Slower bigger props don't lose as much energy.
Not quite correct. I think I know what you are trying to say though. Slower bigger props have less slip in the water. They are more efficient at turning rotary motion into forward motion. Not True about being more efficient though. They take a heck of a lot more energy to turn them. There is mass to take into account, but that part is not as important as surface area. The more surface area, the greater the friction the prop encounters in the water. Thus harder to turn. Slowing the speed down lowers the resistance, but it also means the pitch
has to be increased to compensate. It is the pitch
that governs the forward motion, not the prop size or blade No. of course.
So all in all, propelling a boat hull
forward takes ruffly the same amount of energy, not matter what. It is why a rule
of thumb exists of Hp/lb(Kg) of boat. All calculations should be worked backwards from that. Of which I find most alternative solutions do not do. It should be...energy required to propell boat of X weight at X speed, with X margin of reserve energy for safety
factors. Then the propulsion
system power requirements should be worked out from there. The industry seems to be going the opposite way. ie. smallest amount of power required to push boat along equalls enough.
once you start comparing older engines or new older style engines you start to get parity or advantage to diesel electric.
There is a small advantage in newer engines. But it is not that big. There is a fact of diesel that a given amount of fuel
produces a given amount of power. No matter what age the engine design. What is true is that polution has been greatly reduced, so we have cleaner burning. To obtain cleaner burning, better control of the fuel
dose has been needed. So with better control, slightly less waste of fuel has been obtained. Slightly less weight equates to less fuel used.What the big advantages have been is better power control. Torque bands have been able to be produced over a wider range and flatter respons. This also equates to better efficiency. But the difference in fuel use over all is very small. Once again, the real big saving is in using the smallest motor
you can get away with. But once again, I think this is false economy.
Most motoring around the docks or harbour you will do under battery power alone.
True in some sense. But lets say you have a 10Hp electric motor
. Lets say you take 1/2hr from marina to open water sailing. And 1/2hr from end of sailing to back to marina again. So a total of 2x1/2hr is 1hr and thus 7.5Kw of energy. That is going to take some recharging using solar
and one heck of a battery bank and Genset to keep up. So what is saved in fuel from the Diesel engine propulsion is taken up in generating.
I guess what I am trying to say is, yes there are savings of efficiency in electric drives. Yes there can be advantages with it. But the small savings are far outweighed by other factors, such as additional devices(solar panels
gennies etc), the huge weight and expense and short life of a very complex battery bank system, the comlexity of charging
that bank, the shorter life span of electric drive and the so far greatly reduced power required to drive the boat.