Hey Mike,
No worries. For my calcs, I did the math and thought that the answer sounded familiar, so I looked up my last answer.
Any time that you're doing this kind of work, you can stop and evaluate if your answer sounds reasonable. My 30' 5 ton sailboat uses 5kW to motor at 6kts. So 7.6-8kW seems reasonable for a 37' 16 ton sailboat.
Alternatively, 43hp is 32kW. If 32kW is 6kts, that would mean that you would need a 65kW drive to motor your boat at 90% of hull speed. That does not sound reasonable.
Honestly, this isn't rocket surgery.
Eric
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> i understand, i just like getting the same answer from multiple approaches whenever i can find such alternatives.... i stumbled upon these new data while researching props... thank you for the input
>
>
>
> ________________________________
> From: Eric <ewdysar@...>
> To: electricboats@yahoogroups.com
> Sent: Monday, September 19, 2011 10:36 PM
> Subject: [Electric Boats] Re: please take a stab at sizing a motor....
>
>
> Â
> Hi Michael,
>
> For gasoline, the conversion is GPH = 0.0226 * HP / TE. Diesel contains 115% the BTUs of gasoline.
>
> If we assume a 20% Thermal Efficiency for your engine at low RPM (just a guess), we get 1 * 0.20(TE) / 0.0226 = 8.8hp * 1.15 for diesel = 10.1hp
>
> That's 7.6kW. If you look back at post #19718, my answer was that you could expect a 16kW drive to push your boat at 7kts. Half the power and slow down 1 knot means 6kts = 8kW. Two completely unrelated calculations that come up with the same answer +/- 5%. Weird, isn't it.
>
> You can keep coming up with different ways to ask the question, but the answer is going to keep coming out the same.
>
> At least this answer is better than the 43hp that you guessed on your next post...
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
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