Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1jun01) This issue shows you how to do your sail area math. Next issue, 15jun01, will present some thoughts about types of wooden boat construction.
The 12th ANNUAL MIDWEST HOMEBUILT BOAT MESSABOUT...
... will take place on June 9 and 10 at the Gun Creek Recreation Area at Rend Lake in Southern Illinois. Take exit 77W off I-57, 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 $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.
Barron Wester's QT power skiff.
SAIL AREA MATHBACKGROUND...
I repeat this essay every year because it is one critical area where builders have problems when they start to modify their boats.
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.
We'll start to take a look at types of wooden boat construction.
Power QT Skiff
POWER QT SKIFF, POWER SKIFF, 13' X 45", 90 POUNDS EMPTY
QT has been around as a rowing skiff for a long time now and was last reviewed in the 15nov00 issue. But each set of QT rowing skiff plans also included a set of plans for a powered version of QT. I just got photos of a completed power version of QT from Barron Wester of Smyrna, Ga. The photo shows him in his local waters using a 2.5 hp outboard.
The power QT is nearly identical to the rowing QT except that its bottom runs straight aft from the maximum beam, a "straight run" as they say, with no rocker in the stern at all. This is critical in a power boat that will be getting some dynamic lift from its speed over the water (unlike the rowing version whose lift all comes from "displacement" of the surrounding water as the boat sinks to a level where its weight equals the weight of the water it has "displaced"). I think there are two reasons why the straight run is critical for a power boat. One is that as the boat speeds up it tries to climb over its own bow wave and as it does so it lifts its bow. The boat must have a lot of stern volume or else it will bury deeply at the stern and its bow will point to the sky alarmingly. The second reason is that with the typical outboard motor arrangement, most of the boat's weight is concentrated in the stern, the skipper's weight being usually the largest item. Even with no motion a good rowing boat with its fine stern lines can't handle that weight back there - its bow would point skyward and its stern dangerously close to swamping even without starting the motor. The upshot of this is that a good rowing boat can never be a good power boat, and a good power boat can never be a good rowing boat.
By the way, power QT would be limited to about 5hp (maybe less) by the Coast Guard recommendations, and mine too. If you are lightweight it may plane with that power.
Simple nail and glue construction from three sheets of plywood for either version of QT. No jigs or lofting.
Plans for both the rowing and power versions of QT still come in one package for $25.
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.
Caprice: Chuck Leinweber of Duckworks Magazine has finished the prototype Caprice. Here is a photo. Waiting for some shakedown cruises and a report. There is a chance this boat will be at the Midwest Messabout and I'm looking forward to seeing it.
Skat: I'm told the prototype Skat project is underway again and very close to being done.
Normsboat: This is an 18' sharpie being built by Cullison Smallcraft in Maryland. You should be able to check on it by clicking through to his web site at Cullison SmallCraft (archived copy, actual site no longer active). He is presenting an excellent photo essay of how to assemble a flattie. This boat has been launched and I'm waiting for photos and a test report.
Vamp: I received word that a prototype of the little rowboat Vamp is completed up in Utah. Hopefully photos and test report soon.
Family Skiff: A Family Skiff has been started in Virginia.
HC Skiff: One of these is going together in Massachusetts.
Electron: An Electron has been started in California.
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
Messing About In Boats
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
Herb builds AF3 (archived copy)
JB Builds Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
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