IS ELECTRIC PROPULSION VIABLE?

The automotive industry is leading the charge in the move from internal combustion to hybrid to all-electric vehicles. Are all-electric vessels a feasible follow-on? Well, maybe.

Diesel-electric hybrid vessels have been around for over a decade – Greenline launches and sailing cat manufacturer Lagoon are among its early adopters. In both cases the electric propulsion is auxiliary to the main (diesel) engines.

Configurations vary – from diesel gensets powering electric motors directly, to a battery bank charged from the main engines or gensets.

To my mind these offer the worst of both worlds. You have the significant added weight and cost of an entirely separate electric propulsion system sitting alongside a perfectly serviceable diesel set up, sharing the shafts and props.

The sales pitch would have you believe that, at manoeuvring speeds, the joy of gliding through the marina silently, emitting no exhaust and virtually no vibration is worth the extra dollars and weight.

There’s the ability to go all day long at, say, six knots on a single charge or, if you pull back to four knots, solar panels on the roof will keep up with the discharge rate.

That’s not really viable for a vessel that can cruise under power in the mid-teens.

ENERGY DENSITY

The problem with batteries – even modern lithium models – is they can’t match diesel’s energy density. Energy density measures of the amount of energy in kilowatt-hours

(kwh) per kilogram. The lower the energy density, the more batteries (weight) you have to carry, and more weight means more drag.

Modern lithium-ion batteries have an energy density of around 10 to 12kg/kwh. So, a vessel that at a given speed requires 250hp at the prop will need around 185kw. With an electric motor’s 90 percent conversion efficiency that equates to 205kw.

To run at that speed for one hour would require around 2.5 tonnes of battery. Compare that to diesel, which gives approximately 10kw per litre, albeit at 25 percent efficiency by the time you take mechanical and combustion losses into account. That indicates 82 litres or 70kg for the same speed and running time.

Not exactly an even match, is it? And that’s even before factoring in speed and drag. If you double the speed the drag goes up four-fold. By increasing drag you reduce range – doubling the drag halves the battery life.

REFUELLING

This is another conundrum for those considering switching to electric vessels. An electric vessel needs an appropriate shore-based charging infrastructure – one that matches battery technology to the speed of recharging as well as depth and number of charging cycles. While advances in lithium-ion technology have reduced recharge rates significantly, the batteries aren’t cheap.

What about solar panels? Well, even the best panels put out around 200W (0.2kw) per square metre. So if you had a 10m x 3.5m solar panel array on your 12m launch’s flat roof – this is being optimistic – the 35m2 array would generate 7kw at best. That would run a decent fridge/freezer and all the Tv/audio, but would take at least 30 hours (of daylight) to re-charge our 205kwh battery bank.