Thinking about changing your ageing diesel engine for a new all-electric propulsion system? Duncan Kent offers advice on the pros and cons

There are hundreds, if not thousands of yachts still sailing with 40-year-old diesel engines emitting smoke and pollutants, so is now the time to replace them with a clean, low maintenance, pure electric propulsion instead?

The more I’ve looked at sailing yachts that have been converted from diesel to electric auxiliary drive, the more I wonder why you wouldn’t do it, especially now that the cost of hi-tech boat batteries and solar charging has dropped dramatically.

The immediate advantage of changing to marine pure electric drive (PED) systems is that they’re clean, quiet, smell-free and require minimal maintenance, all of which is very appealing if you’ve ever owned a boat with an ancient, clunky diesel. PEDs offer other benefits too. Their reaction to the throttle is instant, which is a boon when berthing in a cramped marina, and once you’re sailing at a decent lick you can generate free ‘fuel’ from the prop.

The disadvantages are the initial cost of the system (largely due to the batteries) and range issues. On a typical 10m cruising yacht it’s just not possible to store enough energy to motor at full tilt for 10 hours a day, so if you’re planning to make long offshore passages regularly then you’ll either need a lot of patience or enough solar and wind generation to power a small village.

But for coastal cruising, often from marina berth to marina berth (what 85% of today’s boat owners do anyway), a PED system is far more viable, especially once your old iron topsail has reached the point where it’s just so much ballast.

If there’s even a little bit of you that would rather not get your hands dirty tinkering with a temperamental diesel, then electric may just be for you. Photo: Fernhurst Books

The devil you know

A diesel engine can provide heaps of grunt when you mistime the tides and want to escape an oncoming storm, provided, of course, that choppy seas haven’t stirred up the inevitable gloop at the bottom of your diesel tank and blocked up your fuel filters.

It’s also fairly simple tech, so, if you’re mechanically minded and have all the tools and spares on board, you can usually keep it turning over. Fuel can also be carried in jerries too, so if it takes a bit longer to thrash your way back into port you can top it up along the way.

You can also leave the boat for months, even years, on end without much harm coming to it and without needing an umbilical cord permanently attached to the shore to keep the expensive power pack in optimum condition.

A spare charged battery, a pair of jump leads, some fresh fuel and bingo, you’re away. But if there’s just a little bit of you that would love to never have to lift the engine box lid and prefer to start your annual cruise with clean fingernails, then sliding soundlessly out of your berth in the early hours must sound appealing.

Range currently remains an issue with battery capacity the limiting factor, but things are improving all the time

Range anxiety

Just as with electric cars, whenever anyone whose vessel is equipped with a conventional diesel falls into a conversation about electric propulsion for boats, the first question is almost always, ‘How far will it go on a charge?’

Well, the answer I’m afraid is simply, ‘It depends.’ How far do you want it to go? Are you willing to toddle along at four knots, or do you want to charge around from port to port like a Greek charter yacht?

I get that it’s not for everyone and, being a mature seafarer myself, I dread to think about losing all power whilst halfway through the Portland Race in the dark. But as with pretty much anything to do with navigating a vessel, you just need to plan things out carefully in advance.

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There are those that say having a finite source of auxiliary propulsion teaches you to sail better, which in many ways is true. The introduction of the powerful modern marine engine has made us all lazy with our tidal planning, knowing, as we do, that you can always resort to the donk to get you home if the log drops below 4 knots or that extra pint in the pub has made you late for the tidal gate.

So, let’s look at some facts. The typical electric propulsion system for a 10m, 6-tonne cruising yacht will usually be designed to provide around five to six hours of gentle cruising in fairly neutral currents at around 4 knots, or maybe 10 hours of motor-sailing, before requiring a charge.

Unlike a car, however, a boat can have solar panels on its deck, coachroof and arch, that can extend this range by another couple of hours, maybe, in the summer.

And then there’s the possibility of regeneration.

Oceanvolt electric ServoProp propeller has electrically controlled pitch which auto adjusts for your speed, or for regeneration


Nearly all electric motors used in marine propulsion have the capability to turn into a hydro-generator when your speed under sail goes above 4.5 knots or so and the prop/motor is allowed to rotate in reverse.

Regeneration doesn’t make a massive contribution, at best around 750W at 7 knots boat speed per prop, but at 48V (typical electric drive voltage) that’s another useful 15A or so going back into the battery bank.

There are some systems – Oceanvolt’s ServoProp for instance – that incorporate a variable pitch prop. These are capable of a more decent output at lower speeds but are currently seriously expensive.

Either way, I believe solar and hydro-regen should be treated as a bonus when planning your system, not as a given.

If you are unlucky enough to have a few days of no sun or wind you will be back to relying solely on the energy reserves held in your battery bank.

For those who really can’t get over the nagging worry of running out of drive power on a passage, there’s always the option of installing a hybrid system, in which a standby diesel engine is always on hand to get you out of an emergency.

