Electric&Hybrid – A new era without gensets

Hybrid is the name of the yachting game these days. Respect for the environment, in addition to increased efficiency, amazing comfort and lowering operational costs, hybrid often translates to fewer emissions and can mean better compliance with environmental legislation.

But hybrid is not just propulsion: the presence on board of energy storage systems such as batteries can power propulsion in complete respect for the environment and maximum comfort. In addition batteries can be used to power the electrical systems on the ship (hotel load and technical load) during the night, fully respecting the environment (zero emissions), notably increasing comfort (completely eliminating noise and vibration from generators, which are annoying during the night).


Currently Megayachts have two or more or more gensets that produced the electric power needed for technical systems (bow and stern thrusters, air conditioning, cooling systems, other accessories) and the hotel load.

Gensets consist of a diesel engine and an alternator that transforms the mechanical power generated by the diesel into electric power. The technical solutions used for these generators, which are generally of high quality, have increased considerably in recent years, guaranteeing a product that is more efficient, quieter and more comfortable compared with solutions used a few years ago.

But the fact remains that, while it operates, the genset is a source of pollution, creates vibration and noise and so reduces the comfort and quality of life on board. The disturbance is less noticeable during the day (at least when underway) but is very noticeable during the night when, in perfect silence, even small funds and slight vibration can disturb the relaxation of owners and guests.

Another troublesome element generated by gensets are the clouds of smoke and pools of oil on the water close to the hull during operation. In fact the diesel engine of the genset, although it has an exhaust system that respects all the most advanced regulations, produces smells and oily substances that are very annoying for those who want to have fun swimming or playing in the water when the yacht is it anchor.

The “shaft alternator” solution

Shaft generators on board ships are driven by the main engine to supply power to the mains (hotel load and technical load). The mains have to be supplied with constant voltage and frequency by the shaft generator whilst the speed of the main engine changes, i.e. when the vessel travels at different speeds or if the propeller speed strongly varies in heavy seas.
Shaft generator systems with frequency converters supply a three-phase current of constant voltage and frequency to the mains at variable main engine speeds. The useful speed range of the shaft generator can be defined from the ship’s operational requirements. For example, it is possible to select a shaft generator speed range of 40% to 100% of the main engine speed with a constant system output.

The active power required for the mains is generated by the main engine. The power generated by shaft generators are very high, theoreticaly limited only by the main diesel engine power.

Arrangement for a “Shaft generator”

The shaft generator can be arranged and driven by the main engine in two configurations: PTO or SAE.

The shaft generator on PTO is powered via a power take-off on the reduction gear which is arranged between the  main diesel engine and the propeller. The PTO can be defined “primary” or “secondary”. If generator is always powered when the main diesel engine is running, we have a PTO primary. If the alternator is powered only when the propeller is rotating (only when the main gearbox clutch is “clutched in”) the PTO is a “secondary” PTO. It’s clear that to be able to produce electricity not only during the navigation but always, we need a primary PTO. The big limitation of this solution is that in this case the alternator that always can work as motor too (the electric machines are always reversible), cannot work as propulsion because primary PTO cannot be used as PTI (power take IN). This is a big limitation of the PTO solution for shaft alternator

The second solution is to use the configuration SAE or called “AUXILIA” (form the brand of the producer) where the alternator is rigid mounted on the diesel housing and direct connected with the flywheel. “AUXILIA” shaft alternator is “primary” and produce electricity during navigation and when the ship is stationary, without limitation. The big advantage is not only the full operation, but the simplicity of installation (plug & play), space saving and better weight.

Energy stored in the battery

We have seen that by using a “shaft generator” it is possible to produce electrical energy using the main diesel engine. During navigation and any time the main engine is operating, the energy needed to make the ship operate (hotel loads and technical loads) can be obtained from the AUXILIA generator. In these conditions the gensets can be switched off. Given the size of the main engines and the large amount of power they create it is possible to connect very powerful generators. The electrical energy surplus to immediate requirements can be stored in lithium batteries and then used when the main engines are turned off. This solution makes it possible to use the ship without ever switching on the generators (genset-less yacht) with great advantages in terms of reduced maintenance, excellent comfort on-board (no vibration or noise) and no emissions (Zero Emission Mode). It is thus possible to build a megayacht, with a suitable pack of batteries, that can go a whole night (8-10 hours) using only batteries and no generators. In addition to other advantages, thanks to the remarkable power produced by the main engines, a super-fast battery charging system can be developed that can top up the batteries in a very short time (2 C or more).

Cost of energy produced via shaft generators and stored in the battery

The energy produced by the shaft generator costs much less than that produced by the classic diesel generator. In fact, if we look at a consumption curve for the diesel engine we can see that, for the same rpm, there can be a difference in the specific consumption (grams of diesel for each kilowatt of power produced) on the cube of the propeller and the maximum on the brake of as much as 20% (specific consumption on the “brake” curve is less).

This implies that if the production of electrical energy is added to a diesel propulsion engine used only to power the propeller, at the same rpm with a shaft generator that produces, for example, 20% of additional power), if the specific consumption between the total and that that I would have had on the cube of the propeller is lower by 20%, the electric current produced is at zero cost!


The introduction of the “shaft generator“ with batteries opens up a new era in which we can imagine genset-less megayachts or at most ones with a small emergency genset that can be used as a range extender if the batteries need to be charged. In this way it is possible to eliminate vibrations and noise, considerably increasing comfort on board. Thanks to this solution there are no more emissions and great benefits both to health and the environment. A new era has begun!



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