Note: this page was written in the planning/build phase of our project in early 2023 and contains many assumptions. We have updated it with references to our practical experience.

We are currently planning the build of a brand new sailboat, an Alubat OVNI 430, with an expected delivery in late 2023. We feel very excited and grateful about this, and hope to document the process on this website.

OVNI 400 “Polarstern” with the characteristic hard aluminium dodger at the Grand Pavois in La Rochelle, 2021

Our boat will be equipped with a hybrid electric propulsion system, and as we’ve received a number of questions about that and there’s certainly growing interest in such solutions in the marine industry, this page is meant to explain the design of the systems and the reasoning behind our choice, and to give interested readers an idea about what it can and cannot do.

We have looked at what others have been doing in this space to help us come to this decision. The following projects were a great inspiration to us:

• Sailing Uma, who started with a custom built electric system but are now using an Oceanvolt engine

• SY Volt, an innovative 42 ft custom build (now mini series) fitted with Bellmarine motors

• SV Anemis, a Garcia 45 fitted with an Oceanvolt system (check out their experience report on using an electrical system)

We’re quite happy to exchange ideas if you are thinking about getting a similar system or have one already - feel free to contact us.

Specifications

Below are the planned specs of our new boat that relate to the hybrid electric propulsion system and energy management:

• 24k kW Oceanvolt ACX24 shaft drive motor

• 23.2 kWh 48 V LFP battery bank

  • Consisting of 4x 24 V battery modules; ensuring that half of the battery bank is still usable in case one module should fail

  • The 48 V bank also supplies power to the 12 V system (other than the motor, everything else is still in 12 V to keep things simple)

• 910 Wp of solar panels

• Hydro-generation via the Oceanvolt motor

  • Expect ~75 W of regeneration when sailing at 5.5 kt, 200 W at 7 kn, 350 W at 8 kn, etc. (Update: we now have measurements, it’s even slightly higher than that; Update 2: after a bit of long term practical experience, things are looking less bright)

  • An Oceanvolt ServoProp saildrive can do almost 3x of that, but a saildrive seems too risky to us when drying out with the OVNI

• 5 kW inverter & electric induction cooker

  • No propane on board, no refill worries!

• 11 kW DC 48 V generator

• No wind generator

  • Wind generators are too noisy, we think, and not worth it in our setup

  • In general, the large battery bank allows you to better live through a few days of lower solar power when at anchor, with less need to look for alternative energy sources

Capabilities

We should be able to motor at 5 kn in flat water using ~4 kW power, or motor closer to hull speed using ~10 kW (=generator power). These are approximate numbers based on real world testing done with the Viator Explorer 42 DS which is comparable in size. The Oceanvolt Configurator suggests similar figures.

Updated: see our own first performance tests.

The 20 kW engine provides a safety margin higher than the generator power output, and we can motor at 20 kW (full power) for up to 60-90 minutes if necessary (10 kW supplied by the generator, 10 kW by the batteries). We don’t expect we will use this often though, the 4-6 kW power level seems best suited for going in and out of harbors and to escape a calm patch on longer crossings. Heavier seas are better endured under small canvas, or avoided altogether.

Another great use case for this electric motor is when considering whether to motor-sail or not, as we can add small amounts of power silently when the sails are up. E.g., 1 kW would take our sailing from “2.5 kts, not fun” to it being bearable. We can sustain this level of motor power for >10 hours without running the generator. For us, the decision to motor-sail is normally accompanied by thoughts of “I hate the sound of the diesel, let’s not do it” but this will no longer be a problem.

When motoring for a longer duration, the diesel consumption under generator power (hybrid electric system) should be slightly lower compared to a main diesel engine, as the generator runs at optimal RPM, which the main diesel engine usually doesn’t (and the electric motor has a very high efficiency). That means we even get a little more range out of our diesel tank.

We know we will use very little diesel with this setup and we’re happy with that. We think that a sailboat has sails for a reason, and we plan to use them. The patient sailor always enjoys good wind, and we don’t plan to be in a hurry... We will also use diesel for heating when going up north, and having the diesel and generator on board provides a safety net as we’ll be sailing with a child.

That said, building the same setup without a generator and without a diesel tank is entirely feasible (I wouldn’t even increase the battery size), you would just be little more limited in your planning (see Sailing Uma).

One more note about the reliability and maintenance requirements of diesel engines, and fuel quality. Engine problems are more likely to not develop in the harbor, where they’re easy to fix, but out at sea, because the bad diesel will be shaken up from the bottom of the tank by the boat’s movements. This can put you in some tricky situations if the main diesel engine fails at the wrong moment - dual, switchable diesel filters and a day tank can help. In the hybrid system, the battery is your “day tank”, and while a diesel generator failure is still possible, you are not disabled & adrift in the water when it fails. Instead, we can choose when to run the generator (to some extent), we avoid running the diesel for short periods only (which is bad), and overall we will accumulate less generator hours than main engine hours on a normal sailboat, lowering the maintenance frequency that we have to do on the generator compared to main diesel engine maintenance.

Energy Balance

From a stochastic simulation of our future energy generation and consumption under difference usage scenarios, we expect that we will consume between 3-7 kWh per day (250 Ah - 580 Ah in 12 V terms). Below is a summary showing the 10th percentile, median and 90th percentile estimates - we’ll see what reality has to say about that...

Other engine use such as longer motor passages would or could happen under generator power so does not make sense to include in the energy balance. But it’s clear that some of the most frequent uses of the engine, such as going in and out of harbors, can be accomplished within the daily energy budget and without the generator.

Due to the large battery bank, even relatively unfavorable scenarios (e.g. at anchor, no sun) can be sustained for up to 4-5 days before having to run the generator. In many other scenarios, there is an energy surplus that will make running the generator a very rare occurrence - hopefully.

Frequently Asked Questions

What is the equivalent in HP of a 24 kW engine?

By simple unit conversion, 24 kW is 32 HP, but people tend to agree that the higher torque of the electric motor makes it feel more powerful than the equivalent HP diesel engine. Oceanvolt claims the AXC24 electric motor is equivalent to up to 80 diesel HP. 

Do you have to heat the batteries storage space if you go in area where the temperature is below 0˚ C?

No. As long as you live on the boat the interior will be above zero (I hope), and below the floor boards the water will help to keep things above or around zero anyway. So the batteries will only go below zero if you leave the boat for a long time. Even then, they will still work, but maximum power output will be limited until they’re warmed up again. You can probably live with that. If you compare it to a diesel, the batteries will certainly work (start) better at temperatures way below zero.