‘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 opposite 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. 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.