Sam Fortescue examines how renewable energy afloat is benefitting from technical developments in other sectors

As the rest of the world grapples with decarbonisation, the sailing community is benefitting from the various technical developments made in other sectors, and now has more options to use renewable energy afloat.

It is now simpler to harvest and store power on board than ever before – no bad thing when you consider how many power-hungry gadgets fill a modern cruising yacht.

From Nespresso machines to electric winches, sailing consumers are reaping the rewards of the efficient electricity generation.

The core of renewable energy generation for boats remains wind, solar and hydrogeneration, but the last two of these are developing rapidly.

Meanwhile hydrogen is continuing to make inroads into the sailing market.


Jimmy Cornell’s Outremer 4.Zero has covered 1,500 miles from Tenerife to La Grande Motte using renewable energy, but the multihull’s hydro and solar capacity needs to increase before he can take it around the world. Credit: Gilles Foucras

It all comes down to how much power you need: a kilowatt-hour over the day to run the fridge and electronics (83Ah), or 50 times that for induction cooking, air-con and even electric propulsion.

Wind remains an important part of the mix, capable of adding up to 500Wh on a blustery day, but here the technology is more mature.

There may be small incremental improvements – quieter blades or more efficient power transfer.

‘There is not going to be a silver bullet in respect of renewable generation on yachts because the physics tell us that the existing technology is already very efficient,’ says Peter Anderson, MD of Eclectic Energy.

The D400 converts an industry-leading 36% of the kinetic energy in a 12-knot wind stream into electricity

The D400 converts an industry-leading 36% of the kinetic energy in a 12-knot wind stream into electricity

Sam Fortescue, freelance marine journalist and former magazine editor

Sam Fortescue is a freelance marine journalist and former magazine editor who sails a Sadler 34, which has taken his family from the Caribbean to the Baltic

‘For example, our D400 wind generator converts 36% of the kinetic energy in a 12-knot wind stream into electricity. The theoretical maximum (Betz Limit) is 59%, and the latest multi-megawatt commercial turbines achieve around 40% efficiency due to their scale.’

Nonetheless, he believes that a yacht can cruise entirely independently of fossil fuel, and he’s far from alone.

Jimmy Cornell’s Elcano Challenge aims to prove just that, aboard an electrically-powered Outremer catamaran.

True, he has just put the round-the-world voyage on hold, because the regenerating prop could not keep up with demand.

But he thinks the answer is to beef up hydro and solar capacity while trimming power use on board.

‘I am determined to continue my zero-emissions project once certain changes have been made,’ he says.

Solar panels have been with us for decades, and as the technology has matured, so they can produce more power from the same footprint.

Even a small panel putting out a few watts is enough to keep a lead-acid battery bank trickle charging when the boat’s on a swinging mooring. But some have already gone much further than that.

Renewable energy: solar developments

Catamaran builders, in particular, have been trying to capitalise on the extensive deck area of their boats by fitting solar panels.

Silent Yachts is ahead of the curve on this, with a 55ft cat whose 49m2 coachroof and hardtop are carpeted with 10kW of panels.

On a sunny Mediterranean day, that provides enough electricity to run all the boat’s systems and leave plenty for a few hours of electric propulsion.

Luxury cat brand Sunreef has developed cells that can be built into the actual fabric of the boat.

Solar cells built into the hull of a boat to produce renewable energy afloat

Solar cells, built into the structure of luxury Sunreef multihulls, vastly increase the solar power potential

‘They can be easily mounted anywhere on the yacht’s surfaces, including the hulls, mast, superstructure, bimini roof or bow terrace, vastly increasing the amount of solar power,’ says the brand’s Sara Smuczynska.

‘Sunreef Yachts is also the first company to develop a system to recover heat from the panels to heat up the yacht’s boiler.’

With panels in the topsides, decks and coachroof, up to 13kW can be installed on a Sunreef 50.

Monohulls are a different story. Gantries, guard wires and coachroofs can support panels of a few hundred watts – enough for basic systems.

But if you want to generate serious solar power for more ambitious green goals, then you need to think laterally.

That’s what Frenchman Alain Janet did when he launched SolarCloth – a business that sticks solar cells to your sails.

solar cells in sail cloth to help produce renewable energy on board the Spirit 44E

SolarCloth cells on the mainsail of the Spirit 44E produce 560W on a sunny day. Credit: Sam Fortescue

The advantage to this is obvious: the sails offer the largest surface area and their near-vertical alignment can suit the angle at which sunlight falls on them.

The cells are based on proven copper indium gallium selenide (CIGS) technology, capable of around 17% efficiency and very flexible.

Simply glued to the sailcloth in positions that won’t chafe on the spreaders under any reefing conditions, they are robust enough to withstand flogging, folding and all manner of abuse, as demonstrated during the 2016-17 Vendée Globe race by skipper Conrad Colman.

More recently, Spirit Yachts integrated the technology into its beautiful 44E performance cruiser, launched last autumn.

Spirit 44E under sail

The Spirit 44E under sail. Credit: Richard Langdon/Ocean Images

On the Spirit, the cells were arranged in panels 30cm high and about 2m wide on either side of the mainsail, producing 560W on a sunny day.

