Friday, 18 April 2014

Electric Lagoons

Lagoon starts rolling out versions of its main selling models with twin electric motors instead of the usual Yamnar diesels

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When a catamaran production company as big and as mainstream as Lagoon starts rolling out versions of its main selling models with electric twin electric motors instead of the usual Yanmar diesels, you have take note. It means that people with a big stake in the yacht market believe that electric propulsion technology is mature, it is affordable and there is a market willing to buy into the concept.

12 HP Electric wheel installed
12 HP Electric wheel installed

Anyone interested in giving the system a thorough testing could do worse than charter WAYPOINT, the first Lagoon 410 S2E (electric) built – booking through The Catamaran Company in the US. In November 2003, this cat made the first ever Atlantic crossing of a production cat with electric propulsion during delivery to its owners and its charter destination in the BVIs.

The twin electric drives going into the Lagoons are 12 HP STI 74 "Electric Wheels" developed by Solomon Technologies Inc., Florida, US. They replace the much higher rated twin 28 HP diesels. Still, WAYPOINT is said to do 10 kts with the STIs because of the greater efficiency and constant torque of the electric configuration.

Electric propulsion has compelling attractions, besides the kudos of winning one’s “green” credentials. How much more enjoyable to slide silently away from moorings and out of the harbour in silence, without vibration, and with no smelly exhaust fumes fouling the air either.

However, unlike fossil fuels that can be kept in low-tech tanks, electricity is difficult, heavy and expensive to store. WAYPOINT’s twelve 144 volt batteries hold enough charge for 4-5 hours motoring, compared with around 17 hours on the standard Lagoon 410 with diesels and tanks. With the STI system from Solomon there are two ways to replenish stored electricity - other than retuning to the berth and plugging in to the mains.

If there is little wind Waypoint must bow to necessity and run the 15kW diesel generator to recharge her batteries. With good winds the STI system can regenerate power through the spinning prop. The regenerating system is a key technical advance of the STI system, although of course it causes significant drag on the boat. Developed to a high degree of sophistication by Solomon Technologies, it significantly increases the attractiveness of electrical power systems on a sailing boat. With the throttle off the STI system uses the power in the ‘wind-milling’ pop shaft to recharge the batteries. It takes about twice as long to recharge the batteries under sail as it does to drain them while under power. So motoring out of the harbour, sailing for four hours, then motoring back in for a hour would result in no overall change in the charge in the batteries.

Inside the Electric Wheel
Inside the Electric Wheel
Electric Wheel, Solomon Technologies
Electric Wheel, Solomon Technologies

Regeneration power can be taken to another level since the SDI system continuously monitors the load on the propeller and automatically switches from power to generation mode. By tweaking the throttle a point can be found where power is generated in gusts and consumed in lulls. This even works on the surges sailing over waves, so an average speed increase of typically one knot can be achieved with no net power consumption. Hence the newly coined concept of ‘regenerative motor sailing;’ a sexy idea, since though the speed increases achieved are modest they are ‘free’, silent and odourless.

To give a reasonable comparison of electric and conventional diesel engine propulsion, we have to consider the whole domestic and ship service systems in each alternative.

Example conventional system for a cruising cat:

2 x 28 HP sail drives; motor starter battery; fuel tank and fuel; 5 kW generator set and 12 volt batteries for domestic and ship services.

For equivalent electric driven cat:

Batteries and main distribution box for electric d...
Batteries and main distribution box for electric drives

2 x 12 HP electric drives; 15 kW generator set; fuel tank and fuel; 12 144 V batteries for the propulsion system and 12 volt battery for domestic and ship services.

Once the main batteries are fully drained in the electric drive system, the 15 kW generator can drive the boat at full speed as long as diesel lasts. If part of the generator power is used for domestic appliances, the boat will run slower.

There is the option to run the system as hybrid, or completely green, and a generator is not really necessary at all, though not having one reduces the options and risks of running out of power. The 144 V batteries can power domestic AC and DC requirements, all the energy coming from regenerative power and shore recharging (energy from the grid is of course largely not green).

The weight of the electric drive system is more easy and efficient to distribute in the hull, since the drive unit is small and the batteries can be distributed somewhat flexibly. It is possible to divide them between catamaran hulls. Comparing the whole weight of electric and diesel systems, including all batteries, generators and fuel has the electric system coming out either heavier, lighter or the same, depending on assumptions of fuel and so on. Not surprisingly, since the technology is not being manufactured in high volume, the first cost of the electric system, including installation is higher than the equivalent diesel one, by about 30%. Having said that, prices are very likely to come down with time.

Another advantage of the electric system is having ‘instant power’. There is no need to wait for the engine to warm up; there is no gearbox to engage. Just turn on and go. Instant reverse is available too, one can go from full power forward to full reverse in an instant for a very abrupt emergency stop.

The STI system is virtually maintenance free, since there are very few moving parts. The only regular maintenance requirements are changing the shaft bearings every 100000 hours and the batteries every 3-5 years.

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