ELECTRIC VS PETROL OUTBOARDS
Advantages of Electric Outboard Motors
“Electric Outboard Virgins” and indeed “Electric Outboard Cynics” are almost always astonished when they first try a genuine electric outboard (ie. one that’s not a trolling motor). If you’re used to the behaviour of a petrol motor, you will find an electric motor is instant, easy, quiet, smooth and fume-free… a delight to use.
Let’s expand on some of those points. The advantages of electric outboards over petrol include:
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Ease of use. Push button, go.
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No starter cord to pull, no choke, no fuel tap. If you can steer, you can use it, including kids etc. who can’t always start petrol motors.
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Ease of storage. Nothing leaks out or smells. There’s no carburettor to gum up due to lack of use.
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Low running costs
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Zero servicing cost for eight years
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Quietness. It’s just a more pleasant way to travel, and you can talk in the dinghy without the whole world hearing you!
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Zero pollution at point of use. It’s nicer for you (no combustion fumes to breathe in) and the environment. Marine life really doesn’t like ingesting petrol and oil, and, increasingly, many inland waterways are banning combustion engines altogether.
Electric vs petrol outboard comparison
- at time of writing (2021) they are only really cost-viable at small sizes and over distances of 20 miles or less (we’re waiting for battery technology to catch up with motor technology)
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kind of the same point, they are a lot more expensive
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you can’t really fix them yourself. There is less to go wrong, and they are generally much more reliable, but there is nothing to “tinker” with. There’s no choke to adjust, you can’t take the carburettor apart, etc. (Although many people might put that in the advantage column!)
The real elephant in the room is that first point, on batteries. Even the latest, still-in-the-laboratory, innovations do not have anywhere near the energy density of fossil fuels. A litre of petrol weighing roughly 1kg contains roughly 10kWh (10,000 Watt hours) of energy. The best lithium batteries on the market – in production, with waterproof casing etc – are now achieving about 130Wh per kilo. Petrol is still roughly 75 times more energy dense!
Electric outboard motors win some of that back, because they are much more efficient:
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in a petrol outboard liquid fuel is being turned into heat (and indeed running a cooling system to counteract the heat!), noise and overcoming friction – plus some motion. That motion is then transferred via a vertical shaft and right angle gearbox (more losses) to a propeller
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in the electric outboards we’re looking at the motor is in the “bulb” at the bottom of the leg and directly in line with the propeller. They make very little noise and because they’re underwater the sea does the cooling without need for pumps etc.
With an electric outboard motor, a much greater proportion of the stored energy is turned into propulsive effort, rather than heat, noise and other losses.
The absolute best diesel car engines are approaching 40% efficient, but the best small petrol four stroke outboards are not going to be anywhere near that. Somewhere between 10 and 15% is our reasonable working guess (please tell us if you know different), and probably less if you’re looking at a two-stroke petrol. Torqeedo and Epropulsion claim around 50%, in terms of converting energy in the battery to propulsion in the water.
If we were to say the efficiency of an electric motor gives it a 3:1 advantage (45% vs 15%) that reduces the energy density advantage of petrol to… oh, still about 25:1! (75 divided by 3)
So with petrol, undeniably, you can carry a lot more energy in a lighter, smaller container. A 5 litre petrol can contains a huge amount of energy. But now think about how you use your outboard motor – do you actually need more range than an electric outboard can achieve?
Electric Outboard Range & Run Time
The Torqeedo 1103 and Epropulsion Spirit PLUS both have roughly 1kWh batteries, driving 1kW motors. If you run them flat out they last about an hour. Roughly the same as the 1 litre fuel tank on a small petrol outboard. Think about it this way: how often do you do a journey with your small petrol outboard that uses more than 1 litre of fuel?
And just as it’s true of your small petrol outboard, if you “throttle back a bit” on an electric outboard the range and run time increase exponentially.
In the last couple of years the energy density – the amount of electricity that can be stored for a given volume and weight – has improved to the point that the batteries provided with the Epropulsion Spirit and Torqeedo 1103 are more than sufficient for most users’ daily needs (and often two or three days’ use).
As a case in point when the old Torqeedo 1003 was launched (2009), its battery had a capacity of 320Wh (Watt hours). Today they can put up to 916Wh in the same size casing. (Nearly Triple!)
A typical “real world range”, for both the Torqeedo 1103CS and the Epropulsion Spirit, on the back of an inflatable dinghy, running at 4.5 to 5 knots, seems to be 10-12 nautical miles, in fact a bit more for the Epropulsion. For most boaters – we know not all – in most applications, that’s enough.
Going up the scale we recently ran a “can I convert to electric” viability test on a customer’s 26ft lifting keel yacht (roughly 2 tonnes as tested, with 3 blokes on board). With an Epropulsion Navy 3 (3kW) outboard, and two Navy batteries (6kWh capacity) the results were:
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4 knots (750W power) for 8 hours, 32 nautical mile range
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4.5 knots (1000W) for 6 hours, 27 nautical mile range
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5 knots (1500W) for 4 hours, 20 nautical mile range
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6 knots (3000W) for 2 hours, 12 nautical mile range
Note that a small increase in speed requires a lot more power. As we said to him “patience is a virtue, especially if you have finite energy storage capacity!” But with those figures established, the real point to consider was how often – if ever – would he need more than 30 miles motoring range in a weekend (or between opportunities to recharge).
Undeniably a 6hp outboard and a 20 litre can of petrol would give him far more range, but not quietly, smoothly or pleasantly. We also considered whether he would be far more likely to “electric motor sail” in light airs.
Running the petrol motor is always unpleasant, so you’d probably give up on sailing and motor to destination as fast as possible. In contrast a near-silent electric outboard running at 300W would boost boat speed by perhaps 2 knots, and pointing ability by perhaps 10 degrees. “Sail-Electric Hybrid” could well be a massive range extender, not to mention annoying all the people who would think you were some kind of sailing performance god (no noise or fumes to give you away)!
Electric Outboard Costs
Petrol outboards have been made for over 100 years, it’s a “mature technology”. Electric motors have been around longer even than that, but lithium batteries – which have made electric outboards much more viable – are a relatively new technology. Demand for lithium, from the car industry among others, has pushed the cost skyward.
So – mainly because of the batteries – the the initial outlay on an electric outboard motor is much greater than a petrol outboard motor. At the small end, a Torqeedo 1003 retails from about £1300 (depending on battery choice), compared to a Suzuki 2.5hp petrol at around £600.
But the initial outlay is not the whole story. Electricity as a fuel is cheaper than petrol, but petrol outboards don’t use a lot of petrol anyway, so that’s largely irrelevant. However, servicing cost is not irrelevant. To keep up the warranty on your petrol outboard, you need to have it serviced annually by an authorised dealer. On average that will cost £100 a year, plus getting it there and back.
As a rule of thumb, for the small 1kW electric outboards that sell in greatest quantities, we reckon the costs even out after about 5-6 years, and electric is cheaper thereafter.
Unfortunately – with lithium batteries being the greater part of the cost in an electric outboard installation – the cost disadvantage grows almost exponentially with the power requirement. We are getting limited interest and sales up to around 4kW motors, but above that there are very few users who can justify the premium. (Those few tend to be very high-use commercial operators, for whom the fuel cost saving makes a significant difference.)
We’d be delighted to supply you a 10kW motor, with enough battery capacity to make it useful, but don’t expect much change out of £20k!
