When the knot log fouls and the GPS goes on the blink then there is always the Dutchman's Log for estimating the speed through the water. Just count the number of seconds a bit of floating material takes to travel from bow to stern to get knots.
Use the formula:
Boat Length in nautical miles / drift seconds from bow to stern = Knots / 3600 seconds in an hour
Example for 33 ft. boat given 6076 ft. in a nautical mile and 3600 seconds in an hour:
Formula: .00543 NM boat length / drift seconds = knots / 3600 sec.
Knots = .00543 x 3600 / drift sec.
Knots = 19.6 / drift sec.
Table of Speed Though the Water for 33ft. Boat
9s = 2.2 kts.
8s = 2.5 kts.
7s = 2.8 kts.
6s = 3.3 kts.
5s = 3.9 kts.
4s = 4.9 kts.
3s = 6.5 kts.