9th August 2021

Hydrogenation in the marine sector is a relatively new concept that has arisen as part of the electrification of sailing boats. It broadly relates to the generation of electricity by an electric motor by dragging a propeller through the water, causing the motor to spin and creating electrical energy.

Sailing boats have an interesting advantage as a wind powered vessel which allows them to utilise hydrogeneration to charge their batteries – a task traditionally requiring a fossil fueled generator, often supplemented by solar or wind generation.

Before we assess the potential of hydrogenation in the marine sector, let’s look at it’s equivalent in the automotive sector, which is termed regenerative braking.

Regenerative braking in electric vehicles utilises the electric motor to slow a vehicle rather than using the mechanical brakes of the car, and to use the power that the motor generates to charge the battery. This is achieved at the motor by, broadly speaking, applying negative torque to the motor and using the momentum of the vehicle to spin the motor and generate power to decelerate the vehicle.

In some applications this can generate substantial amounts of energy, for example heavy electric trucks in mining operations are able to charge their batteries as they descend into deep mines. The large motors on these heavy trucks operating on steep declines have the potential to regenerate substantial amounts of power. This also has the added benefits of reducing wear on the braking system.

Regenerative braking is often incorrectly perceived as a valuable source for battery charging, and in on-road applications the recovery of energy from regenerative braking is disappointingly small. We often state that the laws of physics that govern energy are cruel. This is particularly relevant when it comes to regenerative braking and the perceived potential energy that can be harvested. Note though, that the energy that is capable of being harvested from a motor in regenerative mode can be the largest amount of charging power that the battery will need to accommodate.

In our electric vehicle work we found that the role of regenerative braking was focused more on the drivability of a vehicle so that it resembled internal combustion powered vehicles. For example, without regenerative braking an electric vehicle would simply continue to coast freely, which can be disconcerting after the habits of driving internal combustion vehicles, particularly as one approaches an intersection and expects the vehicle to decelerate when the accelerator pedal is lifted.

The difference between regenerative braking in vehicles and hydrogenation in sailing boats is that vehicles really only use regenerative braking for a very short period of time – while decelerating. A sailing boat on the other hand, is capable of hydrogenerating for extended periods – while under sail.


Now…. let’s turn our minds to the marine sector where hydrogenation in sailing vessels has the potential to contribute to the energy budget of a sailing boat.


There have already been advances in hydrogenation with products on the market (* see link below). These are relatively popular and being viewed as effective energy generation sources. In addition, most providers of electric propulsion systems include the hydrogenation capabilities in their products.

At this point we need to state some guiding concepts that are associated with battery electric propulsion systems and hydrogenation strategy.

  • As battery electric propulsion systems become normalised in sailing boats, they will effectively replace diesel engines. This trend is occurring rapidly in the automotive sector, driven by a range of persistent variables and these technology advances will ultimately benefit the marine sector.
  • Quality electric motors and their motor controllers are quite capable of utilising the propeller to act as generators to charge the boat’s propulsion battery.


However, there are some detailed aspects of hydrogenation that also need to be considered in this discussion.

  • Electric motors differ considerably in their power and torque curves compared to diesel engines. These performance characteristics will affect motor selection as diesel motors are replaced.
  • The high efficiency RPM zone in electric motors is different to that of diesel engines. The efficient operation RPM of electric motors is vital for optimising the energy efficiency on a sailing boat. It is possible that the move to electric motors may see a decrease in propeller RPM and an increase in propeller diameter. This will improve efficiency and hydrogenation capacity. This may or may not impact on gearbox ratios.
  • Blade design for Thrust and Hydrogeneration are fundamentally at odds. Interestingly, designing propeller blades that are optimised for hydrogenation are actually a mirror of the shape of blades designed for thrust. As you begin to ponder this mind bending quiz, I will save you some mental anguish, hand gesturing and, for some, resorting to paper models  – NO - it is not a matter of simply over-pitching a blade to achieve the correct profile. The blade design is a MIRROR of a thrust blade.

Much of the above requires more work to optimise the efficiency of battery electric propulsion systems, but these types of issues involving the technology transition from fossil fuel to battery electric have been resolved in the automotive sector quite effectively.


So…the question remains – is hydrogenation going to remove fossil fuel generators from sailing boats?


Anecdotally, existing electric propulsion systems, using traditional blade designs are hydrogenerating very useful amounts of energy. These systems are utilising folding blades that were designed for thrust on diesel powered systems, so it is likely that much greater efficiencies can be achieved.

Consistent with the above, the theoretical energy recovery from a propeller is much greater than is currently being reported from systems currently operating in sailing boats. These calculations suggest that the energy generation may be able to easily accommodate the energy budget of a sailing boat, if other aspects of the system, such as storage, are also optimised.

As with all things, the universal laws remain cruel and hydrogenation will impose the ‘cost’ of drag on a sailing boat as it generates energy. This ‘cost’ appears to be in the order of 1 to 3 knots while in full hydrogenation mode. This leads to obvious thoughts on designing hydrogenation, where several possible modes may range from Maximum to Minimum hydrogenation depending on the energy requirements.

Other factors that influence the energy system design of a sailing boat would include; frequency of usage, mooring methods, availability of shore power, ability to accept hydrogenerative drag etc.


My personal view is that, strategically, hydrogenation is well placed to deliver the energy requirements of sailing boats, however, like the automotive sector there may also involve changes in behaviour as to how we utilise sailing boats as we transition from fossil fuels to electric propulsion.


* https://www.wattandsea.com/en/products/cruising-hydrogenerators


Tech Notes are a technical comment on a particular topic. Tech Notes are provided as guiding information only and are not to be relied upon as technical advice.

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