Aluminum boat

Hybrid diesel electric marine propulsion

Available alternatives pros and cons for an exploration sailboat

Who hasn't dreamed of maneuvering silently without polluting in and out a magnificent anchorage ?

The different alternatives available today are reviewed below, with pros and cons, particularly for a sailboat programmed to navigate off the beaten routes.

Two types of hybrid diesel-electric technologies are available: a serial hybrid system installation where a generator charges the batteries for one or two electric motors providing propulsion alone with independent shafts, or a so-called parallel installation where one or two generators are coupled to the main thermal propulsion diesel engine, all driving the same shaft and propeller.

First of all, the advantages of electric propulsion are not only to be able to preserve the pristine environment of a secluded anchorage, thanks to silent fume free motoring, a real joy; but also instant torque and immediate forward/reverse transition for maneuvering.
The disadvantages are a very limited autonomy despite a large bank of lithium batteries, still low power compared to a diesel engine and a compulsory use of electronics, in particular for the management of relatively fragile lithium batteries (see BMS: Battery Management System). For safety reasons on a sailboat, only LFP (LiFePO4) batteries should be used, as there is a risk of fire with other types of Lithium batteries; and preferably a DC voltage of 48V shouldn't be exceeded.

"Parallel" hybrid technology

The company Hybrid Marine Ltd. has developed a comprehensive solution based on the Yanmar 4JH110 engine with an hybrid transmission consisting of a PRM-500 gearbox with no down angle suitable for the Enduro 54 lowered engine configuration with shaft, 2 generators of 10 KW each ("V-Twin" option) that can be used as electric motors for short periods of approximately one to two hours, depending on the speed of the ship and the capacity of the battery bank, perfectly suited to maneuvers in port or anchorages.

Great flexibility of use and versatility particularly suited to blue water cruising and exploration:

  • Possible use of the diesel engine alone with its maximum power and/or its optimum performance/consumption for navigation in rough seas or in ice or even long periods of crossing with anticyclonic conditions. No loss of power due to the constant drive of 1 or 2 alternators. Max power. available when needed.
  • Automatic coupling of the 2 generators with a belt when batteries charge is low and the other charging alternatives (solar panels, wind or hydro generators) are insufficient. Powerful and very fast charge (10 to 15 kW) particularly with Lithium batteries. Thanks to the addition of a clutch between the gearbox and the propeller shaft, it is possible to occasionally use, at anchor or in a port without electrical outlets, the diesel engine and the 2 generators as a genset without driving the propeller shaft.
  • Use of the electric motors alone for mooring or port maneuvers. Autonomy of approximately one hour, depending upon power used and battery bank capacity. With a bank of 4 MG LFP 230 (or 280 Ah) batteries, bringing the usable capacity for electric propulsion to 460/560 Ah at 48V (approximately 15 to 20 kW available at full charge), for a weight/budget compromise and an acceptable autonomy. For greater autonomy with electric propulsion, it will be necessary to reduce the speed of the boat (for example 3 hours at 4N instead of one hour at 6N) or even increase the battery bank (enough space in the bilge under the pilothouse). To cruise longer on electricity, a powerful generator should be unconvincing solution when a diesel engine is already on board.
  • In the event of a diesel engine failure, even minor but unforeseen and occurring at the wrong time i.e. when entering an harbour, for example following air in the fuel circuit or clogged filters or a broken water pump impeller, electric propulsion is available immediately, avoiding an emergency repair or to take the outboard engine of the dinghy and position it on the transom.
  • Under sail, the propeller can operate the 2 generators to charge the batteries without driving the diesel engine's gearbox, which often poses a cooling problem when the engine is not running. This obviously induces additional drag with a rather mediocre efficiency since the propellers are not designed for such use; but can make it possible to store an appreciable quantity of energy with little counterpart (up to 400 W), eg. during long crossings in sustained winds when the sailboat is already reaching its hull speed, often a maximum limit for a highly loaded sailboat on long passages.

In summary, the main advantages of this option are :

  • Clean and silent electric propulsion for port or anchorage maneuvers
  • Extremely fast charge of the lithium batteries (less than an hour for a full charge)
  • Instant backup solution in case the diesel engine fails at a critical time e.g. port entry
Hybrid Marine with Yanmar 4JH110Yanmar 4JH110 engine integration with Hybrid Marine's V-Twin hybrid system


Nanni Diesel also offers a possible hybrid solution with the N4.115 engine with a generator inserted directly between the engine gearbox and the shaft, so without using a belt, but less versatile than the Hybrid Marine solution. New developments are reported to be ongoing at Nanni.

Serial hybrid technology: electric motors plus genset

The configuration which could be used on the Enduro 54 with electrical engines on the port and starboard sides of the boat includes 2 skegs protecting the shafts and propellers. Also has the advantage of protecting the rudders too, against OFNIS at sea or when beaching on unstable grounds which could cause the sailboat to collapse on its stern.

diesel-electric hybrid with 2 motors2 electric motors, shafts and propellers, 2 skegs aligned with the rudders


This solution consists of 2 generators/electric motors, for example from companies such as Bell Marine or Oceanvolt and in constant progress (see also Torqeedo Deep Blue) seems interesting for port maneuvers and cruising along the classic routes. The electric motor not only yields a silent, non-polluting operation, but above all instant torque and an appreciable immediate forward/reverse transition for docking along a quay or a pontoon with difficult access. In addition, the 2 engines on each side of the boat with their propellers in front of the rudders, in principle reduce the need to install a powerful bow-thruster, usually the case with any twin-rudder configuration. As on a catamaran, they can be used with opposite forward/reverse thrust to help the ship pivot.

On the other hand, this solution is not yet fully suitable to blue water cruising and exploration. An acceptable autonomy requires a huge battery bank imposing a quite powerful genset, hence with significant consumption (e.g. Onan-Cummins or Northern Lights for their reliability) for an overall diesel-electric low efficiency (long chain: from diesel to AC current then DC then charging the batteries then operating electric motors) and a not really attractive overall weight. Prolonged use of electric motors requires effective fluid cooling and not just air cooling, and in the event of high power requirements (rough seas or ice) the first parallel hybrid solution is more appropriate. In the event of a general electrical failure (e.g. management by electronics not always completely reliable), there is also no immediate alternative solution for the motorized propulsion of the boat.

This solution with 2 parallel electric motors in series with a generator will undoubtedly continue to progress along with the major developments underway in the automotive industry, and in particular for the batteries. So the need to reassess in the medium term, because today it is not yet completely satisfactory i.e. a range of action of maximum 10NM in electric mode in real sea conditions for sailboats of equivalent size, and a powerful generator (min. 15 kVA) which can even exceed the consumption of the inboard diesel engine in order to maintain an equivalent speed i.e. power available to the propellers in addition to the charge of the large battery bank.


While the solution offered by Hybrid Marine seems convincing thanks to its versatility and multiple options for operations, with an easy switch between propulsion modes, the use of electronics, in particular for the management of the lithium batteries, is certainly a weak point for a sailboat with an actual exploration program.
Some will say "it's urgent to wait" in order to benefit from the ongoing developments in the automotive industry.
One can also wonder, given the budget increase with the hybrid transmission system and the lithium batteries, whether it would not be wiser to keep the Nanni N4.115 engine, more robust, with a larger displacement and revving slower versus the Yanmar 4JH110 Common Rail, and opting for the carbon rig with the performances sailplan instead, making the sailboat faster in light airs and hence reducing the need to start the engine with low winds.