Hybrid propulsion: How does it work?

17 August 2021· Sam Fortescue

Imagine manoeuvring into port where the only sound is the clink of ice in your glass and the gentle strains of your favourite chill-out tracks. Or going for a swim off the beach club at anchor without detecting the throb and cough of the diesel generators. Now imagine getting your yacht delivered from St Tropez to Porto Cervo with a fraction of the expenditure on fuel and a fraction of the emissions.

This is what hybrid propulsion is all about.

How do hybrid propulsion systems work on yachts?

There are many types of hybrid power, but the most developed for large yachts is the combination of diesel and electric. And the clear market leader here, with a unique turnkey solution, is Italy’s e-Motion, a company based in La Spezia. Its systems appear on new sail and motor yachts up to 220ft, as well as in refit projects around the world.

“The comfort that is given by electric power is unbeatable,” says e-Motion founder Michele Maggi. “Once you get used to it, it’s hard to go back.”

Integrating seamlessly with Simrad navigation units, the e-Motion hybrid system allows the Captain to switch between purely electrical propulsion or a traditional combustion engine at the touch of a button. The brains of the system – software that required hundreds of hours of sea trials to perfect – translates commands into action behind the slick control panel.

The DC bus, which Maggi calls a “lake of energy” distributes the power where it needs to go. And e-Motion’s range of powerful but low-profile electric motors fit into the drivetrain between the gearbox and the propshaft, requiring virtually no extra space to install (or, indeed, maintenance).

Silent at anchor

With its extra battery capacity, the e-Motion system can fulfil the yacht’s hotel load overnight without having to start a generator. A bank of super-efficient lithium-ion batteries is installed below the waterline, big enough to supply power for 12 hours of silent operation at anchor.

Electric propulsion

With more battery capacity, it becomes possible to navigate in pure electric mode too – leaving harbour, say, or nosing into a pristine bay for lunch. E-Motion’s zero emissions cruise mode generally allows for up to four hours running on silent electric propulsion.

Save fuel, reduce emissions

Between these two extremes lie a whole realm of possibilities. With e-Motion’s variable speed generators and proprietary power management software, you can run the motors off the generator output far more efficiently than off a diesel engine. Any surplus power is pumped into the battery bank.

On longer voyages it may be more efficient to use Economy Navigation Mode, where just one of the main engines does the job of both. As the engine turns the prop shaft, it spins the electric motor which acts as a huge alternator, generating power to turn the other motor, recharging the batteries and running the hotel load. “This gives you around 30 per cent fuel savings and drastically reduced engine running hours,” says Maggi.

Extra power on demand

Equally, if you need more power for a rapid manoeuvre or a fast passage, the electric motors can provide a boost to the combustion engines. This mode can give you 25 per cent better acceleration and up to 3.5 knots more boat speed.

Ultra-quick charging

And the joy of a high-tech lithium-ion battery bank from e-Motion is that you can fast charge it in as little as 30 minutes. Spin up either the boat’s generators at anchor or the main engines underway to reach some 90 per cent charge in half an hour or less.