Hello,
I want to make a projet to measure the velocity of a boat (no engine boat) and save that record to a sd card on arduino.
What sensor can i use to make this measure?
Is possible to record the results to a sd card?
Thanks
I've seen little propellers under boats, where the rotation speed depends on the speed relative the water. Maybe Google yacht fittings, or if it's not full sized, check the RC community?
I would guess that older ones would have a cable that turns like old car speedos, but imagine modern ones would have a hall effect sensor, or opto-interrupter. Both could probably be made to work on Arduino.
Hi,
GPS will do all you need without any propellers or external sensors.
Google arduino gps sd card
Or by a GPS, most have storage for tracking, in fact there would be special ones for sail boat applications.
Tom..... 
as has been mentioned, there are choices and options.
people who have not been to sea miss that speed is relative to the water.
boating up a river at 5 knots, when the river is flowing at 5 knots, gets you the wonderful result of consuming lots of gas without going anywhere.
that said, speed relative to water can get you nowhere, or be the most important measurement.
if you are a general boater, then the GPS is the way to go.