Dr Vincent Argiro, who commissioned that boat, wanted a fast, energy-efficient design.

‘The stretch goal for the 44E was near total energy self-sufficiency,’ he says.

‘I envision plugging into shore power to be a rare event.’

Janet acknowledges the junction boxes and wires needed to connect the sail to the deck are clunky, but he is developing a sleeker solution.

Meanwhile, a new partnership with One Sails to produce the so-called PowerSails will give the idea fresh impetus and broader distribution.

Continues below…

Janet says a PowerSail costs 50-60% more than a standard sail, while the same technology has also been used to add photovoltaics to biminis and awnings on cruising boats.

All this is based on silicon technology, where the record efficiency for an expensive six-junction cell is 39.2% in natural light.

But further down the line, emerging Perovskite technology could make photovoltaics lighter, cheaper and applicable to any surface by painting or printing.

Researchers at Imperial College, Cambridge University and China’s Soochow University calculate that it has the potential to eclipse silicon with up to 60% efficiency, once the issue of durability has been cracked.

Luca Bondi, technical director of Italian solar panel producer Solbian, says the future lies in the combination of silicon and Perovskite in the same cell.

‘Tandem cells made by crystalline silicon and Perovskite raised close to 30% efficiency,’ he says.

‘The increase in efficiency is important but not disruptive, thus I think we cannot say that we have a big step imminent, but an improvement of the already good existing solar cells.’

For those who dislike the look of solar panels, there is another option.

A printable film has been developed which is stuck on top of the solar panel to disguise it.

Finishes range from monotones that match your paint to a teak-effect that would allow you to add solar panels to decks.

Solbian supplies its monocrystalline panels with this so-called iSP mask, and Bondi says that it does very little to reduce their 24% efficiency.

Hydro power on board

Sails are the most abundant generators of renewable energy on board, propelling tonnes of yacht at a brisk pace.

Converting just a fraction of the boat’s kinetic energy into electricity can yield plenty of power for the loss of less than one quarter of a knot.

Broadly speaking, there are two approaches.

The first is the well-established principle of hydrogeneration, where you lower a dedicated propeller into the sea that is turned by the passing water and used to drive an alternator.

Products in this space are typically mounted on the transom and deployed using a lanyard to generate power.

Hydrogeneration is where you lower a dedicated propeller into the sea that is turned by the passing water and used to drive an alternator, producing renewable energy

Custom deck mount for a Sail-Gen hydrogenerator

They include the Watt & Sea, which comes in 300W and 600W units, Eclectic Energy’s SailGen and Italy’s 600W Swi-Tec.

More recently, regeneration has emerged as an alternative. It harnesses the same principle but uses your auxiliary propeller to generate the power, so no need for a bulky transom unit or the braking effect of a second prop in the water.

There are retrofit options available from the likes of Holland’s Bell Marine, but it is relatively expensive to install, so the more common option at the moment is to fit a new hybrid propulsion system – either diesel-electric or battery-electric.

If your engine needs replacing it’s worth considering.

However you configure it, hydro can be a very efficient way to generate power, especially at scale.

A dedicated propeller of a hydrogenerator is optimised in pitch and diameter for maximum torque

A dedicated propeller of a hydrogenerator is optimised in pitch and diameter for maximum torque

The 350ft Dynarig yacht Black Pearl is able to sail across the Atlantic without burning any fossil fuel – its twin props regenerate hundreds of kW of power.

Cruising yachts, on the other hand, will struggle to generate even a kW, and typical output at five knots doesn’t exceed 100W.

This is because the power out is a cubic function of boat speed, linked to water past the prop, so even a small speed increase hugely increases yield.

Nudge up just a little to seven or eight knots and you can get a more manly 300W from regeneration.

Dedicated hydrogenerators are more efficient because their props are pitched and sized according to your boat’s cruising speed.

With regeneration, your main prop will be optimised just for propulsion. Only a variable pitch prop can excel at both tasks.

The 350ft Dynarig yacht Black Pearl is able to sail across the Atlantic without burning any fossil fuel – its twin props regenerate hundreds of kW of power

Under sail in regeneration mode, the three-masted Black Pearl is capable of crossing the Atlantic without burning any fossil fuel. Credit: Tom Van Oosanen

That is what Finland’s Oceanvolt has achieved with the Servoprop – whose pitch adjusts electronically in real time to extract the greatest possible power from regeneration.

The team behind it claims that it can boost electricity output nearly threefold compared to a fixed prop.

Indeed, at seven to eight knots it produced 1kW of power.

There again, at five knots, output falls to around 200W.

It all depends on how much power you need.

For house loads, 200-300W should be more than enough, but for electric propulsion you’ll need far more.

Servoprop comes as a complete saildrive system with the option of either a 15kW or a 10kW motor.

But electric propulsion rival Torqeedo is sceptical about variable pitch systems on small motors.

‘It’s not possible to get much more than 300-400W because the physics makes it tough to adapt the pitch of the prop and to take care of the waves,’ says sales director Phillip Goethe.

