There are a number of places on the Thames where you can find transit markers spaces exactly 133.3 metres apart. This is because 8 km per hour is equal to 133.3 metes per minute. So, the idea is that you time your boat passing between these markers and if you take less than 1 minute, then you know that you are travelling too fast!
I have added a clipper duet to AViVA which measures depth and speed. The speed measurement is achieved through a spinning wheel under the waterline, but this needed adjustment, so I used the transit markers on Port Meadow, near to Oxford and made the adjustments as I took a few transits across these markers gradually getting a one minute transit with a speed showing 4.2 knots.
The photos above show the speed markers from the river bank, taken on a pleasant spring walk along the river.