Re: Boat Speed Meter
If you are operating in sea water, there is a proven method. Naval vessels use a detector called a Rodmeter.
The rodmeter is a coil inside a waterproof sheath. On the surface of the sheath are two contact buttons. A constant magnetic field is established by the coil.
- As the vessel moves through the water, the conductor (seawater) is cutting through the magnetic field generated by the coil.
- A conductor moving in a magnetic field generates a potential difference between the two ends of the conductor.
- The induced voltage appears across the two contact buttons on the outside of the rodmeter sheath.
- Measurement and display circuitry converts the voltage to speed units.
There are many variations of rodmeter. Some are shaped like small wing shaped antennas that stick out from the boat or ship hull. Some are mounted flush with the hull.
There is another, more sophisticated, method that also can be used with great accuracy in any type of water. You can mount an ultrasonic transducer on both the front and back of the boat. You pulse the front transducer, and measure the time to detect the pulse in the rear transducer. The time required for the ultrasound pulse to reach the rear transducer through water will be a fuction of the speed of the boat. This method of speed measurement is generally accurate to within 0.1knot.