Of course it CAN be done, if your Westerbeak's tranny will support it. The simple method would be to add a pulley of appropriate size onto the shaft between the transmission and the stuffing box, and build a mount for the motor that positions it over the shaft, with a pulley on the output shaft of the motor. Typically you would have a 2:1 reduction ratio but careful engineering would have you optimizing the ratio and not just going, "well, 2 to 1 oughter doit". A chain works but its messy. You want a quick way to reduce tension on the belt to take it off, for when you are just running your diesel only. When the belts are on and the diesel is running, the electric motor can charge the batteries. When the belts are off and the diesel is running, its as if the electric wasnt even there. When the diesel is not running and the belts are on, if the transmission is in Neutral, you are in pure electric mode. And maybe, just maybe, the motor could be used to start the diesel, I don't know. The trick is matching all the power levels and RPMs and reduction ratios for optimum efficiency and motor life.
The most elegant method would be to have a slow wound motor actually in line with the shaft, direct drive, with the motor shaft sticking out both fore and aft so it couples between the prop shaft and the engine. The controller would either load up the motor as an alternator or reduce regen to zero for minimal loading of the diesel. With the transmission in Neutral the electric motor would either be spinning in regen mode, or pushing the boat through the water on battery. I have not seen or heard of this setup being successfully implemented yet. There are not a lot of motors to choose from, actually, that present both ends of the shaft outside the casing and operate well under load at such low speeds. The offset hybrid type in the first paragraph has achieved some success, I believe, and there are actually turnkey systems that work like that.
decoupling the diesel altogether and using it only as a generator could also work. And yet another way would be to fit one of those nifty little electric pod drives, and leave the diesel, shaft, and prop alone. Or maybe switch to a folding prop on the diesel.
FWIW, I opted to remove my infernal combustion engine and mount my electric motor via a 2:1 Baldor enclosed reduction gear using the original prop. I am pretty happy with the general layout though I did recently decide to upgrade, and I am in the process of that upgrade now. My 4201 motor and Kelly KBL48301 controller at 48v was not quite getting me up to hull speed and anyway I just wanted to try a more powerful system. The simplicity and space/weight reduction to me make up for the lack of redundancy. I got the old controller and motor for spares, anyway.
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