Electric tenders have been a long time coming, but, says Sam Fortescue, these frontrunners are well worth the investment...
Let’s get this out of the way up front. In the blunt words of Ernest Menten at Tenderworks: “You have to accept that an electric tender is going to be slower than an internal combustion engine boat.”
Even a state-of-the-art lithium-ion battery supplies just 10 per cent of the power of the same weight of petrol, so electric tenders are more expensive and need frequent charging. And yet they are flying off the shelves, with most manufacturers reporting they have filled build slots for all of this year, next year and even into 2024.
“We have already seen a tenfold increase in the electric boat order books of the current manufacturers compared to this time last year,” says Alex Bamberg, CEO of fast-charging specialist Aqua superPower. “We see that in the next three years, an unstoppable and confident transition will be well under way.”
The attraction is clear. These boats are silent to operate, fumeless and welcome in a growing number of locations – from Norway’s fjords to Amsterdam’s canals – which are closed to polluting craft. “The environmental benefits, noise and vibration mitigation and reduced service requirements make electric particularly suitable for tenders,” adds Audrey Hodgdon of Hodgdon Yachts. The company has adapted its 10.5-metre and 12-metre limo tenders to run on electric power.
Speed vs Range
Technological development has reduced, but not eliminated, a fundamental compromise at the heart of every electric boat: a hull’s drag increases exponentially with speed, so you can either drive fast or drive long – but not both.
“Even with the best lithium-ion batteries on the market, conventional planing boats will not reach more than 20 to 25 nautical miles of range at high speeds, making them inferior to conventional gasoline-powered speedboats in terms of performance,” explains Teodor Hällestrand of electric boatbuilder Candela.
Some say that doesn’t matter. “How often have you sat in a limo tender doing 40 knots?” asks Menten of Tenderworks. “It never happens. Not even when the tender driver is alone! Smaller tenders just don’t get used for extensive driving. They wait idling for hours behind the main yacht and then drive 10 minutes to the shore.”
Candela chose a different solution. In the C-8, a trio of foils lift the hull above the water and allow the drivetrain to achieve 30 knots using just 37kW of power. At a still-impressive 22 knots, it can knock off 50 nautical miles with its single 40kWh battery pack. There is a virtuous circle at work here, because a lighter boat requires smaller motors and smaller batteries, in turn making the boat lighter still. The only complication is garaging a boat with foils – even retractable ones.
Berlin-based startup Voltaire has gone down another path. Its boats have yet to be built, but designer Jonas Hertwig has drawn a twin-hulled tender with razor-fine hulls for minimum drag. Its twin 50kW Deep Blue motors are driven by 85kWh of BMW i3 batteries, giving it a top speed of 20 knots and a respectable range of 100 nautical miles at nine knots. As a kicker, the hull is laminated in sustainable flax-fibre composite.
Standard monohulls need to pack in anything up to 230kWh of battery capacity. “You have to put enough batteries in, but if you put too many in, you actually reduce the range,” says Menten.
There are other weight concerns, too. “With tenders, you’re very much limited by the hoisting capabilities of the motherships,” says Massimo Kovacic, head of engineering at Bremen-based Yachtwerft Meyer, which has designed a number of electric tenders but is yet to build one. “We have had a number of requests, but when we made all the analysis, the clients decided to stick to traditional.”
Fast Pit Stops
So the electric tender is no all-rounder. You won’t be wakeboarding one moment and ferrying guests to a deserted beach with a champagne grill the next. But as a means of luxurious, short-range transport, it is almost perfectly suited, says Audrey Hodgdon. “The operational profile of tenders makes them a great candidate for electric propulsion, with the short ship-to-shore-type trips and the ability to charge both from the mothership and when at shore.”
Until very recently, big battery packs could only be charged overnight using low-power AC from dockside supplies, and the DC systems on board were slow. But, just as in the automotive sector, burgeoning boat demand has spurred rapid development. Batteries at 400v accept more charge than their low-voltage counterparts, and there are already moves towards 800v batteries with better cooling for even faster charging.
