‘Electric yacht propulsion opens doors to a completely different ways of thinking about sailing’

Sailing with electric propulsion is very different to doing so with a diesel engine and may cause you to reconsider your diesel usage. Rupert Holmes reports

Electric propulsion is rapidly becoming established in the mainstream yacht market. This is increasingly driven by customer demand, often by people who own electric or plug-in hybrid cars and are more than happy with the balance between their advantages and shortcomings.

In my role testing new yachts I’ve sailed an increasing number of boats with electric and hybrid propulsion over the past six years, ranging from lightweight daysailers through smaller cruising yachts to heavy aluminium bluewater cruisers and large cruising catamarans.

I quickly discovered that electric yacht propulsion opens doors to completely different ways of thinking to the diesel engines we’re accustomed to using, especially when motor sailing.

Throttling back

Many diesel powered yachts carry enough fuel for the rate of consumption to never be a worry. However, the further you voyage the greater the chances you’ll opt to throttle back to extend range. It’s incredibly easy to underestimate the dramatic effect this has on energy use, whatever type of fuel is used.

The wave making resistance of a hull increases exponentially at speeds above 1x the square root of the effective waterline length in feet. It therefore takes three times more energy to push a boat with a 36ft (10.9m) waterline at 8 knots than at 6 knots.

Additional efficiencies can be gained by throttling back further. As with air resistance when driving on a motorway, frictional resistance of the water on the hull varies with the square of boat speed. Reducing the speed of a 36-footer from 6 knots to 5 reduces total energy consumed by roughly a further 30%. At 5 knots it therefore requires less than a quarter of the energy needed to push it at hull speed of 8 knots.

This helps explain why many boats with electric propulsion appear to have relatively small battery banks. The same is also true of a Tesla – a Model S with 500kW of propulsion power may have an 80kW battery, yet the car doesn’t run out of power after less than 15 minutes – its range is over 300 miles.

As with a boat, most of the time only a small proportion of the potential power is being used. With a Tesla it’s on tap to provide supercar levels of acceleration for less than five seconds, while on a boat it’s there for manoeuvring and for punching head seas for short periods, such as leaving harbour or rounding choppy headlands in strong wind over tide conditions.

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Sail power

Modern reaching and downwind sails have revolutionised the sail area it’s feasible for a lightly crewed cruising yacht to carry in light airs. When using a dedicated reaching sail, such as a Code 0, many yachts can now sail at close to the true wind speed in breezes of 4-8 knots. These sails are just as relevant to older cruising yachts and heavy displacement new designs as to very light, high performance craft.

This ability to maintain speed under sail reduces the amount of time spent under power while on passage and therefore effectively increases range, whether you’re using diesel or electric propulsion. In the case of the latter, most electric propulsion systems also have a regeneration function, capable of putting significant amounts of power back into the battery bank while under sail.

The power these produce also increases exponentially as boat speed rises. In lighter airs this can make gybing wide angles to increase apparent wind when sailing downwind an important strategy. If doing so increases boat speed from 5 to 7 knots, for instance, the rate of charge doubles, which helps explain why some all-electric yachts can work as long-distance cruisers.

Depending on the boat, in less than 10 knots of true wind optimal downwind VMG is likely to be found at true wind angles of around 135°-140°. This means you’re sailing 30% further, but expecting the increase in speed to be larger than that figure.

Motor sailing with electric power is very different to doing so with a diesel engine. Unlike a diesel, electric motors can be used to give just a small extra push that helps increase apparent wind in a virtuous circle. When I tested a hybrid Ovni 430, for instance, reaching in less than 4 knots of true wind with mainsail and Code 0 just 3.3kW of push from the electric motor – less than 14% of its rated output – was enough to double boat speed from 2.2 to 4.5 knots. By contrast, 11kW is needed for the boat to maintain a 6.8 knot cruising speed under power alone.

Nevertheless, making good progress towards a downwind destination can be notoriously difficult so a big asymmetric spinnaker designed for downwind VMG angles is a huge advantage. Unlike a race sail this doesn’t need to be made of super light material – regular 0.75oz, or even a more robust 0.9oz cloth, works well.

