With good sailing conditions dominating in Les Sables-d’Olonne in the second half of April, we were finally able to do the second part of the certification of our Oceanvolt propulsion system, which is testing the regeneration performance under sail (in November, we only tested and certified the propulsion performance). In 12-16 knots of wind and good sea state (around 1 meter of wind waves), and using our genoa or Code D sail, we were able to test regeneration at different true wind angles to the wind, and different speeds.

How it works: as we have a 19 inch Flex-o-fold folding propeller (on a shaft drive, not a sail drive), the propeller needs to unfold or open from its folded sailing position in order to start generating power. This is achieved by letting the Oceanvolt engine produce some forward thrust for a few seconds, spinning the propeller at 300 RPM or above so that it opens from centrifugal force. As long as the boat moves above a minimum speed, the propeller then stays open and the engine can generate power that charges our 48 V main battery. Once speed drops below a minimum, and the propeller RPM drops below 250-300 RPM, the propeller folds and regeneration stops. Regeneration then has to be restarted again using the forward propulsion method.

The minimum boat speed for regeneration depends on sea state, angle to the wind and how stable the boat speed is. Sailing upwind, it was easier to maintain regeneration, down to approximately 4.5 knots of boat speed (still producing around 200 W on average). Sailing downwind, when the boat accelerates and slows down more with waves, an average speed of 5.5 knots is required to avoid folding the propeller during slowdowns.

The average power achieved during regeneration depends on boat speed, starting at 200 W at 4.5 or 5.5 knots (upwind or downwind, respectively) and reaching or exceeding 500 W at 7.5 knots which was the highest we could achieve on average on this day. For brief moments, we saw over 700 W of regeneration power at 7.9 knots of boat speed (at 110 degrees TWA, 16 kt TWS).

We will see how the system performs in more wind, if higher boat speeds can produce even more regeneration power or if the corresponding bigger state and disturbed flow of water state will make it harder to achieve high averages in power.

300-400 W of regeneration power at mildly ambitious cruising speeds of 6-6.5 knots is significant, though, and is a very nice addition to our energy balance and exceeds our expectations. For context, our 910 Wp solar panels produce 500-700 W in good midday sun and when not shaded by sails. Our fridge, instrument systems and autopilot consume around 200 W (perhaps more in warmer climate and bigger seas). The big advantage of regeneration is that is is able to produce power day and night, which is a big deal on longer passages. Even during a daysail, a few hours of regeneration will easily power all the induction cooking we do on board in a day, or partially restore the energy needed to motor in/out of the harbor on that day.

We will have to collect more data to be able to tell more precisely how much regeneration slows the boat down. So far, it does not appear to be significant.