We previously reported how our electric engine has a very interesting capability in theory, that is to regenerate power while sailing, by converting the movement of the propeller through water into valuable energy. In practice, the initial propeller choice on our boat, a 19″ 3 blade Flexofold folding propeller, was unable to sustain that power generation in real world scenarios. We therefore soon started the search for a better propeller.

Any folding propeller will have the same issue - it needs to spin above a certain RPM threshold to stay open. Bellmarine electric engines seem to have a feature where motor power is used to keep the propeller open, but this requires energy and reduces the net output of regeneration (and is not available on our Oceanvolt motor). Still, it’s nice if you can keep regeneration going for many hours with no intervention. Also, Gori has developed a new Hybrid folding propeller that appears to be able to lock in the opened position - sounds great if it works, but this propeller was not available to us in summer of 2025.

Feathering propellers are more promising as they don’t fold, and we started to research what people’s experience was with regeneration with Variprop and Max-Prop propellers. These propellers rotate their blades so they don’t produce drag in the water when sailing, but some models can be locked in reverse thrust orientation so they don’t feather and are able to sustain regeneration.

Another great option is the Autoprop, which is a self-pitching propeller with a unique design. It has very good regeneration capabilities in the reverse thrust position of the propeller, but their helpful support person pointed out to us that in order to put the propeller in a low-drag sailing position, we need to stop the shaft. Since we don’t have a gearbox like with a diesel engine, and the resistance of the electric motor is not high, we would’ve needed to add a shaft brake to our boat to use the Autoprop in both regeneration and sailing modes reliably. This was out of the question for our planned short haul-out, but quite doable in general and probably not a bad choice. Also the Autoprop needs bearing replacements in regular intervals.

Out of pure luck, we got our hands on a never used Max-Prop in exactly the right size (19″) and shaft diameter/taper (ISO 30 mm) at a very compelling price, that a fellow cruiser needed to get rid of. So we decided to try it and see how regeneration would perform on the the next big passage.

In short - it worked quite well! On a lovely 2 1/2 day or 343 nm passage from Halifax (Nova Scotia) to Ramea (Newfoundland), we produced 7.1 kWh in 24 hours (295 W average) without a single intervention or having to restart regeneration. That’s pretty good! Wind was between 14 - 22 knots true at 140 - 150 degrees TWA. Later in the passage the wind died down a little and we stopped regeneration, but this was a very nice above-average showcase what is possible in decent conditions. Of course, not all passages have steady and strong winds like this.

To put those 7.1 kWh from regeneration in perspective, in the same 24 hours we produced 4.2 kWh from solar power (more than usual), and we consumed 8.3 kWh in total (no motor, only 12 V and 230 V consumers). 7.1 kWh also equals about 45 minutes of generator runtime or 2.5 liters of diesel which we didn’t need to burn because we had that regeneration.

To operate the regeneration feature of the Max-Prop, we need to engage reverse power briefly while sailing to rotate the blades of the propeller in reverse position, then stop the motor. In this position the blades (mostly) don’t feather, and regeneration will start. In many cases, regeneration will continue until stopped, but we also noticed that in some conditions (speed and/or sea state dependent) regeneration may also stop unintentionally. Perhaps the water manages to rotate the blades back into a feathered position, we’re not sure what happens. Locking the blades is not super reliable, but reliable enough, and a lot better that with the Flexofold for sure.

To stop regeneration or feather the propeller, we briefly operate the motor in forward power to rotate the blades back into forward position. In that position, the movement of the boat through water will feather the blades so they no longer produce drag, and the shaft stops spinning. Most of the time, this works even with the low resistance of the electric engine in zero power mode. Sometimes we have to play around with a little reverse power to stop the shaft, but we can always get it to feather without a shaft brake.

Pros of the Max-Prop compared to Flexofold:

• Regeneration is a lot more stable as described above, and can run non-stop for hours in most conditions. Sometimes we have to restart regeneration, but much less often.

• Regeneration will still produce steady power at low speeds - still over 100 W between 4.5 and 5 knots.

• Reverse thrust is more immediate - a general advantage of feathering propellers over folding propellers. Not a big deal to us as we don’t maneuver in tight spaces very often.

Cons of Max-Prop compared to Flexofold:

• It is a lot more noisy when motoring. Since the blades of the Max-Prop rotate 180 degrees, they are flat and don’t have an ideal profile. This causes issues of disconnected flow (a pre-stage of cavitation) and grinding/bubbling like sounds which are most notable when accelerating or in speeds between 3-5 knots. The Flexofold was virtually silent at any speed. Luckily, regeneration with Max-Prop is not noisy at all.

• Blades don’t fold fully and can catch lines in the water more easily. However we have a rope cutter installed on our shaft, so hopefully this will mitigate the risk of fouling our propeller a little.

Areas where Max-Prop and Flexofold are equally strong

• Forward propulsion efficiency is more or less the same

• Momentary regeneration power at higher speeds is also very similar, except that Max-Prop can actually maintain this regeneration

For us, the Max-Prop propeller is a big improvement over the Flexofold propeller due to its regeneration capability. It still feels like a compromise and has weaknesses (mainly blade shape/noise and not perfect locking in regeneration position), but it’s much better.

If we had more time to add a shaft brake to the boat and to get our hands on an Autoprop, this could be, purely from my theoretical understanding at this point, almost the perfect propeller for this type of boat. Regeneration should be very reliable, it’s efficient and quiet in regeneration and propulsion, the only downside is the bearing maintenance where I’m not sure how excessive regeneration shortens the lifespan of bearings. Perhaps someone else wants to try this and report on their findings.