Leisure marine engines are generally used so little that they tend to corrode out rather than wear out, so until recently there has been little incentive to retrofit a hybrid installation.

Prices remain high and deep-cycle lithium batteries are equally expensive, so most boaters stick with what they’ve got.

But things are changing as more production boatbuilders begin fitting hybrids as options, and the manufacturers of the internal combustion engine see hybrid – or even full-electric – as the future.

Economies of scale in the automotive sector are also bringing prices down, and increasing numbers of boaters have come to enjoy the benefits of silent electric cruising.

The large battery banks required also replaces the need to run generators overnight at anchor.

Here are some of the most promising advances for retrofit, as seen by PBO at recent boat shows.

1. Multi-fuel marine hybrid propulsion

If you’re looking for a replacement for a small marine engine, Berlin-based Arens has developed a 379cc two-cylinder 11kW (14.3hp) hybrid that can switch between fuels.

In development for eight years, the Arens Syprop can be used in several modes, namely thermal, pure electric, as a 4kW generator, a hydro-regenerator and a booster.

In booster mode, both internal combustion engine (ICE) and electric drives are combined to deliver 16kW/20.8hp of peak output.

The thermal engine is a twin ‘Boxer’ design configured to run on either gasoline or methanol with a seamless electronic switchover.

The electric motor is connected directly to the shaft by a belt to reduce noise and vibration and there is no gearbox.

Reverse is selected by the e-drive spinning backwards.

Aimed at smaller sailboats, the Syprop weighs 65kg (143lb) and basic maintenance is only required every 500 hours/5 years.

Arens is currently producing about 30 units a year, averaging €15,000 each, but is looking to increase production.

2. Gearbox hybrid

Mounting an electric motor between the gearbox and an internal combustion engine allows for simple installation as you usually only need to shorten the prop shaft.

Steyr was the pioneer in this field, and now UK-based AR Peachment has developed its own parallel hybrid designed to retrofit to most popular makes of marine diesel engines.

Configured to 48V, the hybrid offers a 10kw (13hp) electric drive which automatically recharges the batteries when in diesel mode.

The same throttle lever can be used in both diesel and electric drives, and the propeller can also act as a hydro-regenerator when under sail.

3. e-Saildrive

Several manufacturers have developed versions of the fixed or steerable saildrive, the pioneer being Finland-based Oceanvolt.

The serial system is a direct replacement for an existing saildrive and can be harnessed to a separate generator or simply run off battery-stored renewables.

The variable-pitch Servoprop has been designed to harness as much hydro-regen as possible, so software helps to twists the optimised blades to the ideal angle when under sail.

Capable of being mounted forward or aft facing, the Servoprop 15 can generate 1kW at 7-8 knots, but the new and larger Servoprop 25 can generate 1kW at 6 knots, and a claimed 5kW at 10 knots.

4. Retractable drive

‘Why hybridise the engine when you can just hybridise the boat?’ So says the French company Blue Nav, which has a system ideally suited for smaller motorboats and yachts.

A pair of retractable 48V pod motors are bolted to the transom and can be lowered and remotely steered for silent, precision docking or gentle electric cruising.

Touch a button, and they retract to reduce drag when the internal combustion engine is being used.

Operation is by a joystick with a real-time electronic display of orientation and energy use.

5. Pod drives

Fully sealed submersible pod drive units can supplement the internal combustion engine by offering electric cruising in serial mode.

The added advantage is that manufacturers are now configuring the low-maintenance brushless motors and swept blades to generate more electricity under sail.

Pod drives are increasingly being mounted facing forwards, where they are more efficient in undisturbed water.

Some are steerable, so can double as docking thrusters.

By locating the pod drives under the hull, a great deal of accommodation space is freed up, with the generator being housed in any convenient locker space on board.

6. Bolt-on parallel

A true parallel hybrid simply requires a physical connection to the existing prop shaft, which can be achieved with relatively minor modifications.

Several motor manufacturers also provide the necessary brackets and belts.

Some also make the clutch mechanisms needed so the electric drive can be remotely detached, if necessary, to reduce resistance.

The electric motor, often an efficient ‘pancake’ design, can either sit above the shaft or in some cases become part of the shaft itself.

Marine hybrid drive systems: the bottom line

Prices for a hybrid unit vary widely but remain quite high.

AR Peachment’s hybrid, for example, starts at £20,000 and the Servoprop 15 starts at €38,500, but DIY enthusiasts have developed hybrids from industrial motors for just a few thousand.

Due to high cost, the decision to go hybrid will centre around the quality of life on board rather than any paybacks in fuel saving or engine maintenance.

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However, retrofitting can be straightforward, obviating the need for a separate generator, with the large battery bank providing ample power for domestic loads, slow but fume-free cruising or even emergency propulsion.

The ability to harvest some power from the free-wheeling prop when sailing is another advantage.

The extra batteries will be a major part of the expense of retrofit but can also add weight where needed.

The bank can be increased in capacity as budgets allow.

Meanwhile, internal combustion engines will continue to banish range anxiety and provide hot water via a calorifier, so a marine hybrid can offer the best of both worlds.

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