Aqua superPower is in the vanguard here. By the time you read this, it will have installed 14 high-powered DC chargers in marinas in Europe and the US, with a further 90 expected in the next year in key hotspots around Italy and the French Riviera, plus in MDL marinas around the UK. Often requiring local infrastructure upgrades, these are capable of supplying 125kW to two plugs simultaneously.
Before any power comes aboard, the boat must complete an electronic handshake that also validates the user’s profile. Only when connected does the plug become live, supplying AC and DC current. Aqua can also replicate this performance with an onboard charging system, creating the prospect of a 60 per cent recharge in the time it takes to drink a cup of coffee.
“Currently the network is in the build-out phase,” says Bamberg. “We know there are very significant boat sales in the sub-15-metre market and these will drive the usage next year in the leisure and workboat space.”
Dutch yard Tenderworks has also dipped a toe in the electric tender market. Under the codename V-AMP, it has already built two fully custom 14-metre electric tenders for a client who was focused on setting a positive example. “As a billionaire you are an opinion leader, so you’d better make your choices count,” says CEO Ernest Menten.
Potentially competing with the Vita Lion, the V-AMP nevertheless features a Vita electric drivetrain, which the two companies collaborated on. All the parameters were meticulously studied, from the shape of the hull and the location of the shafts to the positioning of the batteries. Her naval architecture is completely different, however, with a semi-displacement hull designed for comfort and range. “On the electrical range of the naval architecture, it lifts with little resistance and is flat on the water, but cuts through the water with a sharp bow,” says Menten. “You cut the wave in two at a very early stage and it creates a speed with the least amount of power.”
It means that her top speed is a little lower at 20 knots, but perfectly sufficient for making comfortable runs between the ship and the shore. “The V-AMP really impressed with her stability when cruising, cutting through the waves in total comfort with nine passengers on board,” says BOAT International’s Raphaël Montigneaux. “The boat was as if on rails. Even though the technology on board comes from Vita, there is a clear feeling of being on a very different boat, proving that Vita has created a really flexible technology that’s easily adaptable to various platforms.”
After launching in 2017, Vita Power set about designing a series of electric concept boats, which proved hugely successful in the Yacht Club de Monaco’s Energy Boat Challenge. The new 10.5-metre Lion is the first production dayboat, built by JFA Yachts, and incorporating many lessons learned over four years. “Built in carbon, the fit-out of the boat is neat,” says Montigneaux, who tested the boat first-hand. “We see here the experience of the superyacht builder. Her layout is very classic, with large sunpads aft followed by a cockpit with seven seats to cruise comfortably.” The cockpit converts into a U-shaped dining area, and a little cabin forward offers a double bed and a dayhead. “I especially liked the beautiful work done to link the deck and bathing platform,” he says. “Linked to this pure design, the use of white throughout reinforces the feeling that they are pollution-free.” All the control systems have been developed by Vita and are managed through two large touchscreen displays at the helm.
And the feel of the boat? Well, it was rather different. “We left the port of Saint-Tropez in complete silence. At the beginning, it gives the feeling that something is missing. But after a few minutes you start to really enjoy it.” Montigneaux never reached the boat’s top speed of 35 knots because of restrictions in the bay, but the Lion quickly settled into its cruising stride of 22 knots. Despite the confused, choppy seas that often prevail here, the ride was dry and comfortable. This is not a lightweight boat, packing an industry-leading 233kWh of battery power – this alone amounts to well over a tonne. But it gives the boat 90 minutes of operation at 22 knots, and Vita CEO Rory Trahair says that the boat recharges in less than an hour using Aqua superPower’s new fast-charging stations.
Every Watt Counts
In the quest for efficiency, lossless motors and slippery hull design take on giant importance, because just a few percentage points’ improvement adds another mile to the range.