‘When your speed through the water is changing often – from stalled to surfing, it is very hard to have the optimum pitch.’

Instead, Torqeedo’s notion is to spec a fixed-pitch propeller that strikes a compromise between propulsion and regeneration.

‘Perhaps you lose 2% [in propulsion], but gain two digits in hydrogeneration efficiency,’ says Goethe.

‘But for cruising applications, it doesn’t need to be optimised for propulsion above seven knots.’

Oceanvolt 15SP: from €46,660 ex-VAT
Hybrid Marine: from £14,980 ex-VAT for a 30hp engine and 10kW motor.

The cost of hydrogenerators

Watt & Sea: £3,504.10 (300W)
SailGen: £2,464.69
Swi-Tec: £3,080

Hybrid options for renewable energy onboard

Isle of Wight-based Hybrid Marine specialises in diesel-electric parallel hybrid systems built around new Beta and Yanmar engines.

They can take advantage of regeneration and allow limited manoeuvring using the electric motor, with the diesel for longer passages.

Hybrid Marine specialises in diesel-electric parallel hybrid systems built around new Beta and Yanmar engines

Hybrid Marine specialises in diesel-electric parallel hybrid systems

‘Retrofits are tricky. It takes a lot of work to reliably convert an engine and means it has to be removed to make the conversion. Accumulated costs work out close to a new system,’ says MD Graeme Hawksley.

Hydrogen propulsion

Hydrogen fuel cells can be used either to provide small amounts of electricity to charge a battery, or at larger scale to power an electric drivetrain.

Either way, they can be emissions-free if they use hydrogen produced using renewable energy.

Hydrogen is attractive because it is three times as energy dense as diesel, but being a gas in ambient conditions, it must be stored under tremendous pressure – up to 350 bar on boats, requiring voluminous storage cylinders.

Efoy leads the market for marinised low-power fuel cells, with a 40W and 75W unit available.

French company Genevos has already started selling a 15kW hydrogen fuel cell tested by singlehanded racer Phil Sharp during a Solitaire du Figaro campaign

Phil Sharp with the Genevos hydrogen power module

It burns methanol supplied in 5lt and 10lt ‘cartridges’ that are available from distributors across Europe.

You can simply clip the output wires to a suitable charging point on your battery system, but for optimum efficiency, Efoy also supplies its own Lithium batteries in 70 and 105aH capacities.

Though Efoy doesn’t quantify the benefit, it describes this combination of fuel cell and battery as ‘particularly efficient’ by avoiding unnecessary charging cycles.

A 10lt canister yields just over 11.1kWh of usable power – enough for four weeks of typical use, according to Efoy.

Beyond that there is a bit of a void in the market until you reach a power output of 15kW, where the purpose is to supply an electric motor for propulsion, as well as covering the boat’s domestic load.

French company Genevos has already started selling a 15kW fuel cell tested by singlehanded racer Phil Sharp during a Solitaire du Figaro campaign.

‘We’re going to see quite a lot of private projects as retrofits in coming years, and by 2025, there’ll be production boats with hydrogen energy systems,’ he says.

That’s despite typical costs of around €100,000 to supply and install a system.

Artist impression of a hydrogen fuelling station offering renewable energy to yachts

French firm EOD is developing plans for futuristic looking floating hydrogen fuel stations that actually generate H2 from seawater

Rival Energy Observer Developments (EOD) is designing fuel cells in the 60kW to 1MW range for larger vessels.

It stems from a project that demonstrated how solar and wind power could be harnessed to make hydrogen from seawater on a round-the-world prototype.

Other than the sheer cost, the current stumbling block is that hydrogen gas is not yet widely available in ports or marinas.

‘However, we’re going to see much wider access to hydrogen in five years’ time,’ promises Sharp.

EOD is developing futuristic-looking hydrogen fuelling stations that float in a corner of the marina and generate hydrogen from seawater using green mains electricity.

And British firm Fuel Cell Systems says its first marina hydrogen pumps should be installed in the south of France this summer.

‘Although the UK will be very slow to pick it up in my experience,’ cautions CEO Tom Sperrey.

Efoy Fuel Cell 80: £2,195
Efoy 5lt methanol £37.80
Efoy 10lt methanol £53.40

Battery tech

Lithium is still the performance choice for storing renewable energy on board.

Advances in chemistry and design driven by the automotive sector are making it possible to store more energy in the same footprint.

So the capacity of the BMW i3 battery that Torqeedo offers has risen from 30kWh to 40kWh over five years.

Oceanvolt lithium batteries on a Feeling 32

Oceanvolt lithium batteries on a Feeling 32

Promising technologies have been demonstrated in the lab. California’s QuantumScope has developed a stable battery that uses solid lithium as the anode, and offers four times the energy density of current lithium batteries plus lightning-fast recharge speeds.

Other approaches use graphene, salt, aluminium and even ceramic, as well as solid electrolytes.

‘The technological development of batteries is really fast,’ says Oceanvolt’s head of R&D Marko Mäki.

‘We believe that in the future, the combination of battery price, capacity and safety will only improve.’

Expect performance gains of 5-10% per year.

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