Hi Mary...

Doesn't it seem that things are getting overwhelmed with too much technology? Why not try it the old fashioned way? Get a long piece of light rope or cord, tie a knot in it (or mark it) every 47 feet, 3 inches. Attach a heavy piece of wood or other floating sea anchoring device to one end. When underway, toss the wood off the stern (the rope must be able to pass freely over the stern).
After the first knot passes over the stern, time for 28 seconds, then stop the rope. You could turn your boat around and collect your wood or just drag it aboard. Count the number of knots that went over the stern during the 28 seconds (after the first knot). You have your speed in knots.

A thousand feet of rope should be enough to show up to 20 knots. I suppose 500 feet would work if you timed for 14 sec. and then doubled the number of knots that were counted.
1500 feet of rope (or 750 feet then double) would be needed to determine a speed of 30 knots.

Any current helping your boat along would also be helping your piece of wood along, cancelling out any bonus speed reading.

Of course, GPS is a bit easier!


Mark Owens
1976 H16 sail # 19857