Good questions. Short answer, a larger motor will not use less amps to drive your boat at a given speed. It will probably not run cooler at the same given amperage (different motor, different design, who knows?).
My 30' 10,200 lb Bermuda 30 ketch uses 5.1kW (measured as draw from the batteries) to motor at 6 kts at full throttle. At 5kts (2500W), the motor windings of my Propulsion Marine 5.5kW drive never get to 80C, Motenergy says the max temp for the ME913 motor is 150C. At full throttle, the 6.8hp of power going into the electric drive powers my boat 1/2kt faster than the old 12hp diesel could push the boat at full throttle. At 28'and 8000lbs, do you think you need more power than that?
Coincidentally, I believe that the Motenergy ME913 is the motor in Electric Yacht's "10kW" drive. In my opinion, that is a little over-stated. The motor is rated for 125A continuous, so a typical 48V system maxes at 6.2kW continuous. Moving up to 72V will only get one to a 9kW continuous rating, so I read 10kW with a grain of salt (yes, I know that they state 10kW as "maximum current" but what does that really mean?) . Even so, the Propulsion Marine drive in my boat that uses the same major components as the EY system was conservatively rated at 5.5kW on purpose to allow for less than optimal cooling in the bilge of an auxiliary sailboat.
With all that said, I believe that using peak current ratings (1 minute rating or 5 minute rating) is misleading in the marine market. In a car or motorcycle, peak ratings make more sense (when was the last time that you drove your car for 5 minutes at full throttle?). But we "set and forget" boat throttles, running for long periods at 90% or more (gas and diesels) or 40-50% (electrics). Nobody wants to hit the throttle to combat a strong current and have the motor do a thermal shutdown 4 or 5 minutes later. Since we operate our drives in a continuous mode, we should only be looking at continuous ratings. Please note that the most important single specification for sizing an electric marine drive has proven to be displacement. Length of the boat (LOA) does not appear to have much of a direct correlation to how much power you need because LOA doesn't say much about the boat's design.
Back to the group's recommended ratings, 1kW per ton of displacement will power a typical displacement boat to 90-95% of theoretical hull speed with a properly matched driveline. You can put in more motor, but it really doesn't do much besides raise your cost. There are some in this group that believe that a larger motor is actually less efficient being driven at a lower percentage of it's design, though I believe that may be more theoretical than practical.
Going back to my boat, the ME913 (remember that Motenergy says 125A continuous) will push the boat to 95% of the theoretical hull speed at 106A as long as I've got power and it will never overheat. With 8kWh of Lithium, that would be 69 minutes from a full charge to 80% depth of discharge including losses to Peukert's Effect. If I doubled my battery back to 16kWh (320Ah @ 50V), I could run at full throttle for 140 minutes and still not overheat. I've posted my detailed performance observations many times here and my measured "watts to knots" efficiency is better than any verifiable performance numbers that have been posted here for a mid-size sailing auxiliary. However, beating the performance of my boat should be doable. She's a full keel, prop in an aperture, rudder that is a 3" thick slab of teak, and her hull is far from fair so a modern design should do better.
We've got a rare collection of practical information about electric boats in this group. We've discovered that D. Gerr's calculations don't work well for what we're doing here (his predictions are more than 300% off for my boat, and D. Gerr himself admitted that his work is more applicable to faster boats). We've got electric boaters with hundreds of hours (or more) of electric motoring experience. We know how much power it really takes to push boats like ours to practical speeds. We've got a number of vendors that are a reasonably reliable lot (as vendors, they probably have at least a subtle bias). We've got a number of experimenters that want to build a better, or at least a different, mousetrap. There is more unbiased practical experience in the field of electric marine drives here than anywhere else on the planet. There are also a lot of unsubstantiated opinions too. But if you go slow and read carefully, you can collect enough real information to build or buy an effective electric drive for your boat. So welcome and feel free to ask anything.
Fair winds and smooth seas,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion marine drive, 8kW LiFePO4 battery pack
Marina del Rey, CA
---In electricboats@yahoogroups.com, <sabre281@...> wrote :
Maybe this is a stupid question but I've wondered about the advantages/disadvantages to having a 10 kw motor vs. 5 kw. My boat is 28', 8000#'s of displacement so let's assume hull speed is 6 knots at full throttle. From what I've read I know the 5kw will do the job. If you wanted to motor lets say at 3.5-4 knots for a long period of time would a 10KW unit run cooler & use less amps from the battery bank when compared to a 5kw?
Thanks,
Bob
