It is a more difficult problem than it seems.
The sensor is fairly simple to solve. Speedtech, a Swiss company with a vendor in the US, sells a speedometer system for sailboards. Their impeller, which is a basically a tiny plastic prop with a magnet imbedded in the plastic, is a good place to start. The rest of their system is too bulky and heavy for a model.
I believe telemetering is, or should be, illegal for racing – but it excludes itself anyway. If you put an impeller under your racing hull, it will be draggy, and the electronics add significant weight. My thought was to use it for tuning. Accordingly, I attached the sensor to a dinghy, and put the counting chip and the transmitter in the dinghy’s hull. I used a tugboat hull from MicroGlass in Buffalo. It is about 12 inches long.
The Speedtech system uses a coil under the board sailor’s feet. The impeller spins up, induces a pulsating voltage in the coil; on board electronics count the rotations and display the result in knots, mph, kph, whatever you prefer, on a readout clipped to the mast.
The coil is impractical for a model. I used a Hall Detector chip. It reads right through the fiberglass hull, and kicks out an nice, sharp waveform. (It contains a Schmitt trigger, if you are into this stuff.)
For the counter and scaling I used a basic stamp.
To get the count back to shore, I stripped a 27 Mhz FM pistol tranmitter from HiTech. Oddly, the PC board is nicely boat shaped, and slips right into the Tugboat hull.
How to modulate it? I replaced the joystick pot on one channel with a pot-on-a-chip, that is, a digital potentiometer. This works like an ordinary pot, only the wiper is incremented up and down by digital command, instead of by your fingers. The basic stamp processor counts the rotations of the impeller, and then drives the pot wiper up or down as a function of impeller speed. The R/C transmitter sends a signal as usual to a Hi-Tec R/C receiver on shore.
On the dock, a second processor chip measures the pulse width of the incoming R/C signal, converts it to a number (speed) and displays it on a little readout.
The net of it is, you can fiddle with the controls on your yacht, especially third channel controls like twist, backstay, or slot, and get an immediate readout on whether or not you have helped your speed.
It is not finished. The transmitting and receiving and all the processing electronics work on the bench (blowing on the impeller, running it under a faucet, etc, to simulate real boatspeed). But I have not yet put the tugboat into the water. The simple mechanical problem of mounting the hall detector is the latest sticking point, but I don’t doubt the system will work in practice.
Incidentally, you should probably have an FCC license to do some of this. But anyway, there is the basic system.