It’s no drag

Gideon Goudsmit of African Cats explains to BB that electrical energy is generated and stored in lithium-ion batteries giving sufficient energy to be used for propulsion, hot water, cooking and all the other electrical equipment onboard. When batteries are fully charged or high speed is required for racing, the propulsion system is lifted in its entirety, eliminating the drag and increasing the speed of the yacht between 5% and 15%.

It came about from looking anew at the basic design. Its development meant a few tricky issues had to be overcome, explains Mr Goudsmit, but actually the resolution gave the boat some “interesting” advantages.

“On preliminary investigation, we immediately found a problem with available electrical propulsion systems; an electrical motor has a very high torque, the draw starts at the point it starts turning over, and further, when fitted to a boat it also needs a large propeller to work efficiently.”

However, this, in turn, creates its own difficulties as a large propeller on an electrical system acts as a brake in the water. To get round this, African Cats decided to develop a retractable propulsion and generation system – and the benefits of the novel approach began to add up.

In the flow
To start with the obvious, the actual sailing speed increases because no sail drives, propellers, props shafts or shaft supports are in the flow of the water. Not so obviously, this allowed locating the motors closer to the centre of gravity, making the yacht safer and more comfortable.

The - virtually silent - electric motors are usually located in the water under the bridge deck or - in the case of a monohull - under the boat, so noise is minimal. Overall weight is substantially lower compared to the normal electric drive or diesel propulsion system too - and the maintenance, fuel and downtime are almost nonexistent.

At its heart, Green Motion made use of the Motogen, an electric underwater motor that functions as a generator when sailing and can be retracted when batteries are fully charged.

It’s a lightweight and extremely efficient propulsion system – Mr Goudsmit is insistent it's the best-ever so far designed for yachts and adds that extensive testing has been done with a "proof of concept" sailing in excess of 10,000 nautical miles.

Certainly it’s flexible, and can be fitted in all sailing yachts from 30 to 90ft whether monohull, catamaran or trimaran construction – the only difference being that installation is inside in a monohull or on a retractable arm for multihulls. While predominantly for new-builds, it's possible to retrofit the system fairly comfortably.

Three different Motogens are available, ranging in power from 10 to 30kW (giving 25 to 100hp) and covering yachts from 30 up to 90ft.

Mr Goudsmit adds the electricity production on board is sufficient to power navigation systems, cook, make hot water and motor the boat in case of harbour water movement or a drop in wind.

Perpetually mobile
Further, a boat fitted out like Green Motion could be almost be perpetually mobile as, in addition to the power from the Motogen system, it also has solar panels and a wind generator to supply extra energy when at anchor.

The company found fitting a 100 - 500W wind generator was useful: the advantage in mounting the unit on top being threefold: less noise; higher efficiency and a good sized wind vane. Added to this, highly efficient solar panels can be mounted on the (bimini) roof to help keep the batteries charged when moored.

On top of this, a 150V DC water heater can be included in the system, cutting out the necessity to change over to 12/24V DC and then to 110 or 220V AC, reducing energy loss from the conversions.

There are other benefits. Because the shaft is connected directly to the motor on one side and to the propeller on the other side, no through-hull bearings or P-brackets are needed, so water resistance comes down.

On the propeller side you get a cost saving because expensive folding or feathering props are no longer needed – and maintenance is generally reduced.

Grounding is very limited so prop anodes will wear very slowly if at all, and because the propellers are out of the water they don’t corrode as much. In any case, the motor housing is made from corrosion-proof aluminium 6082 and coated in Coppercoat so if you forget to lift the motors there will be limited growth on the motor housing and strut. On top of this, the propellers can be cleaned of fouling very easily.

The systems are independent: when under power from one of the motors or charging a single Motogen while sailing, the other side can be lifted to decrease resistance.

Prop placement
The specially designed, large four-blade lightweight aluminium propellers have a higher efficiency than the normal folding or feathering props. On a twin prop installation they are counter rotating, so there is no drag induced “wheel” effect.

Further, more efficiency is gained because the props are not mounted close to the hull or in the wake of the keel, but at a three foot distance - so there is a better water flow. And this means they also are kept at a distance from the rudder, which in turn gains in efficiency – the whole set up has the effect of reducing vibration.

The electronic controllers are seawater cooled with an aluminium heat exchanger to increase efficiency, keeping the noise and heat to a minimum.

Maximum and minimum rpm settings can be preset in order to use the least energy or to charge most efficiently, with charging normally starting when the motor reaches 400rpm. But there’s a big curve to the trade-off between pace and energy, so the maximum settings are best tailored to the hull.

Mr Goudsmit says: “The maximum propeller rotation is usually set to a little over the hull speed. So, for instance if the maximum hull speed is 9kts and it takes 1,200 motor rpm to get it there, the maximum rpm setting is 1,300.”

He explains: “It is a waste to set it any higher since, given this example, 1,400rpm will only let you have another 0.2 knots extra - but the energy consumption will go up by 40%.”

Easy to see
However, keeping an eye on the whole thing is fairly easy since an LCD screen shows monitor temperatures of the motor and controllers, motor rpm, torque, electrical consumption and generation. Happily, the system has built in safety for overheating or stuck propellers, if overheating occurs first the motor output is lowered, and if the situation continues it switches down until cooled. The same is done with the controllers.

The batteries used are the Mastervolt lithium  ion with a total weight of 270kgs for 25.8kW of usable power- or five hours at a speed of 6kts. (This is not taking into account any recharging taking place from the wind generator or solar panels).

Finally, it is futureproof. “Since the system is all electric it is ready for future hydrogen generators when these become financially affordable,” says Mr Goudsmit.