You're right, there are many other factors that should be considered, but I'll try to answer this at a generic level.
ICE engines suffer efficiency problems at lower speeds due to unfavorable torque curves, but are more efficient at higher speeds.
Let's take a look at 2 types of drive systems.
First, a Yanmar 1GM10 that the manufacturer rates at 9hp/6.7kW. If propped correctly, this engine should drive your 26' sailboat to hull speed at 3000rpm burning a little more than 1/3 gallon per hour. Slowing down to 1500rpm reduces the fuel consumption to less than 0.1 gph and should slow the boat to 1/2 hull speed. Since diesel contains 38kWh/gal, then less than 1/10 gph is about 3kWh consumed.
Now lets look at the performance results from most of the typical drive systems available today. A 5kW drive should drive your boat at 2.5-3kts using less than 1000W. Do it for an hour, and you're under 1kWh consumed. Some drives may draw only 600W (12.5A @ 48V) at this speed.
Therefore, at slow speeds the electric drive is between 3 to 5 times as efficient as the diesel engine. Gasoline engines are considered to be less efficient still. But, higher speeds leads to different results.
At hull speed (3000rpm), the diesel consumes 13kWh of fuel in an hour. Alternatively, the electric drive will probably be close to full power or may not be able to achieve hull speed at all. If the drive is consuming 5kWh from the batteries to produce the same speed, the electric drive is only 2.5 times as efficient as the diesel engine at these power settings.
So its obvious that electric drives are way more efficient than ICE engines. Our real challenge is the size of our fuel tanks (batteries). With only the typical equivalent of less than 1 gallon of usable energy on board, our range is limited, but more than 4-6 hours of slow speed is not unreasonable. That can drop to about 1 hour at high speeds, reducing your range by a factor of 2 or 3.
A diesel tank of merely 10 gallons provides a much greater safety factor. That said, I didn't burn a whole gallon of fuel at one time in the first 10 years that I've owned my 30' ketch. So I think that electric drive makes sense for the way I use my boat. It won't make sense for everyone, but this fact is what finally convinced me to convert to electric. Time will tell if I made the right choice...
Fair winds,
Eric
Marina del Rey
--- In electricboats@
>
> I'm curious to know if anyone can tell me how much energy(percentage-
> of top-rated energy(such as optimum RPM's for an IC motor & optimum RPM's
> for an electric motor), a 26 boat(5000k) running at 1/2 hull speed, would
> require with a gasoline engine and an electric motor, respectively.
>
> I know the answer is dependent on more information, but what I'm trying to
> compare is if the economy(energy-
> than hull-speed begins to somewhat approach the economy(energy-
> IC engine, since the IC engine seems to require much more speed(RPM's) for
> the same amount of torque of an electric motor.
>
> I'm hoping that the electric motor will result in a much lower percentage of
> the optimum/maximum to go 1/2 hull speed than a gasoline engine would, such
> that an electric motor at the lower speeds might be 5, 6 or more times more
> efficient than an IC engine would be.
>
>
> Just curious
>
> John Francis
> Pearson 26
> Port Clinton, Ohio
>
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