Latest Thoughts on Diesel/Electric Propulsion

Some updated thoughts on diesel/electric propulsion…
 
According to the internet, more sailboaters are switching to electric propulsion. You will find “ground up” DIY systems from off-the-shelf components (thanks to the growing DIY electric vehicle movement) to full “turnkey” installation products.
 
Now, I am not about to debate electric vs diesel propulsion. But since I started following this topic a few years back, one hurdle has remained when considering this Westsail build: a properly sized electric motor. Many of the sailboat installations tend to be catamarans or mono-hulls under 35 feet (with a modern underbody). Many of the advertised electric propulsion motors just do not have the necessary power for a full keeled heavy displacement mono-hull like the Westsail 42.
 
One difficulty is one cannot directly compare a traditional diesel engine (typically rated in HP at nominal RPM) to an electric motor (often rated in kW), even when converting units. This due to how each engine type delivers torque. An electric motor’s torque curve (vs RPM) is fairly flat. A diesel’s torque curve is a hump peaked (ideally) around peak HP at a certain RPM. This lack of torque at lower RPMs is why diesel engines need a reduction gear (transmission), otherwise risk ‘stalling’. A properly sized electric motor should not need a reduction gear. Undersize an electric motor and you risk overheating (a reduction gear would help, but at the cost of reaching hull speed).
BETA Marine 75 Diesel engine. A candidate for powering a Westsail 42.
 
Proper comparison calculations (and there are many examples on the web) must consider other factors: LWL, prop-pitch, displacement, etc. But a general rule of thumb is, for converting to electric, an electric motor roughly half the “power” (in HP) of an otherwise properly sized diesel engine should be enough to drive the boat to hull speed.
 
Having said all that, it appears the Electric Yacht people may have just the system that would be a good match. Specifically their “QuietTorque™ 45.0 LC – 72Vdc Liquid Cooled Plug-N-Play System”.
 
QuietTorque 45 LC
 
They have even larger 96VDC systems. Electric propulsion for this Westsail 42 is more viable today!