What about a thing like an electric motor powering a shaft driving a pair of mini paddle wheels sitting/fixed on the top of the board as well as ballast to get it to say 80kg. A lot of the weight could be a couple of car batteries that would hold so much charge you could get quite a few runs in without much voltage variation.
Just charge it up and time it over a known distance or put a gps on to measure speed. Change postion of load centre, change fins change boards etc all with a pretty constant power input.
Seems like measuring strain on a paddle would be very tricky to interpret.
Hang on...might be a new product here - electric powered SUP
. Catch a wave then silently power back out to the lineup.
