Would you make the switch? We ask two sailors who have looked at how the real-world numbers stack up on electric propulsion but have reached different conclusions
Ian Thomson: ‘I am an electric propulsion convert’
Let’s be clear, electric propulsion is nowhere near a 300-mile range at 6 knots on a 38ft yacht.
But I think it is viable for a typical 30 footer in the Solent and crossing the Channel occasionally, as long as you don’t motor long distances against full-bore spring tides, and use the sails most of the time.
For example, a 60-mile range under power in a flat calm at 4.5 knots looks achievable.
That’s why I have decided to convert my Sadler 29 to electric propulsion.
You may need a more traditional approach to cruising, but for me having no noise, no vibration, no smell and no maintenance is worth the compromise.
After a season of real-world testing in our Sadler 29 here’s what we found.
We fitted a prototype 10kW pod in August, with 18kWh of lithium batteries where the Bukh DV20 used to be.
This gave us the following results:
- 3.3 knots at 1kW output, 54-mile range
- 4.5 knots at 2kW, 40-mile range
- 5-5.5 knots at 3kW, 30-mile range
Additionally, the power generated under sail often puts nearly as much charge in on passage as we’ve used coming in or out of harbour.
On a sail of more than a couple of hours, we might even make a net gain in battery charge.
Cost aside, it would be possible to add another 50% in battery capacity and still come in just under the weight of the Bukh and diesel tank removed, and double that again if you took out all the 12V batteries.
This would make 80-100 miles on battery alone achievable, although I didn’t feel the need for that much capacity.
If you accept 4 knots as a useful speed then a 2kW petrol suitcase generator small enough to keep in a locker would charge the batteries at about the same rate that you’re using electricity, getting you home if the wind died mid-Channel.
Electric motors are incredibly efficient, but you also don’t seem to need as much power as a direct conversion of horsepower to kilowatts would suggest.
3kW is about 4.5hp, and we can achieve 5 knots with 3kW.
We’re actually going to change the 10kW pod for a 6kW pod, as there are no circumstances in which 10kW does any more than 6kW, just as you very rarely use all 20hp of a diesel engine.
And the 6kW pod is smaller (less drag) and lighter, so there are incremental gains there too.
This disparity between rating and power delivered could be due to a couple of different reasons:
- The torque is instantaneous so you can turn a propeller of a larger more efficient size for cruising that would stall an equivalent power diesel at idle.
- The propeller itself, in a pod installation, is vertical rather than at an angle as on a shaft drive, converting all the power into forwards rather than upwards thrust.
Another advantage when manoeuvring is the complete controllability.
You don’t have to knock it in and out of gear at idle, you can run it at 50rpm if you wish.
The lack of noise and vibration is fantastic.
Similarly, motoring downwind in light airs with a Sunday morning hangover, there are no diesel fumes in the cockpit, and there’s no oil in the bilge.
What’s not to like?
The components we fitted are as follows:
- Epropulsion 6kW EVO pod drive, £2,500 (inc VAT)
- 2x Epropulsion E-175 9kWh lithium batteries, £3,800 each
- Epropulsion 30A mains charger, £400
- Remote control throttle, £250
- Miscellaneous connection cables, £300
- Total system cost £11,050
The range figures quoted are with two batteries as above – a perfectly viable system could be rigged up with one battery (and half the range) at about £7,000.
The charger will charge each battery if completely flat in about six hours, or two could be rigged in series to double the charging speed.
Recharge cost of the complete 18kWh system is approximately £2!
For transparency, Ian’s company is an Epropulsion supplier, but the boat, and the costs, are his own.
Alan Kohler: ‘It’s horses for courses
Boats come in a wide range of sizes, as do our cruising aspirations, from marina day-sailing to high latitude exploration.
The suitability of electric propulsion technology depends largely on these two factors.
For dinghies, electric outboards are proven and attractive solutions.
Inland waterways craft with low speeds and easy access to shore power are also ideal candidates.
And the technology is advancing quickly.
Recent trends include electric drive options on many day-sailing boats, which only need power to get in and out of their berth.
The ‘maths’ gets more challenging for larger coastal cruising yachts on longer passages, anchoring ‘off-grid’.
So what are the sums? Roughly:
- 1 litre of diesel (0.83kg) = 2.9kWh net engine equivalent (allowing for engine efficiency)
- Thus the energy density of diesel is circa 0.3kg.kWh
- The best car batteries (eg. Tesla) are circa 6.6kg/kWh
- Hence net energy density of diesel is 23 times better than current battery technology.
Additionally, electric engine ranges are typically quoted in flat water, with no headwind and no tide – how often does this equate to your typical cruise?
Many boat owners use their engines more extensively, motoring into strong headwinds, plugging round tidal headlands or rushing to get back for work on Monday.
Power consumption rises sharply, and range plummets in these circumstances.
The case study above shows electric power can now be considered for smaller yachts, if the reduced range and power fits with a primarily local cruising style (or using a generator as a DIY hybrid solution).
The 18kWh battery equates to just 6.2 litres of diesel (compared to the 70 litre tank).
My X-Yachts Xc38 is typical of many larger yachts, with a 55hp (41kW) engine and a 200-litre (166kg) diesel tank, giving power to handle adverse weather and a range of nearly 400 miles – handy for cruising the Hebrides or crossing the Bay of Biscay.
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The equivalent sized battery would be 580kWh, weighing 3.8 tonnes.
Clearly this would be too large, heavy and costly to be practical.
This is where hybrid solutions start to become attractive, perhaps using a generator or fuel cell for longer range with a more modest battery bank.
Indeed many blue water cruising cats are now offered with hybrid drives, their speed aiding power regeneration.
Adapting cruising style also helps.
Perhaps we need to drop the fixation with ‘6-knot passage planning’?
Realistically, the cost of large battery packs, drives and all the other systems mean that most of us are unlikely to be able to justify replacing the diesel engines in our existing cruising boats for some time, though hybrids are an interesting option for new boats or major refit.
Don’t get me wrong, I am excited about silent, low emission electric power and hope to have an electric motor to replace my 55hp Volvo when it reaches end of life in 10-15 years.
Also I will likely buy electric to replace my dinghy outboard.
We should, however, avoid rose-tinted spectacles when assessing whether an electric system will suit our boat and cruising plans.
Enjoyed reading Would you switch to electric propulsion?
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