Motor considerations
One of the main advantages of an electric drive is that it doesn’t need a gearbox, so virtually any type of electric motor can be coupled directly to a propeller shaft. However, to gain the full advantage of e-propulsion, there are certain factors to consider:
Voltage
The usual voltage for a sailing yacht installation is 48V DC, giving plenty of torque but unlikely to prove lethal if you get a shock from an open circuit. It’s easy to step this voltage up or down via inverters or DC converters for 240V mains and 12V appliances on board.
Regeneration
Some DIY-sourced motors, such as from scrapped fork-lift trucks, work fine for propulsion but can’t be used for generation. For full regen, you need a motor type that can also operate as a generator, especially if you wish to harness a freewheeling propeller.
Cooling
Electric motors can run hot, which reduces their efficiency. Many have a simple cooling system already installed that will need hooking up to a suitable skin fitting. The saildrive units tend to have all this already built into the leg.
Electric prop
With the torque available usually less than from a diesel, the role of the propeller becomes more critical. Companies such as Darglow, Gori and Bruntons all have feathering propellers that lend themselves ideally to a combination of electric drive, motor-sailing or regen. Bruntons’ Ecostar has been designed specifically for electric boats. The blades pivot outwards with a variable pitch, automatically adjusting for the forces in play. The Oceanvolt Servoprop does much the same thing, adjusting the pitch in response to the power input and the water speed to give the optimum blade angle. When in regen mode, the blades open in the opposite direction to harvest as much energy as possible, but only when there is sufficient speed to negate the drag.