Tennis Ball Knot Meter for Sailboats

Article By John Cody

If you don't have a GPS or other speedometer, you can make a speedometer with a string attached to a tennis ball. What you do is toss the ball overboard and when it crosses the transom, start a timer (or just start counting in your head). When the string is fully extended (pulled taught), you stop timing and figure out your speed.

If you have 101.2 feet of rope, the following would be accurate:

60 seconds = 1 knot
30 seconds = 2 knots
20 seconds = 3 knots
15 seconds = 4 knots
12.5 seconds = 5 knots
10 seconds = 6 knots

If you do have a GPS, something you might do is take a reading with your gps and then try the tennis ball and string method to measure the speed you are going in the water. Going back and forth between them will give you a good eye for using the tennis ball, and then next time you go sailing you can leave the GPS and home and just use the ball and string for measuring your speed on the water.

And while you are experimenting with speed... make sure to take a look at the wave your hull is producing. Sailboats naturally produce a wave that the hull sits in as you sail through the water. After a while of checking your speed, you will be able to get a pretty good estimate of how fast you are going just by looking at the length of trough, of the wave your boat is in while sailing.

Rough estimates like this are usually good enough since the speed a sailboat travels is continually changing, and most of the time when I am doing speed calculations, I am trying to figure how long it will take to get to the next waypoint and estimating is good enough.