Final answer:
A Doppler speed log measures the speed of a boat through the water using the Doppler effect.
Step-by-step explanation:
A Doppler speed log measures the speed of a boat through the water. It uses the Doppler effect, which is a change in frequency or wavelength of waves due to the motion of the source or observer. In this case, the speed log measures the change in frequency of sound waves reflected off the moving boat and calculates the boat's speed through the water based on that change.