Jim Michalak's Boat Designs

118 E Randall, Lebanon, IL 62254

A page of boat designs and essays.

(1Aug03) This issue will take a look at some real life sail area math. The 15Aug03 issue will rerun the taped seam essay.



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....


...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.


Greg Flemming's Scram project looks like an orange peel before its future cruising grounds at Port Stevens, Australia.




Contact info:


Jim Michalak
118 E Randall,
Lebanon, IL 62254

Send $1 for info on 20 boats.



Real Life Sail Area Math

I get letters all the time asking if a certain sailboat can be rigged in a different way. For that I present each year the Sail Area Math essay but the letters keep coming. Here is a real world application of the Sail Area Math essay.

Let's say you had a Bolger Birdwatcher since 1989 with its sharpie sprit rig set on a 24' mast. You found from the first day that the mast is the largest that a fellow can step by himself and even then only in good conditions when the boat is solid on a beach or on its trailer. Recently reality has convinced you to put an outboard motor on this boat such that sailing in light winds isn't a big deal. In fact the original Birdwatcher rig was not that great in light winds anyway.

Now let's say you also had an old very modified Bolger Jinni which in its final days had this lug rig which was back in your polytarp sail testing days, this one being the largest you could get out of a 12' x 16' tarp.

The Jinni finally gave up after 10 years of outside storage under a tree with no cover. It was burned but not before it gave up its vital organs for transplant, including the complete sail rig.

So the question you have is can you put the polytarp lug sail on the Birdwatcher. Then you would have a rig set on a 16' mast instead of a 24' mast, now a one handed job to step. The sail can now be hoisted with a halyard like a normal sail where the original was tied right to the mast.


...you need to figure the sail area and locate its center of area (its centroid). So you need to get a scale drawing, side view, of your boat and then make a scale drawing of the new sail to the same scale. All this is done with pencil and paper to scale, no computer is used. So in this case I found the dimensions of the Jinni lug sail and made a scale drawing like this:

It shows the dimensions needed to make the actual full sized sail.

To find the area of the sail you need to divide it into triangles, two needed in this case like this:

Of course if your new sail is triangular you need only one triangle. But here we have two to define this four sided sail. Each triangle has its area calculated and the total area of the lug sail is the sum of the two triangles. To calculate the area of a triangle you need a "base" and a "height" which is perpendicular to the base. You multiply the base times the height and divide the result by 2 to get the area of a triangle. In the top triangle here the base is 13' and the height is 11.3' so the area of that triangle is 13 x 11.3 / 2 = 73.4 square feet. The bottom triangle figures to be 42.2 square feet so the total is 73.4 + 42.2 = 115.6 square feet. Note that all the dimensions are measured off the scale drawing.

Next you will need to find the location of the center (centroid) of the area of the sail. Again you start by finding the centroid of the two triangles that make up the lug sail. To find the centroid of a triangle you make a tick mark at the mid point of each side, then draw a line from that tick mark to the opposite vertex. Do that for each side and you will have three lines that all intersect at the centroid of the triangle. (Actually you only need to find it with two lines but the third line makes a good check.) Again all of this is done with pencil and ruler on the scale drawing. Here is the job for the Jinni lug sail:

Next you use that info to figure the centroid of the total sail. It lies on the line that spans the centroids of the two triangles. So first you draw that line. Then measure the length of that line. The total sail centroid is located by using the ratio of a triangle's area to the total area, times the length of the line. In the example the ratio is 73.4/115.6 measured away from the centroid of the smaller triangle. On my scale drawing the length of the line between triangle centroids measured at 60mm so the total sail centroid is 60 x 73.4/115.6 = 38 mm measured from the centroid of the smaller triangle. You could also figure it using the smaller triangle, giving a ratio of 42.2/115.6. That times the 60mm length gives 22mm measure down from the centroid of the larger triangle. It is easier seen that written, like this:


...get out the scale drawing of the original rig of your boat. Here is the drawing of the Birdwatcher copied from Phil Bolger's great book BOATS WITH AN OPEN MIND. (Any book you can find by Bolger is worth going after.) Draw a vertical line up from the waterline that passes through the original sail area centroid, like this:

Next tape the scale drawing of your new proposed rig over the boat drawing such that the centroid you calculated falls right on that line, like this:

There you have it! For the boat to handle correctly the sail MUST be placed in this position. It can't go anywhere else and have the boat balance correctly.

In this case I also show the location of the lug sail's mast. It MUST cross the yard very close to 40% back from the sail's luff. Otherwise that yard won't behave well in practice. I also show the location of the original mast and it is clear that the lug sail mast will need a change in partner and step. There is no getting around it although on a slot top boat like Birdwatcher it is actually pretty easy to shift a mast fore and aft.

I guess your rerigged Birdwatcher will look like this.

The mast is clearly a lot shorter than the original, if fact the entire rig will easily stow inside the boat. Sail area is a bit smaller than the original, 116 square feet vs 126 square feet. Hopefully by the end of the sailing season here I will have an update on how well the swap worked. To be emphasized is that the original mast partner and step are still in the boat and the boat could be used with either rig.


...We'll repeat the essay on making a taped seam hull.




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.

Blobster uses taped seam construction. Five sheets of 1/4" plywood, eleven sheets of 3/8" plywood and one sheet of 1/2" plywood.

Prototype plans for Blobster 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:

The Alabama Skat is in the paint stage:

This Seal Cove Skiff was completed a while back, built within a few weeks of when the plans were released. There was a delay in launch until the Canadian ice melted. Write up soon.

The Texas Ladybug builder prepares for taping seams:

Am upside down plywood box I'm told is the true 16' Shanteuse (at last) is going together in Tennessee.





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