Skip to main content

The world of cruising yachts is dominated by diesel power, but before diesel came on the scene there were boats powered by electric motors. These were boats designed for rivers and lakes, but the concept of electric power for seagoing boats was a non-starter because the theory was that electricity and water do not mix. And when they do mix, water always wins. All that has changed. Modern technology and the new greener world in which we live have re-energized the future of electric propulsion or a combination of electric and diesel.

The Greenline 36 uses a combination of diesel and electric power, offering the options of operating on one or the other of these fuels. This is just one of several ways to incorporate electric power into the boat’s propulsion and it is perhaps this variety of possible ways to use electric power that confuses boat owners about the advantages and disadvantages of hybrid power.

One method of hybrid power.

One method of hybrid power.

Things have changed dramatically since those early days of electric propulsion—120 years ago. Lithium batteries have transformed the way that electricity can be stored, electric motors have become compact and powerful, control systems have improved, and above all, electricity has overcome its fear of water by using modern materials and technology. So let’s have a look at the various systems and how they can be used.

The Tender Electro by utilizes Torqeedo's Deep Blue hybrid system.

The Tender Electro by utilizes Torqeedo's Deep Blue hybrid system.


Electric propulsion and hybrid systems are presented as the way forward for green yachting and the technology has developed rapidly in the past few years. Today there are sport boats with electric outboards that are capable of speeds exceeding 45 knots, as well as commercial tourist boats capable of operating all day on one battery charge. In a world where green is good, this newfound capability of operating boats with electric power could mark a significant change for the cruising sector. But the savvy buyer still must ask: What exactly are the costs and benefit of hybrid technology?

A fully electric propulsion system is actually quite small but it is limited by its capacity.

A fully electric propulsion system is actually quite small but it is limited by its capacity.

A fully electric system that combines electric propulsion with stored electricity from batteries that are topped up with solar power is an option if you just cruise for short day-trips. For serious cruisers, though, the limited range and lack of backup tend to make this option a non-starter. This leaves us with the various hybrid systems that combine electric and diesel power and here are several options.

Here are the three main systems that can incorporate electric propulsion: 1.) An electric motor connects to the main diesel propulsion to offer electric auxiliary propulsion with batteries as an integral part of the system. 2.) Electric motors are the main propulsion with power supplied by batteries and a generator. 3.) A simple battery and electric motor system provides all of the power and does not rely on any diesel backup.


The modern electric propulsion revolution started with hybrid systems where a combined generator/electric motor was introduced into the transmission system of a boat so that it could generate electric power when the diesel engine was running. This power could be used to charge a bank of batteries. The generator is also capable of acting as an electric motor for propulsion, taking its power from the batteries when the diesel engines are stopped. This hybrid system requires a considerable bank of batteries to store the electric power between the point of power generation and the power being used.

There are various ways in which the motor/generator can be incorporated into the system. Fitting it between the diesel engine and the gearbox works well when smaller power requirements are involved. This is a compact system with a clutch that allows the motor/generator to be switched in on demand and the diesel isolated when electric-only propulsion is required.

Torqeedo hybrid system overview

An overview of the Torqeedo Deep Blue Hybrid System

Such systems are promoted as being green because they allow silent electric propulsion at low speeds, and no running diesels translates to no emissions. In reality, however, such a system would burn more fuel than a straightforward diesel-only system because every time you generate power, store it in batteries, and then use it, you are losing between 5- and 10% of the power in the conversion between generation, storage, and use. This means that such a hybrid system would be less efficient than using diesel-only power, and the fuel consumption would rise if you do not take electric power from other sources such as shore power and solar panels. The big advantage of such a hybrid system is that when running under electric power alone, you have peace and quiet ,and no harmful exhaust emissions which could be a benefit for operating in environmentally sensitive areas. They can also enhance low-speed maneuverability.


When we look at hybrid cars, the attraction is mainly found in improved fuel consumption. Hybrid cars have the advantage of being able to recover energy through braking and going downhill that would other wise be lost. Obviously boats offer no such benefit..

