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Vic-
 I think this is one instrument where construction is highly a matter of 
convenience. The boards I have seen pictured are triangular in shape, perhaps 
4"-12" along each side. In each corner you drill a hole and attach perhaps 3' 
of any convenient light line, i.e. "parachute cord". Take those three lines 
and even them out (i.e. stand on the board and pull all three up over the 
center of it) then tie a knot where they join. You could insert a ring at 
that point to join it up to your knot line, or simply tie the line on.

In this age of synthetic cloths, I would think "ballistic nylon" or any heavy 
synthetic canvas could be used instead of the wooden log, just like a very 
small sea anchor, with a fishing weight on one corner perhaps to make it 
submerge promptly.

The knot line can again be any convenient cordage with a knot tied every 
increment along it. The increments can also be at your convenience, so you 
can measure only full knots, or place one every 1/10th of a knot (10x more 
often) etc. Instead of actually tying knots in such a long cord, consider 
just putting a stitch of yarn through it to mark every position.

The distance between knots would be simple math: One "knot" of speed being one 
nautical mile per hour, which is 6076.115 feet of line that would pass 
through your fingers in one hour.

Dividing the ungainly 6076.115 feet of line by 60 (the number of minutes in 
one hour) we can arrive at 101.27 feet of line that must pay out in one 
minute, to measure a speed of one knot. Or 50.56 feet of line between the 
physical knots, to measure a speed of one knot in thirty seconds.

You can, after all, stream the line out for a full minute or any convenient 
portion of it. This is custom instrument building and your preferences rule. 
I would encourage you to build your line literally on the scale that suits 
your boatspeed and patience.

If you are on a sailboat making typical speeds of perhaps 6 knots, that is 
36,456 feet traveled in one hour. That could require hauling in 607 feet of 
line in one minute. Or, 303 feet in thirty seconds, or 151.5 feet in fifteen 
seconds. So, you can place your physical knots at whatever distance makes 
them scale nicely for the time frame you plan to use.

In order to calibrate your line for tenths of a knot, simply place ten knots 
equally along each "knot" worth of line. Or a hundred tiny knots, if you want 
to be really compulsive about it.

Don't take any of my math for granted...I'm not licensed to practice it in this state.

   

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