Here's a nifty way to mark fin spacings on BTs:

Wrap a piece of paper around the tube, and draw a line at the overlap

point.

Lay a ruler on the paper so that the zero mark rests on the edge of the

paper, and aome number divisible by the number of fins is exactly on the

overlap line. This will be a diagonal of some sort, but it doesn't have

to go corner-to-corner.

Mark the paper at the appropriate divisors.

Wrap it around the tube again, and transfer those marks as fin

positions.

For example, let's say I want to put 5 fins on a BT-60.

When I wrap and mark the paper, I end up with a length of just about

13cm (41.6mm * pi = 13.11cm). Now, I lay my ruler down so that the zero

point is on the edge of the paper, and then rotate it until some number

divisible by 5 is just touching the line. OK, 15cm. Now I mark at 3,

6, 9, and 12cm. Then, wrap that paper around the BT-60 again, and

transfer those marks plus the edge and voila! Five evenly-spaced fins.

It may sound complicated, but it's pretty simple if you once _see_ it

done. This technique can be used to mark off any arbitrary length into

any arbitrary number of equal divisions. Its accuracy is limited only

by the accuracy of your ruler, and by your ability to mark the paper

precisely. There's no measuring so there's no rounding error.

--

Gordon S. Hlavenka