Speedometers - what are the options?

I'm in the middle of building a human powered boat and I'm just wondering if anyone can suggest any form of speedometer set up at all. I know the Hobie-flappie-thingie has one which I'm guessing is a pitot tube type from the pictures,and obviously there's the GPS option too, but has anyone got anything else?

I was wondering about using a bike computer, magnet on crank arm, sensor mounted somewhere and then setting the computer so that it matched the combination of gear ratio and prop pitch. My thought was to intially set it so that it thought 1 revolution of the cranks was 1 unit (probably in metres). Then pedal over a fixed distance to see how many revolutions it took to cover and from then work out the "real" distance per crank revolution, with which to calibrate the computer.

I can see it'll only give me a water speed not a land speed, and obviously it will do nothing when not pedalling, but I'd quite like something cheap which gave me a rough idea.

Can anyone think of a better way?

 

Fluid Speed

How important is accuracy?? Everything you are doing is in a fluid, flowing, slipping environment. A cheap GPS is the only highly-accurate speed-over-ground option, I think....

 

Accuracy isn't that important really, the boat's going to be used on inland waterways and half of the time the canal has kilometre posts anyway. It's as much about keeping me amused during a long day's pedalling as it is about actually knowing how fast I'm going.

 

If you are serious about maximum performance as opposed to SOG, get a paddle knotlog and a bike computer. then work up a chart of cadence vs waterspeed. This is how we did it on a couple of HPV's.

 

A cheap mapping GPS will provide hours of entertainment. Through-hull transducers will give water speed, take one from the other and you have the river or tidal flow speed.

I use a Magellan Explorist 210 which is cheap and rugged, but there are a variety around.

Easy, eh?

Tim B.