Spinning Propeller
As you sail along, the water flowing over your propeller will cause it to spin. Some sailboats are fitted with a special alternator that will create electricity from a turning propeller shaft.
The energy gained is taken by creating drag force on the propeller blades to make it spin which acts to slow your boat. Albeit minimal.
With this type of device, sailing at 5-7 knots, it is possible to generate 5 amps or so at 12 volts which equates to 60 watts. In 24 hours of sailing, you could create 1400 watt.hours which is enough to charge 1 small 100 amp.hour (1.2 kilowatt.hr) battery.
