Reprinted from his article ‘Some Dinghy Cruising Problems’ in Bulletin No.9, summer 1959.

‘He stepped to the rail and cleared his throat noisily, standing, waiting. “Go,” came the Admiral’s high clear voice from the poop, and Perez spat into the indigo sea.

The Admiral was by the rail on the poop, the finger of his right hand clasping his left wrist. He was counting the number of times his pulse beat whilst the fleck of mucus drifted back to him, which would enable him to estimate the speed of the ship through the water.’ The Earthly Paradise by C S Forester

Losing my first boat through inexperience, I set about reading — among other things — pilotage and navigation. It is soon evident that to set a course accounting for wind and tide, one must first know the boat’s speed through the water. The Admiral had one way of doing it, but to rely on your pulse whilst your crew is leaning over the side dropping matchsticks into the water is not reliable. A stop watch or second hand is. This method is surprisingly accurate as a number of drops are made and an average taken.

The formula:-

6 x length = speed in knots 10 x seconds

is easily worked out in your head, especially if a dropping spot is marked a measured 10 feet from where you sit at the tiller. E.g. average time of matchstick to float by 10 feet of boat is 2 seconds:-

6 x 10 = 60 = 3 knots 10 x 2 20

Using the same formula, a piece of wood attached to a line 20 or 30 feet long, paid out over the stern and timed will give a slightly more accurate result, and is useful if you are single-handed.

(A Wayfarer owner uses a tennis ball attached to a line. Looped on one finger you don’t even have to watch the ball after you’ve thrown it overboard — it will give you a tug at the right moment — you can also write the formula on it to remind you — PB)