Anonymous wrote:
"Efficiency"............. What does that actually mean? Do you get more mechanical power per watt? 1 horsepower is approximately equal to 750 watts
The question should be where do you measure that "power"? The proper place to do this is how far forward does a boat with a certain mass move in a certain amount of time. Motor input power, motor shaft power (after electrical losses) and propeller shaft power (after bearings, reduction, etc.) do not even start to tell the story. All of these things have a long history of being the best compromise for an IC engine which has 0 torque at 0 rpm and has to be running at some speed to have any torque at all. An electric motor has maximum torque at 0rpm and that torque remains constant to a set speed after which it starts to decline at a rate that gives constant hp. It has more in common with a steam engine.
So an IC powered setup will look for the rpm within it's best range of torque and power which is too high for a propeller to run efficiently. So a gearbox is added to reduce the prop rpm but that gearbox looses efficiency as the reduction ratio goes up so the propeller runs faster than best efficiency to counter this. So it is a smaller radius and smaller pitch too. So when installing an electric conversion, the boat owner is stuck with the size of the propeller aperture the boat is designed with and the companies who make these units in general use the same running gear and a gearbox (often with belts) to make the best use of the stock propeller.
Getting the best efficiency out of an electric motor would mean redesigning the boat with a bigger aperture for a bigger propeller with a larger pitch probably. Though an adjustable pitch propeller would be best and, in the same way an mppt charge controller maximizes solar charging, the motor controller should do the controlling of the pitch for best efficiency. There is another use for variable pitch:
To me the beauties of electric on a boat are regeneration.... The ability to store and use solar.
The best regeneration will happen at a different pitch than the best motive force.
So the best electric motor for a boat is one that gets it's power at the best propeller speed which is slow. That is, torque rather than power is the best measure. A higher torque at the same input power at a lower speed will move the boat faster or farther, maybe both (remembering that boat movement over time is the true measure of power). The problem with this (time for another compromise), is that high torque motors that run at slow speed are big. Bigger in diameter and bigger costs more and weighs more too. There are a lot of 10kw motors around that are (relatively) cheap, small (easy to fit in various places), light enough for a person to lift, etc. Torque is not great, but pretty good and with a speed conversion, they work well with currently installed shaft and prop.
The other HUGE benefit IMHO is the ability to entirely eliminate through hulls and motor shaft by using an outboard, which also means you can lift the prop entirely out of the water gaining a huge efficiency advantage over a folding prop with their abysmal efficiency. In an ideal situation there would be zero through hulls below the WL.
An electric outboard can be done in two ways, slap a motor on an outboard leg or put the motor in the water to direct drive the prop.
Most small drives put the motor in the water (most really big cargo ship size drives too). It is simple, there is no transmission loss, cooling is all around, the unit can be designed with no preconceived IC constraints. However, the motor does present more frontal area to the water flow which is a loss. The higher the torque, the more frontal area there is.
electric sail-drives, put the motor inside the boat and a gearbox in the water. Because the motor shaft is vertical, the width can be larger and more torque applied. Also, the whole unit, being of one design, can have a larger prop with adjustable pitch. (see some of the "Uma" videos over the years)
Outboards using a lower gearbox are more constrained by overall motor size (so it can tilt up or be put in a motor well) and so will have a less efficient torque to play with. Also, they are all (that I know of) based on IC legs which add frontal area for things like exhaust and water pump stuff as well an exhaust hole either in the middle of the prop or elsewhere that while adding to forward movement with exhaust flowing, create drag otherwise. Some custom solution would be best.
Probably way more than you wanted to know... sorry.
The part you wanted, higher voltage vs. lower voltage all has to do with wire size. Not just in the wiring outside the motor but inside as well. It allows windings to be closer spaced for smoother running at low speed to make the best use of torque. (the quick version)
Engines and their cooling systems, transmissions, control systems, prop shafts & glands and cutlass bearings fall in with standing rigging and complex steering mechanisms as maintenance intensive trouble spots to be eliminated. Outboards with regeneration are available.
This is all true. The truth is that even though electric motors for boats are in their infancy, they still do quite well. I would say that says something about IC drives.