Regeneration takes place whenever water rushes past the prop and spins it faster than it has been turning under power. If there is an increase in sailing speed, a puff of wind, or if the boat surfs down a wave, the prop is then driven by water force. The instant this happens, the motor becomes a generator, electricity is produced, and the controller(s) sends electricity back to the battery bank. When boat speed drops again, the controller automatically feeds more power to the motor to replace what is no longer being supplemented by regeneration.

Regeneration Monitoring

The controller can do this all day long – continuously adjusting power flow from regeneration to power to regeneration again. And all the while, there’s no pollution, no noise, no onboard indication that the motor is in regeneration mode, except for the plus (+) or minus (-) digital amperage readout number on the E-meter. On any given sailing day, the E-meter readings may cycle from negative amperage if the motor is consuming energy to help push the boat, to positive readings during regenerative sailing periods.

By monitoring , you can adjust motor speed to take advantage of regeneration for battery bank recharging or to use regenerated power to supplement motor cruising speed. A zero reading on your E-meter amperage indicates that the regenerated electricity being stored in your battery bank is offsetting any and all motor power and onboard appliance use that is tied to your battery bank. When throttle speed is increased to the point where the boat travels faster than with the sails alone, it’s a net energy loss. When motor speed is kept low and electricity is regenerated under sail, it’s a net energy gain. On a day with a fresh breeze, plenty of puffs and/or waves to surf down, the boat can return to the dock with more charge in the battery bank than when it left.

Using regeneration data, you can determine how you wish to use the regenerative energy produced. Leaving your throttle just forward of neutral (or in feathering prop boats, in neutral) you can maximize electrical power by allowing the power that your motor has generated to be stored in the battery bank. Or you can throttle forward and use regenerated energy to increase boat speed. Monitoring will tell you the net gain or loss in energy to or from your battery bank at any given time. It’s free energy, provided by the wind, momentum, and water. Use it to help power your boat and your onboard appliances, or store it for future use.

Airfoils + hydrofoils

Regenerative motor sailing is completely different from conventional motor sailing with a noisy diesel trailing a cloud of smoke and an oil slick in the wake. Wind energy is not just converted to motion through the airfoils of your boat’s sails. It’s also harnessed by the steel or bronze hydrofoils of the boat’s propeller. Both regeneration and motoring, however, need an aggressively sized propeller to function most efficiently. That’s why we recommend a large, three-bladed, high-pitch, fixed prop (see Prop Size Matters) for those who wish to maximize regenerative performance.

Regeneration does reduce maximum speed under sail since the energy produced results in drag. We estimate the decrease to be anywhere from one-half knot to a full knot. But average speed over time can be maximized by taking advantage of throttle power to (1) increase apparent wind through increased boat speed and (2) to work the large diameter prop that keeps the boat powering through the lulls.