Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15May99) This issue will show ways of figuring sail area and finding the center of the area. Next issue, 1Jun99, will have some thoughts about making rudders for small sailboats.
MIDWEST MESSABOUT NOTICE...
The 10th annual Midwest Messabout will take place at the Gun Creek Recreation Area at Rend Lake in Southern Illinois on June 11,12, and 13. The usual routine is that some folks come on Friday and leave early on Sunday. To get to Gun Creek take Interstate 57 to exit 77 West, head towards the golf course on the south side of the road and follow the signs. This is a Corps of Engineers rec area with a $2 launch fee. Camping is 10$ a night and that will cover the launch fee. There is no schedule of events - we just wing it. Email me at email@example.com if you have questions.
THE JAMES BROTHERS CAMPING OUT WEST, JEWELBOX AND PETESBOAT
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 missle 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.
I'll show how I make rudders for small boats.
HC SKIFF, SAILING SKIFF, 18' X 5', 450 POUNDS EMPTY
HC Skiff is a rework of the "typical sharpie skiff" that appears on page 101 of Howard Chapelle's great book AMERICAN SMALL SAILING CRAFT. I can almost say that if you own only one book about small sailing boats that this is the one to have. The lines of my version are straight from Howard's offset table. I found it all very interesting. For one thing it is by today's standards a long narrow hull, 18' long and only about 4' wide on the bottom. The stem (which is not quite vertical) rides an inch or so over the water, then the bottom goes more or less straight down to the deepest and widest point of the hull which is at about half the length of the hull. Then the bottom sweeps way up above the waterline for a very small transom indeed. There were working hulls. Heavy loads could be carried without immersing the transom, always harmful to a boat with no motor. You just put everything toward the stern and kept the stem just clear of the water. The fact that the widest and deepest point of the hull is at the halfway point should produce an all around better handling boat than one which has most of its beam towards the stern. I think the hull shape has a lot to admire and is little improved upon by any modern flattie skiff thinking. Chapelle says the shape was very common al along the East Coast from 1860 untill 1900.
I used more modern plywood construction on HC Skiff. The old ones were nailed together from 1" boards. Then they were left outside in the water until they died. They must have been very heavy both in their construction and soakage, not to mention the payloads of clams and oysters. My version uses five sheets of 1/4" plywood and five sheets of 1/2" plywood and will weight a lot less than the original. It's done with prefab parts and glue and nails, no jigs or lofting required.
I had to tinker with the sail rig which was of the same style shown here but at least 50% larger. On the old heavy boats that might have been OK but I'm sure it would be way too much for the lighter ply version. I kept the centerboard, large and effective. I added hull air boxes front and stern for a chance at self rescue after a swamping. I kept the large deadwood aft and barndoor rudder. Also added some seats (the originals had simply three thwarts and stern seating).
Plans for HC Skiff are $20 until one is built and tested.
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.
Usually when a design from the Catalog of Prototypes starts getting built and is close to launch I pull it from the catalog and replace it with another prototype. So that boat often goes into limbo until the builder finishes and sends a test report and a photo.
Here are the prototypes abuilding that I know of:
Fatcat2: There is an old timer (80 years +) in Minnesota who has completed the hull of a Fatcat2. Fatcat2 is a simple 15' x 6' catboat, gaff rigged and multichined. The boat is totally done now, waiting for Green Bay to warm a bit.
Mixer2: Mixer2 is more or less the original Mixer with a rough water bow like Toto's. 12' x 3'9" and about 90 pounds. Has a sail rig. I got a call from a Colorado builder who wanter to learn to sail and apparently has the boat well along. He also built last year a Smoar which is a 12' version of Roar2 and was very satisfied with it. As I talked with him on the phone I got down on my knees and begged for photos.
Frolic2: I recently heard of two of these going together. One is in Oregon , hoping for an early summer launch. The other one is in Colorado, hoping for a late summer launch. The builder sent me this foto in which the boat's Toto heritage is evident. You can see he is using nylon wire ties to fasten the panels together instead of wire twists.
RB42: This is an 18' rowboat meant for two. It's never been in any of my catalogs but the Canadian builder has it well underway. Imagine the Oracle shown above stretched to 18' and you have it.
BACK ISSUES LISTED BY DATE
Herb builds AF3 (archived copy)
Hullforms Download (archived copy)
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