy cabin. The cabin decks slope upward towards the center so the "headroom" in the middle is about 3', usually that is enough to allow sitting up while seated on the floor. There is a slotted walkway on the cabin centerline just like I've used on my AF sharpies, but this one also has a step through to the front deck. The front deck itself is fairly large and easier to use as a boarding platform than with the AF sharpies. But it is very important to NEVER let anyone ride on a front deck like this when the boat is in motion. If they slip off the deck they have no option but to slide under the boat full length and through the prop in an instant.
The prototype Campjon was built by Luke Spreadborough in Queensland Australia. Here he is using a 2.5hp motor.
I doubt if the increased freeboard makes the boat any safer since a flat boat like this is not good in rough water in any case. The higher sides do give a feeling of greater security since this will be a boat you are "in" instead of "on". This would be a better family boat since the kids can be kept cornered in the cuddy while underway. They can stand in the slot, or look out the front or out the windows. I suspect the cuddy will find a lot more use as a spot for the portapotti than as a camping cabin. The cockpit as drawn has no seats and is itself a big playpen. For seating I suggest using common folding chairs like I have used with satisfaction on my AF4. When the boat is not underway the chairs are folded and stowed, or relocated to another part of the boat or the shore.
As for power I would suggest a good 15 hp motor or a 10hp to keep the capability of using the smaller lakes which are wisely limited to 10 hp. With 10hp and with two normal sized adults the boat should plane, and be very lively with one aboard. But I've noticed with my own AF4 that weight gets added as time goes by and I've noticed that folks usually build their boats heavier than I do. I should add that the Coast Guard has suggestions about powering small boats and they come down hard on flat bottomed hard chine boats like jonboats because of handling concerns. I think they would advise a max of 25 hp in a hull like this. (My AF4, which is about the same weight as Campjon, went 22mph with a 40 year old Johnson 18. Greg Rinaca's Jonsboat went 26mph with 18hp.) There is a draining motor well in the stern.
Constuction is nail and glue, with no jigs or lofting. From four sheets of 1/4" plywood, two sheets of 3/8" plywood, and two sheets of 1/2" plywood.
Plans for Campjon are $30.
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.
Here are the prototypes abuilding that I know of:
Electron: The California Electron is coming along. Right now a four cycle 2 hp outboard has been purchased so the original electric idea may wait a while.
The Florida Shanteuse just needs windows and top paint. Remember, this one stretched from 16' to 24', a big boat on a big trailer.
AN INDEX OF PAST ISSUES
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds RB42 (archived copy)