I have a slightly different take on the generator sizing.
Chances are, the bottleneck for this whole system is going to be the charger. Shore power in the best circumstances: 240vac at 50 amps, 240vac is around 152vdc, and the charger is not going to actually pull all 50 amps or it risks tipping the circuit breaker. 45 amps is closer to the max current on a charger connected to shore power. That's around 6800 watts, give or take. A 7kw charger would be the ideal fit. I chose a 6kw charger because stepping up to an 8kw charger added another 30 percent to that component cost.
The same charger can use the (AC or DC) output from a gen set. So, in my case, it made sense to just keep the existing 7kw Phasor gen set I already have. Well, to be totally honest, I've pulled that gen set and set it on a bench in my shop. I might not put any generator at all back on the boat. I had a heck of a time finding a generator that was 1) well documented by the manufacturer and 2) available at a reasonable price. If prices come down about 25 percent, and that documentation problem gets corrected, I'll revisit installing a new generator.
At 4 knots I should have a range of about 16 nm under battery power alone. That's a pretty good range for an auxiliary propulsion system that can be recharged just by sailing, and sailing is what I prefer to do anyway.
I guess the bottom line here is, when you are designing your PBBPS, you need to first identify how much power (in watts) is required to move your vessel at different speeds. Work from there. You may find out that 15hp is more than you need for a 33 foot vessel. One thing I do recommend considering, very strongly, is the nominal voltage of your battery pack. It is very easy to find converters, inverters, and chargers for 48v packs. Above that, however, and the options suddenly narrow quite a bit.
---In electricboats@yahoogroups.com, <gvanderhart@...> wrote:
Preston,
I do not have an electric propulsion system, so you may want to take my advice with a grain of salt. However, I am an electrical engineer, so I know something about creating electrical systems that work. Perhaps my comments will nudge other responses.
Since you want to power a fairly large sailboat and since you are removing an inboard motor, I would recommend a 10-20hp electric motor. To get that kind of power requires at least 48 volts, or one bank of four 12V batteries. The principle for voltage selection is: the higher the voltage, the lower the current. Large currents can require more copper than is practical. The best connection to the propeller is an AC motor with a motor controller. The input to the motor controller is DC, but you get the advantage of a reliable brushless motor and more importantly, speed control.
There are a lot of tradeoffs with regard to how many banks you need (adds Ah) and how fast you can recharge those banks. At least you will need a charger that connects to shore power. If you already have a device that changes AC to charging DC, an AC gen set is probably the best way to avoid duplication of equipment. If you choose the charger carefully (make sure the manual says you can connect other electrical equipment while it is charging), then you can use the power from the gen set/charger to run the motor. In this case, the batteries keep the charger from shutting down (which it would do if it did not detect a minimum voltage), but the current from the charger is used to power the motor. It makes sense to me to approximately match the charger output (volts * amps = watts) with the cruising volts * amps needed by the motor.
Gary
---In electricboats@yahoogroups.com, <electricboats@yahoogroups.com> wrote:So, I'd love to "electrify" my old faithful Pearson 10M (33 ft.) sloop.The Atomic 4 that has served for so many years is shot.I'd like to put in an electric motor (what kind, voltage, etc?), a water cooled generator or motor to run an alternator, a battery bank (of what kind of batteries?) and a controller of some sort.I'll pull the old engine, get rid of the gas tank, and start over.Multiples of 6 volts? A 48 volt sweeper or golf cart motor? A bank of 6 volt batteries, what type? Generator, or motor w/alternator?I'm figuring 4,000 watts for the generation system, max 800-1200 rpms at the shaft…15hp would drive the boat nicely...(In my fantasy, a Hydrogen fuel cell system, fed by cng or something, would be cool…)But, when I wake up and look at my checking account… probably a diesel gen set is the answer…Thoughts, recommendations, guidance?Best,Preston
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