Oceanvolt’s award-winning HighPower ServoProp 25 electric saildrive

Variable pitch props

A normal fixed propeller is designed for propelling the boat and not for generating power as with a hydro-generator. However, Oceanvolt’s DAME award-winning ServoProp saildrive, suitable for monohulls and multihulls from 35-90ft LOA, features a variable pitch prop that combines a high efficiency saildrive with a powerful hydro-generator.

With its unique ability to rotate its propeller blades through more than 180°, ServoProp’s control software adjusts the pitch of the propeller blades to automatically optimise all hydro-generation and propulsion needs.

Oceanvolt claims that a normal fixed propeller generates less than half the power of ServoProp, which it states can produce more than 1kW at 6-8 knots, and 5kW at 10 knots.

It also estimates a 30% increase in forward propulsion, +100% in reverse, and +300% in hydro-generation mode.

Renowned marine propeller supplier, Bruntons, has also launched its Autoprop Eco*Star, designed specifically to propel the rapidly growing number of hybrid and pure electric craft, both motor and sail.

Autoprops are highly efficient because of their ability to auto-pitch depending on engine speed and sea conditions, in order to provide optimum thrust at all times. But with electric propulsion the advantages provided by the new Eco*Star propellers are further extended.

Electric motors provide a constant torque from zero to maximum rpm. Using its auto-pitching ability, the Eco*Star can match its own efficiency curve with that of the electric motor, resulting in more boat speed with less energy.

Eco*Star can spin when under sail allowing the motor to become a generator for recharging the batteries.

Battery setup in an Oceanvolt electric propulsion unit

Drive types

Shaft drive – In most cases it’s possible to retain the shaft and stern gland from an existing shaft drive ICE system, although you’ll probably need a new shaft bearing and coupler unless it is a direct-drive system. The propeller will also probably need to be changed if you want regeneration when sailing.

Saildrive – In many ways electric saildrive units are easier than shaft drives because, as with pods, they come as a complete package with the correct propeller for optimum regeneration. They’re also more efficient at regeneration as the propshaft can be aligned horizontally, unlike shafts, which are usually angled slightly downwards. Some owners choose to convert from a shaft to a saildrive for this reason.

Pod drive – Electric drive pods are similar to saildrives, except they have a direct-drive motor at the bottom of the leg, which allows them to benefit from automatic cooling. Electric drive pods also only require a few small holes in the hull, usually two to four for mounting the unit and one for the routing of the power cable. Pod props and motors are also designed with optimum regeneration in mind.


Most PED systems on small to medium-sized yachts utilise a 48V battery bank. Although there are some that still use deep-cycle AGM battery technology, the only realistic battery chemistry currently available for marine PED systems is lithium-ion.

The safest of the li-ion group is lithium iron phosphate (LiFePO4). While not the most energy dense of the lithium-ion group, they have been proven to be the safest type of li-ion cells to install into a boat, where a battery fire would be catastrophic.

Originally, many professionally built and installed marine electric drive systems utilised other, more volatile types of li-ion batteries such as nickel manganese cobalt (NMC) or lithium manganese oxide (LMO).

System displays can give real time readouts of battery capacity, power usage and remaining range

These types of li-ion are commonly used in the automotive industry for their high capacity, but are wholly reliant on their highly sophisticated battery management system (BMS) to keep them operating within safe parameters.

Apart from preventing a fire risk, the problem with relying on a BMS to monitor the batteries is what happens if something triggers a safety shutdown, causing your boat to lose power completely. A battery shutdown isn’t as drastic as having a lithium battery fire on board, but the total loss of propulsion isn’t ideal, even on a sailing boat.

It’s worse, of course, if you use the same power bank for your house power. Then the loss of your navigation and communications facilities at the same time can be extremely dangerous. For this reason, I would always recommend keeping motor and house banks entirely separate from each other.

If you’re happy to have the more volatile batteries on board or are forced to in order to achieve the desired range, then I highly recommend you have your system professionally designed, installed and commissioned.

Yes, it’ll be expensive, but you’ll sleep at night!

The battery monitoring system will control charge from all sources

Charging a motor bank

Having a much lower internal resistance, li-ion batteries will charge more rapidly than lead-acid batteries because they can accept a much higher rate of charge.

A good quality battery can usually accept a charge up to its own capacity, meaning a 100Ah battery can be charged at 100A. This is called a capacity acceptance rate (CAR) of 1C, and is important when it comes to charging overnight, with the intention of continuing your cruise early the next day.

It also enables various different methods of charging to be accepted simultaneously, provided the combined charge doesn’t exceed 1C, so you can be using solar to its max while also charging from shore power. Another advantage of using li-ion over lead-acid batteries is the BMS can be networked to all charging sources to provide control over the charging regime, plus the data can provide power monitoring at a glance.

Costs of converting

The cost of installing a complete pure electric drive system on a yacht depends on how far and how fast you want to motor between battery charges. The greater the range or faster the speed required, the more battery capacity you’ll need.

A typical 40hp diesel replacement will cost close to £20k including all the parts and installation. A DIY-installed PED replacement would cost £15-£25k, depending on the number of batteries specified, whereas a professionally designed and installed pure electric drive system is more likely to be in the region of £30-£45k.

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