Even then it’s not impossible to imagine a scenario in which an all electric long-distance cruising yacht without a back-up generator might reach back and forth with the Code 0 in very light airs just to generate enough electricity to keep the freezer cold.

But that may not be as much of an absurd idea as it initially sounds. When racing offshore we often use a similar strategy in a near calm, having first identified where the next new (stronger) breeze will be found. Instead of aiming solely to reduce the distance sailed, this is usually a faster way to get downwind in very light airs than pointing directly at the next waypoint and is a strategy cruisers can also employ to good effect.

Into the wind

The other situation in which motor sailing is common is to improve progress when close-hauled. In light airs a small push using electric propulsion can again be used to boost speed without a big power drain. However, the situation is very different when punching a big head sea in strong winds, when a considerable amount of power may be needed.

This is arguably a weakness of electric propulsion, other than in yachts with large range-extending generators. It’s a much smaller problem for designs that sail efficiently to windward and have quality sails that maintain their shape even in gusts than for those that tack through big angles and have badly stretched Dacron sails.

The extent to which getting in and out of harbour and manoeuvring in port typically depletes the battery of course varies in different places. There’s a big difference between a marina berth a few hundred metres from open water and those that are a long slog up miles of tidal estuary.

Last autumn I checked out two yachts weighing 8.8 and 15 tonnes respectively in this respect: the Bestevaer 36 in La Rochelle and an Xc 47 at Port Ginesta near Barcelona. With the former we had no need to use more than 2kW of the 11kW of available power at any time while entering port and manoeuvring, while average consumption was closer to 500W. This reflected time manoeuvring at slow speed in a confined space, handing sails and preparing fenders and lines.

Manoeuvring the Xc 47 in harbour, holding station against a 15-knot headwind while we sorted lines and fenders, used only 300-500W of propulsion power, despite the boat’s high topsides and we could have done that for 100 hours before depleting the battery bank.

Is electric propulsion always greener?

There’s no one-size-fits-all strategy for the most sustainable solution for a yacht’s auxiliary propulsion. The extra carbon emissions associated with manufacturing an electric car is often offset within 15,000 miles thanks to their impressive efficiency compared to internal combustion engine vehicles.

However a recent well publicised ICOMIA report found privately owned sailing yachts clock up an average of only 24 engine hours annually, so the carbon footprint associated with producing the battery pack is never recovered.

The recommendation to minimise life cycle carbon emissions is therefore to use a second generation biofuel such as HVO, which is typically produced from biowaste material. These are very different to first generation biofuels, which were very hygroscopic and provided a fertile breeding ground for microorganisms that were prone to blocking fuel filters.

The equation is massively different for charter yachts and owners who spend considerable periods of time on board. An independent study commissioned by Fountaine-Pajot showed that 80% of the yard’s carbon emissions stem not from manufacturing processes, but from the use of its yachts during only the first 20 years of their life.

In this situation electric or hybrid propulsion is likely to make a very sustainable proposition.

Nevertheless, just as environmental considerations aren’t always a factor for owners of enormously powerful electric cars with 0-60mph acceleration times well under four seconds, a growing number of yacht buyers are opting for electric propulsion for reasons other than its apparent green credentials.

Benefits include quiet, odour-free running, space savings, reduced maintenance, the ability to recharge batteries – effectively refuelling while sailing using only the wind – and a huge amount of torque at low speeds that can make difficult manoeuvres in tight spaces a lot less daunting.

Key rules for sailing an electric boat

1. Don’t try to motor or motor sail too fast – trying to get close to hull speed takes an enormous amount of energy.
2. Good reaching and downwind sails help minimise the time spent under power, while maximising opportunities for regeneration while under way.
3. Don’t try to sail dead downwind in light airs – when true wind is under 10 knots a true wind angle around 135°-140° invariably gives better results.
4. Use the on-screen data to optimise boat speed and power consumption. Often only a very small push from the motor will result in a useful speed gain when motor sailing.

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