I don't have any good links for you, but I considered doing the same thing before pulling my old atomic. There should be a couple of feet of shaft exposed between your transmission and stuffing box. So the simple way would be to key a pulley onto the shaft, fabricate a motor mount that places the motor above the shaft and keying a pulley onto one end of a stub shaft ran through a bearing and keyed to a small lever operated clutch, similar to what you might use for an engine driven wash down or bilge pump, or reefer compressor. The other end of the clutch coupled to the motor. So the motor drives the stub shaft with the hand clutch, which spins in the bearing and drives the belt or belts via the pulley keyed to the end. The belts drive the pulley mounted on the prop shaft. And vice versa.
With the motor clutch engaged and diesel trans in neutral, motor can turn the shaft or be turned by the shaft, and only has to turn the declutched output stub of the trans and not the engine. With motor clutch disengaged and diesel in gear and running, diesel turns the shaft but not the motor. With diesel turning the shaft and motor clutch engaged, engine turns shaft and also turns motor. Most motor and controller combinations allow regeneration which is simply driving the motor by mechanical means and using it as a generator. So the diesel turns the motor and this charges the batteries.
I suppose in theory you might even be able to use the motor to start the diesel should the starter fail. If it can be hand cranked by a human delivering 1/3hp then it should be possible to do it with a 6hp motor... but I am kinda just speculating on that.
The big PITA of this setup is having to decouple the shaft to replace the belt. A spare or two could be kept secured around but not touching the shaft, for quick changes.
The use of belts and pulleys allow experimentation and customization of the reduction ratio.
It must also be determined whether the diesel transmission can serve as the shaft thrust bearing while in neutral. Don't assume... verify.
Another method that has been used is to couple the transmission output shaft to one end of a motor shaft, and the other end of the motor shaft coupled to the propellor shaft. This of course means the motor must be run in direct drive mode, and be suited to running at low speeds under load. Also most of the motors in common use do not have access to both ends of the shaft. This method has potential except for the lack of suitable motors.
Simply moving the diesel and coupling it to a suitable generator while the electric motor alone is coupled to the shaft would be a very good option if long range motoring is seldom done. The inefficiency of using the diesel to turn a generator to power a motor to turn the shaft are obviously greater than having the same diesel coupled to the propellor shaft, but if the battery bank is big enough to hold enough shore power charge for most trips, then it isn't an issue. And you would have a powerful and dependable genset standing ready to pick up the slack. If there is significant solar charging capability then even better.
You could just add a outboard motor mount at the transom, and buy or build an electric outboard. A big trolling motor would work for a smaller boat. By big I mean 36v+ and 100lb+ thrust. The star of the show here is probably the torqueedo but I suspect a diy 5kw electric outboard based on an old mass produced outboard with a bad engine might be more suitable. YMMV. Anyway, electric on the transom leaves the diesel installation undisturbed except for installing a modified alternator for charging your 48v or 36v bank. You would want to store the outboard below on an ocean passage, obviously.
Most of us just pull the old infernal combustion engine and go with a motor only. Solves a lot of issues. This works especially well for a sailboat, where range under power and battery weight are not problems. When my fuel tank sprang a leak I was only too happy to get rid of that noisy smelly gas sucking smoke belching mechanical monster and go full electric. No more bleeding knuckles or mashed thumbs from playing Mr Goodwrench, either.
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