First of all, motor ratings based on bench testing are questionably relevant in our applications. Sure you can run motor at high amps and voltage in a motorcycle where cooling is everywhere, but that same motor may have cooling issues, and therefore, longevity issues, when running in a bilge or enclosed engine compartment. If you installed a 10kW rated motor and found that it overheats (my controller can display internal motor temp) in your boat when run for more than an hour at 140A @ 50V, then that drive system only rates to 7kW, regardless of the manufacturer's motor rating. I have found that the published motor ratings to be optimistic at best.
How about our real world? I've been running a ME0913 for over 10 years, it is similar to the ME115 included in the Thunderstruck 10kW kit. Both motors are rated to 12kW@96V, which doesn't mean much to most of us. They are also rated to 180A DC continuous into the controller, so right there, we're de-rating to 9kW for our 50V nominal systems, in an optimized test environment. So for a 48V installation, the Thunderstruck 10kW kit is only rated to 9kW at best, based on the motor manufacturer's specs. Start adding limitations of the controller, cooling, and cabling, and the max continuous power goes down further.
And in practice, installed systems will rarely be able to pull the rated power, even at full throttle. To explain that statement, I have question to everyone who has installed a 10kW drive at 48V. Do you ever pull 200+ amps, even at full throttle? My experience is probably not. Your ability to apply power to your boat depends on the motor, controller, gear reduction (if any), propeller, and how the entire package interacts with your specific boat. The good news is that similar boats have similar power demands.
About 10 years ago, the members of this group developed a "rule of thumb" that states that displacement type, sailing auxiliaries need about 1kW per ton of displacement. That "guideline" was later picked up by various retailers like West Marine. Others will recommend more powerful systems, but just like most diesels installed in sailboats today, that recommendation may be overkill. Certainly, 1kW/ton will drive most hulls to 90%+ of hull speed, if a well balanced system is installed.
How does that work in practice? My boat is a 1964 Cheoy Lee Bermuda 30 ketch, with a full keel and the prop in an aperture, not the most efficient design. The boat displaces 10,200# or 5.1 tons. When I got her, she was powered with a Yanmar SVE12 single cylinder diesel. Top speed at full throttle was 5.5kts. I installed a drive system from Propulsion Marine in Santa Barbara that was realistically rated as 5.5kW. Each component, the controller, motor, gear reduction, etc. is individually rated for considerably more power (see my comments about the ME0913 above), but the system as a whole, to be installed in a space with less than optimal ventilation will run at 5.5kW as long as there is power to run it. And for you old timers here, you probably remember the extensive performance testing that I conducted and the results that I published on this list.
I'm not going to explain the testing process and equipment now, but here are some basic results that I published here in 2012
| Average results | |||
| Speed | Amps | Volts | Watts |
| 3 | 9.54 | 52.4 | 500 |
| 3.9 | 19.21 | 52 | 999 |
| 4.4 | 28.9 | 51.5 | 1488 |
| 5 | 48.5 | 50.7 | 2459 |
| 5.7 | 78.8 | 49.6 | 3908 |
| 5.9 | 106.2 | 48.7 | 5172 |
We can see that max throttle pulls 5127W from the battery and the boat is almost 1/2 knot faster than with the 12hp diesel. After an hour at full throttle, the controller is just warm to the touch, and the motor temp stabilizes below 100 deg C, well under the manufacturer limit of 150C. My theoretical hull speed is 6.5 kts, this system delivers more than 90% hull speed which was my intended performance target. Prop, gearing, motor, controller, batteries, and cabling are well matched and deliver exceptional efficiency and performance with more power on tap than when she was diesel powered. And yes, we can also see that as installed, I am running a real life 5.5kW system.
Bottom line is take retailer's system ratings with a grain of salt.
Fair winds and following seas,
Eric
1964 Cheoy Lee Bermuda 30, 5.5kW drive, 8kWh LiFePO4 battery bank
Marina del Rey, CA
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