Boatbuilders now offer hybrid propulsion that incorporates a bank of batteries for energy storage. As far as can be judged, they have been slow to gain a significant portion of the market among cruising yachts. Probably one of the reasons for this is that hybrid propulsion adds about 10% to the overall cost of the boat—a significant increase when you consider the limited benefits the system offers. The main cost increase comes from expensive lithium-based batteries required to get any worthwhile cruising time under electric power. Then there is the possible additional cost of having to replace the batteries a few years down the line, so hybrid does not come cheap. There is one area where you can gain from having a hybrid system and that comes when you are in harbor and you can charge up the large battery bank from the marina shore supply. This is generally cheap electricity and it certainly reduces emissions. On the other hand, generating stations ashore powerplants negate this benefit.

You can also add solar panels to the system if you have space onboard to install them. These will give you free energy once you have paid for the installation, but the contribution is likely to be small and you may not recoup the cost of the installation for a few years. The main advantage of a hybrid system will be the ability to reduce emissions in sensitive areas and the ability to motor along in peace and quiet.

However, don’t dismiss hybrid systems completely in terms of cost. The latest hybrid systems are changing their approach and instead of offering the option of diesel or electric propulsion, these new systems have full electric propulsion with the diesel engine acting only as a generator. This can increase the efficiency of the system considerably, because the diesel generator is running at a constant speed where it is most efficient and there are much lower losses because it can supply the motors directly while any surplus electric power outside the propulsion requirements is stored in the batteries. Such a system can offer several options, such as just running for limited periods on electric, running on a combination of electric and diesel power with the engine supplying power direct to the motors, running with the diesel supplying power to recharge the batteries and supplying the motors. With modern computer control for the whole system the operation can be completely automated so all the operator has to do is open and close the throttles and chose the option he requires.

French builder Rhea developed a small trawler type boat with this system installed. Power was generated from a single diesel generator and stored in a considerable bank of batteries. Twin 200-horsepower electric motors provided the drive. Although the system worked well, its higher cost meant it was not a commercial success.

Rhea trawler design.

Rhea trawler design.

A simpler version of such a system would be to remove the batteries and just have a diesel electric system. Savings can come by not having to install a gearbox because electric motors are reversible. The maneuvering of the boat can also improve because unlike a diesel where you have a minimum rpm of perhaps around 800, an electric motor can operate right down to nearly zero rpm, making harbor maneuvering a lot more subtle. Another advantage is that the diesel generators can be installed anywhere in the boat so this could free up the valuable space in the center of the hull, which is often taken up by the engine compartment. Diesel-electric propulsion can make a lot of sense, particularly if several small generators are used so you have redundancy and the ability to match the load to the speed required.

For diesel-electric systems there are now compact electric motors of considerable power which do not take up a lot of space inside the hull, although these do require a supply of cooling water. The more powerful motors that might be required to propel a 50-foot cruising boat are still quite compact and they tend to operate at quite high voltages, often around 400-500 volts AC. Having such a high voltage on board might put you off but this is much more efficient, because it suffers lower heat losses and requires smaller wiring than when using lower voltages. Control is by frequency modulation, and this is well proven technology.

If you are not keen on the idea of having these powerful electric systems on board at least consider them for your tender. Electric outboards are now widely available and portable batteries can hold enough power to get you to and from the shore with power to spare. This can save you carrying gasoline on board, which your insurance company is likely to welcome.

A tender powered by a Torqeedo electric outboard motor

A tender powered by a Torqeedo electric outboard motor

Electric propulsion is definitely here to stay in one form or another and the technology is now mature enough to allow you to have confidence in the system. Many of the major engine or gearbox manufacturers have introduced their own version of a hybrid system, either by having motor-generators installed between the engine and gearbox or by having the unit incorporated into the gearbox with a clutch to link into the unit. There are also electric pod drives available where the electric motor is contained within the hub of the lower unit, removing the need for any prop shaft or gears and by being immersed in the water, no cooling is required. These are small versions of the mighty units that many cruise ships use, but at present, pleasure boat versions tend to be limited to around 20 horsepower. If and when more powerful versions hit the market, this could be the ideal propulsion for a diesel-electric system for the cruising boat.