Engine
Diesel, diesel-electric or electric
The choice could probably be made by comparing the cost and the cruising style: motor-sail vs. sail. I'm leaning towards sailing most of the time and using the engine in emergency cases only.
Diesel
Some disadvantages of diesels are:
- low torque at low rpm, therefore small propeller has to be used
- running diesel at low rpm for extended periods when charging batteries
has negative impact on engine life expectancy due to:
- low temperature, which causes water condensation
- low oil pressure, hence poor lubrication
- low load, which causes extensive soot (carbon) creation and its deposition on moving parts increases friction and accelerates wear
Diesel-electric
Diesel-electric seems to be a good choice because the diesel provides power to run the electric motor as long as the tanks are full and off-loads batteries. Therefore, no need to worry about a huge bank of batteries running in parallel to keep the current under 40% of the bank's capacity to protect the batteries from overheating. The other advantage is that, the whole boat could be electrical: cooking, refrigerating, making fresh water, running air compressor for scuba, having an electrical windlass, etc. - all the luxury stuff.
The drawback is of course, the complexity of all the systems associated with having a diesel.
Electric
Pure electrical besides carrying lots of batteries, has minimum motoring time but is the simplest system of all. If it is used only for emergency maneuvering for a short period of time, then the batteries could be topped up by using wind, solar and hydro sources without the necessity of shore power charging. Therefore, if I choose sailing and anchoring vs. motor-sailing and mooring style of cruising, then most likely I will go with the pure electric motor.