Auxiliary power system

[Stormboy]

Looking to do some long distance cruising in a 60 tonne, 65'
boat, and trying to work out the best way to provide an
emergency back up propulsion system, for my 6 L3B Gardner.
The Gardner is in great condition and incredibly reliable, but I have a belt and braces approach where safety is concerned.
I have considered an
-electric drive ( outboard barge type drive)
-dual outboards on my 17' aluminium skiff
-smaller diesel as a donkey engine above the prop shaft.
They all have pros and cons. but, I prefer an external system that will work even if the prop/gearbox or shaft is compromised.
Sail power is not possible, as I have no room for a mast.
I have 4 kw of solar panels on my cabin top, and around
3000 ah of AGM battery storage.
Looking to the brains trust for possible solutions, and better ideas than mine.
Many thanks in advance for your input.......

[Lepke]

Twin engines are the easiest.

There are companies that make aux setups that drive the shaft. Some using hydraulics I have seen. There probably are ones for electric.
It's not practical to be prepared for every possible failure. You don't loose the engine, trans, shaft, and prop all at the same time. If you do proper maintenance and inspection of the prop, tail shaft, and rudder, the problem is least likely there. Having engine spares, pumps, injector pump, injectors, allows the most common engine problems to be repaired at sea. Don't depend on an engine that is dependent on electricity.