Is there any reason why I cannot hook an electric motor direct to a BW velvet drive?

[tomsnrg]

There are some checks to do beforehand. The maximum torque needs to be compatible, electric motors can be much better in this compartment. Some leeway is possible, you are not dealing with a crane.

if you want to reverse electrically check whether the gearbox can handle it or has any freewheeling equipment - if you do not reverse electrically you will need to handle the control mechanisms for gearbox and motor inverter accordingly.
Biggest drawback for me is noise from the gearbox that you never heard over the running engine but will be a huge nuisance when going electric. You may have also relevant mechanical losses.

You may want to check the real power requirement of the hull and dimension the electric propulsion accordingly, the prop may also need to be changed for better efficiency.

[redneckrob]

Originally Posted by David Rainbow

As to the question of constant speed, A vessel could still be controlled by such a single speed unit with the addition of a variable speed propeller. Up until very recently a Island Shipping company based in Cairns Australia used a container vessel with many power hungry refrigerated containers consuming a great deal of their requirements from a shaft alternator. The vessel thus maintained a constant shaft (RPM) speed even whilst undertaking manoeuvring tasks like docking. Being controlled by propeller pitch and rudder with a short burst of forward tunnel thruster to straighten her up once alongside.

Yes, a variable pitch prop does allow for constant speed. It also requires a significant increase in complexity and would require the OP to remove and replace the entire drive train (usually the pitch is changed through oil pressure fed through a special hollow shaft), which I think is the opposite of what they were trying to accomplish by leaving the transmission in. I have never heard of someone doing this on a small, older sailboat where the variable pitch part of the project alone would probably cost more than the boat, but I suppose if you had a lot of time on your hands and loved to tinker it could be an interesting project.