A Boat with Spark
An electric-powered workhorse with stamina.
The limitations of battery capacity, weight and cost have confined electric motors mostly to small trolling applications, or limited freshwater uses like slow-speed ferries on pristine European lakes.
Sean Kelly of Tauranga’s Electric Boats Limited aims to change this. With what’s claimed to be New Zealand’s first all-electric commercial boat, his recentlylaunched runabout has a top speed of 25 knots and is capable of running for a couple of hours at cruise speed. The result of years of R&D, not to mention gallons of blood, sweat and tears, the boat was unveiled at this year’s Hutchwilco New Zealand Boat Show. Curious, I headed out to Tauranga recently to try it out. Using existing marine technology – an aluminium hull and conventional sternleg and steering gear – Kelly has added high-capacity lithium-ion batteries and high-output digital AC induction motors.
This kind of solution is becoming increasingly viable – technically and financially – thanks to the ongoing improvements in the energy density of batteries (more capacity for the same weight and size) and the downwardtrending costs that battery companies like Tesla are bringing to the market.
The starting point for Kelly’s boat is an efficient hull, based on conventional design and construction and specifically intended to be easily-driven. Designed by Herley Boats, the very fine-entry bow splits the force of an oncoming wave.
An axe bow, also known as a vertical stem, provides additional waterline length and adds buoyancy despite the fine entry. The hull’s relatively narrow, but with a flat aft area the boat’s stable while moving and at rest.
Propulsion comes from a tandem power unit providing 110kw of oil-cooled electric power running at 115 volts. Internally, the unit is actually two motors in the same housing sharing a common drive shaft. They provide exceptional torque as well as reliability, should either unit develop a fault.
The unit’s compact size is extraordinary, as is the amount of instant torque available. A conventional heat exchanger cools the oil, ensuring no salt water comes into contact with the motor. When the boat’s at rest the only sound from the propulsion unit is the raw water pump circulating the water.
Kelly’s elected to use a conventional Mercruiser sternleg to convert the power into thrust and steering. In fact, the gearbox is redundant – the motor has no
need to idle, and provides almost full torque from zero RPM, so a direct drive is possible.
The motor rotation can also be reversed by means of the electronics, again removing the need for a gearbox. By far the noisiest part on the boat when underway is the whine of the gears meshing, and undoubtedly a small amount of the available power is lost by the box.
On the other hand, a sternleg provides steering capability and the ability to tilt the prop up for trailering, both practical considerations in a vessel this size.
The battery compartment is completely sealed under the main deck, and under normal conditions never needs to be opened. Because this boat has been built to survey standards the batteries sit in a solid tray raised above the hull and are completely isolated.
A double lip around the edge of the hatch opening provides an additional safety margin to ensure any drips that get through the first barrier will run out before they get to the second barrier.
‘Fuel’ on this boat is a single bank of 12 lithium-ion cells, arranged as six banks of two cells each. Each set of two cells is connected in series, with the six banks in parallel. This allows the pack to provide a maximum current of around 950 amps at 115 volts, although typical cruising speeds only draw between a third and a half of this maximum. The boat’s current batteries only have a 36kwh capacity – they are a few years old.
With the continual developments in battery technology, and its downward trending costs, this capacity is easily increased. There is
The unit’s compact size is extraordinary, as is the amount of instant torque available.
enough under-floor space to fit a second bank and, with newer battery models the total capacity could possibly get up to 100kwh.
This would provide two to three hours of running at high speeds. On the other hand, running at seven knots the boat would keep going for nearly 12 hours, making this a perfect work boat or day-fishing vessel.
Having six parallel banks also provides reliability, in that failure of any single cell may reduce top speed but allow the boat to continue operating. The relatively high voltage is designed to reduce cable thickness: the
same kilowatt rating in a 12-volt system would draw around 9,000 amps and require cables several centimetres in diameter!
Electric motors are something of a revelation to anyone used to petrol or diesel power, and the instant torque available from them is staggering. Certainly, some electric sports cars have embarrassed the world’s fastest petrol-powered cars, comfortably out-accelerating them.
This raises a potential problem in a boat, where a hammerdown acceleration from rest will produce full torque at a low RPM. The motor used by Kelly in this boat develops 271Nm maximum torque, equivalent or better than a diesel engine of similar horsepower (and considerably more than any similar petrol engine).
The difference is that torque comes on very quickly from zero revs, when the inertia required to spin the prop and shaft is at its greatest. This will stress propeller and bearings and could potentially distort the drive shaft, so a more restrained power-up approach is called for.
Throttle control on the boat is a conventional unit from
Mercruiser – the difference is the actual throttle provides signals to the motor controller. The gear control and tilt/trim buttons operate as usual.
What is a bit disconcerting is that the boat is almost silent (apart from the water pump) until you push the throttle forward, and as you start to move the main noise is the whine of the gearbox.
Despite the need to avoid hard-out throttle movements, the boat accelerates well and we were soon scooting around at a cruising speed of around 16 knots. Kelly has taken the boat to 25 knots with several people on board, although like all boats there’s a balance between fuel consumption and speed.
In the case of an electric boat, the current draw is the critical aspect, and at 16 knots we were drawing 450 amps. Although instrumentation on this boat is fairly basic, much more complex visual displays are possible which could show remaining range and times on a colour touch-screen.
Even with the relatively narrow 1.9m beam the boat’s surprisingly stable, helped no doubt by the battery weight located low in the hull. The 12 cells weigh around 240kg, and with the sternleg and motor added the total power train weighs around 350kg.
We only ventured out into the fairly flat Tauranga Harbour, so I cannot comment on the Herley hull’s seagoing ability, but I have no concerns over its ability to handle rougher conditions.
Overall, this is a very capable boat which clearly shows the enormous potential of electric motors and batteries. The battery bank can be charged overnight from a domestic power point, or quick-charged from a high-output industrial connection.
A common question Kelly gets asked is: “What happens when you run out of battery power.”
Well actually, it’s no different to a fossil-fuel boat: You plan your trip to be safely within the capacity of the ‘fuel’ available. If you run out of petrol you have to call for help. It’s no different for a battery-powered boat. And for maximum range you stick to the most economical speed rather than going flat-out all the time.
In addition to building completely new electric boats, Kelly is also interested in working with boat owners to convert existing hulls to hybrid or all-electric systems. A hybrid solution would implement an electric motor and smaller battery pack, with a high-efficiency generator to develop the electric power.
This is the approach used by diesel-electric locomotives, and even some of our Cook Strait passenger ferries use hybrid propulsion systems. On the other hand, a fully-electric system would implement a larger battery bank and use solar or wind generation for recharging.
The result would be a quiet vessel with running costs just a fraction of your current fuel bill.