Duckworks - Articles

Sail Dart Calculator
by Thomas Hamernik

A while back, I issued a challenge to the Michalak discussion group to come up with a calculator or spreadsheet to do the math involved with Jim Michalak's dart method of making sails from polytarp. I used this technique to build my Caprice and ended up with great sails for not much money. I am still using those sails three years later. The only drawback to the system is the math. We had a few submissions, but Tom Hamernik's spreadsheet is the best of the bunch. It is presented here for the first time, so try it and let Tom know how it worked for you.

Click HERE to download the file for current versions of Excel
Click HERE to download the file for older versions of Excel

SailDart is a MS Excel spreadsheet intended to assist the home boat-builder with the design and construction of sails using the Michalak dart technique for adding shape.

The user enters basic sail parameters such as luff, head, leech and foot dimensions, plus the throat-to-clew distance, to generate the sail shape.

click image for larger view - you may want to print this graphic if you are using the spreadsheet for older versions of Excel as it is not included with the file and is the keys to the dimensions.

The user also specifies the camber ratio and the position of the maximum draft. The spreadsheet calculates the sail's area, the location of the center of effort and the size and positions of the darts.

For jib headed sails:

Assume the luff of the triangular sail is equal to the the sum of the lengths of the luff and head entered into the spreadsheet. I suggest you decide on what the triangular sail's luff is, then enter 1/2 of that value into each the luff and head cells in the spreadsheet.
Then, simply adjust the "throat" to clew dimension until the luff and head line segments line up vertically - the values for the upper dart will go to zero since only the lower dart is required for a triangular sail.

I created this spreadsheet in response to Chuck's challenge made to the Jim Michalak Yahoo group awhile back. Since Chuck had not posted anything to his site, I decided to give it a try. Since the spreadsheet is essentially new and untested I welcome all feedback. Please let me know if you have any comments or suggestions or, especially, if you find any errors.

Thanks,

Tom Hamernik
thamernik@hotmail.com

Editors Note: The first time I tried the Sail Dart Calculator, I did not know that you have to navigate it with tabs at the bottom of the page. Here is a screen shot of the lower left hand area of the page:

As you can see, there are six tabs - Instructions, Plan, Pattern, Key (no Key tab in version for older applications), Center of Effort, and Version History. Click those to move from one function of the calculator to the next.

	vvvv
	The Online Magazine For Amateur Boat Builders