system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do speedometers work on boats?
gpt
Besides GPS, a ship uses a speed log to determine speed.  There are multiple types of speed logs:

- Impeller log: A small device with a propeller is towed or attached to the ship.  When the ship moves along the water, the force turns the propeller.  The rotational speed of the propeller is proportional to the speed of the ship

- Pitometer: A tube faces the direction of the ship and another tube faces 90 degrees.  When the ship moves, the water pressure builds up in the forward facing tube while the 90 degree tube remains at static water pressure.  The meter calculates the difference in pressure between the two tubes, which is proportional to the speed of the vessel.

- Doppler log: The Doppler log emits a sonar wave (TX).  The wave travels out, bounces back, and returns to the receiver of the log (RX).  If the ship is still, the TX and RX have the same and frequency.  However, due to the Doppler effect, the forward motion of a ship compresses the waves closer together, changing the frequency.  The change in frequency is proportional to the change in speed.

Finally, on larger ships, you can determine the speed by doing calculations using the engine RPM, the pitch, and the load.  This will give you the mathematical speed, but won't account for friction and other variables.