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If your boat doesn't sport a nifty electronic knotmeter, or the dang thing packed it in, you can use your GPS speed reading if you average the results over a few minutes for a reasonably accurate SOG (speed over ground).
If you don't have a GPS, or the dang thing packed it in, or you need velocity through water (to account for current effects on SOG), here's a little trick I picked up from "The Complete Sailor".
1. Tie a 51 foot 1/4” nylon braid line to a tennis ball and cleat the loose end off at the stern.
2.Toss the ball overboard (ahead of the stern cleat).
3. Time how long it takes the 50 ft cord (1 foot for cleat tie off and angle compensation) to become taut.
30 sec = 1 knot/1.2 mph/1.8 kmph
15 sec = 2 knots/2.3 mph/3.7 kmph
10 sec = 3 knots/3.5 mph/5.5 kmph
7.5 sec = 4 knots/4.6 mph/7.4 kmph
6 sec = 5 knots/5.8 mph/9.2 kmph
5 sec = 6 knots/6.9 mph/11.0 kmph
4.25 sec = 7 knots/8.1 mph/12.9 kmph
3.75 sec = 8 knots/9.2 mph/ 14.7 kmph
Now here's where I can start taking some credit for this wonderfully simple yet highly accurate (especially at slower speeds where most other devices suffer) idea.
We printed the speed/time chart (above) and put it inside the clear tennis ball container (facing out) so it was always handy, and have tied a lead to the cord bitter end for a manual 50' sounder.
We always take two speed measurements, and use it if they're identical. (The first one being slower could be due to a kink in the line paying out). If the first reading is slower than the second we make a third, and if the latter two are identical we use it.
The lead, line and tennis ball all stow away in the container.
So there you have it, the most reliable (no batteries or vibration/corrosion sensitive electronics) anti-theft/vandal resistant (when stowed) depthsounder/knotmeter combo available, at the lowest possible cost.
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