Hi Andrew,
I think I know where you're going with the throttle setting question, but throttle on an ICE is rarely linearly correlated with power output. I found that looking at fuel consumption (which some regular people track in their boats) is a better gauge of power setting.
The simple formula that I use is (Gallons Per Hour * Thermal Efficiency)/Fuel Constant = HP or (GPH * TE)/FC = HP. This should give you shaft power at the flywheel.
The fuel constant is 0.0226 for gasoline and 0.0197 for diesel. The thermal efficiency is your guess, but for all but peak settings, you can pretty much count on 25% or less from most marine inboards. At very low throttle settings, the TE(including accessory losses) may be as low as 10%.
At cruising speeds with my old diesel, she burned about 0.5 GPH. Using 0.25 for thermal efficiency in the (GPH * TE)/FC, we get (0.5 * 0.25)/0.0197 = 6.3hp. That thermal efficiency might be a little optimistic at partial throttle settings with the old one cylinder, so my power output was probably less at those burn rates. At full throttle, she burned 0.9 GPH. (0.9 * 0.25)/0.0197 = 11.42, the engine was Yanmar SVE12 12hp model. Using 0.265 for the TE makes the calculation come out to 12hp. So this sounds reasonably accurate. Of course, using small numbers increases the chance of inaccuracies due to measuring variances, but I've done the same calcs for a large cabin cruiser and the numbers matched up with the engine manufacturer power specs at cruising speeds.
Anyway, I thought that you might like another formula to add to your bag of tricks. I've used this one in this group to explain the difficulties of convering large cabin cruisers, ski boats and houseboats to electric. It's surprising the number of people that actually know their fuel usage, that makes the conversation easier.
Fair winds,
Eric
Marina del Rey, CA
PS. This is off-topic: Try doing the math for a car that gets 45mpg at 60mph (1.33gph). With a 35% thermal efficiency, you only get 20hp to overcome all the wind and rolling resistance. Since you don't want to be running the engine at peak efficiency RPM (3500-4000 rpm?) while cruising at 60mph, the problem gets pretty tough. This makes my simple old 1990 Geo Metro that got 52mpg averaged over 130,000 miles pretty impressive. They don't make 'em like that anymore... ;)
--- In electricboats@yahoogroups.com, Andrew Gilchrist <andrew@...> wrote:
>
> Chris
>
> i will look forward to hearing about the prop
>
> BTW why is electric your only option?
>
> when you did that test that got you 825prop rpm for the 7 knots on the
> diesel can you remember your throttle setting?
>
> all the best
>
> Andrew
>
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