On the propulsion side, some boatbuilders have gone back to basics to develop the most efficient motors, propellers and gearboxes. “With internal combustion vessels, the tendency is to just add more power rather than improving efficiency of the hull form or build,” says Rory Trahair of electric drivetrain integrator Vita. “We have to balance all of the above, and have designed our hulls around the particularities of an electric power train.”Read More/The best electric tenders for your superyacht
With its foiling boat, Candela quickly realised that it couldn’t simply rely on electric outboard technology. It created the C-POD, whose two contra-rotating propellers are much more efficient than a single prop, while the direct drive reduces friction to a minimum. The result delivers 23 per cent more range than the best electric outboards.
In the quest to strip away every surplus kilogram from the hull, carbon fibre is usually the material of choice. “To accommodate enough batteries to ensure efficient propulsion, you must make the boat as light as possible, so you need to use the latest composite technology and techniques,” says Henry Ward of launch builder Cockwells. Of course, carbon raises its own problems because it has a much bigger carbon footprint than GRP. Care is also needed, as lighter boats can be more uncomfortable in any kind of seas.
You must also optimise hull shape and balance for an electric drivetrain. “Just shoving an electric motor into a boat doesn’t make it an electric yacht,” says Ernest Menten of Tenderworks. “If you don’t have the appropriate naval architecture, you’ll have so much resistance that your batteries will go flat very quickly. It is a whole discipline of naval architecture and hull design – a combination of elements makes it work.”
This is a problem whose solution has so far eluded some RIB makers – particularly those using jet drives, as the ubiquitous Williams Jet Tenders builder admits. “We have spent the last three years developing various prototypes and system studies,” says the brand’s Ollie Taylor. “The big stumbling block is the battery. Due to the jet pump and hull design of our existing tenders, the running time would be very, very low, and recharge time on board would be very high.”
RS Electric, a newcomer with a name familiar to those in the RIB-racing world, claims to have cracked the problem with its more utilitarian boats. Naval architecture was the biggest challenge, according to CEO Jon Partridge. “The RIB needed to have an ultra-efficient running surface that could smoothly transition from displacement to planing mode; too much displacement would be inefficient at high speeds, and too little displacement would be inefficient at slow speeds.”
The result is the Pulse 63, with a 23-knot top speed courtesy of its RAD 40 electric drive and 46kWh of high-voltage batteries. Even at 10 knots it manages a very decent 70 nautical miles. Uniquely, RS has gone eco on the lay-up, too, with recycled PET foam core, recycled carbon fibre and sustainable flax-fibre.
Truly eco or greenwashing?
Aqua superPower’s onshore network is supplied by renewable energy, but on the mothership it’s an entirely different matter. The tender charging station typically draws power from the yacht’s generators, which invariably run on polluting diesel. So do electric tenders simply displace emissions from the small boat to the big?
Not so, according to many in the industry. “Excess energy generated by the mothership can be used to charge the tenders, thus avoiding that energy going to waste,” says Rory Trahair of Vita. “Tenders largely operate during the day and will often require charging at night when the loads on the mothership are lower. This energy can be optimised.”
That may well be the case with older yachts. Their generators are inefficient unless they operate at optimum RPM, so there is effectively free energy to be had during moments of light load. But many modern yachts use hybrid technology, where battery banks mop up any excess electricity then switch off the generator.
“If the power requirements of an electric tender can be factored into the power-generating systems of the mothership to optimise and reduce wastage, then the electric tender serves as an additional use of that energy,” adds Trahair.
Truly renewable energy is restricted to giant sailing yachts such as Black Pearl and Ethereal, which can generate totally renewable electricity as they sail. Lürssen is one of several boatbuilders also spearheading the use of cleaner hydrogen fuel cells to cover hotel loads.
But perhaps the last word should go to Henry Ward of Cockwells, builder of those achingly beautiful wooden launches. “In reality, you won’t save the world with your electric tender,” he tells me candidly. “But you will enjoy a much more pleasant experience in your limousine.”
First published in the March 2022 edition of BOAT International.