Hi Orest,
Yes, 350W is close to 1/2 hp. And yes, that is probably fairly close to the estimated shaft HP that my 5.1 ton ketch uses at 3kts. This figure is considerably less than the estimated power that many formulas predict for my boat, including D. Gerr's work. Search this forum for "Gerr predictions" and you'll find a number of posts where the accuracy of calculated power predictions are discussed in more detail.
And if you look carefully, there are probably more system losses that I am not including in this rough estimate, so my actual shaft power used may be even less than the 350W already mentioned. This makes the "accepted" calculated predictions even farther off....
This is why I really encourage anyone with a converted boat to measure and then post their observed performance figures. These practical observations are the only "good" numbers that someone considering a conversion can use.
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Orest Iwaszko <orestyko@...> wrote:
>
> Bear with me I want to be on the same page Is 350 Watts at the shaft approx 1/2 HP since .745 KW = 1HP TY Orest
>
>
> On 2013-04-09, at 2:50 PM, dan.randolph wrote:
>
> > Ok, Eric, once again you clearly state the situation. Thanks, I will stop worrying about the lower efficiency of the larger motor at the lower speeds.
> >
> > - Dan
> >
> > --- In electricboats@yahoogroups.com, "Eric" <ewdysar@> wrote:
> > >
> > > Hi Eric,
> > >
> > > I think that you missed my point on efficiencies. The question becomes "If I make this system more complicated by "optimizing" components for partial throttle, how much energy will I save?" Followewd by " Is it worth it?"
> > >
> > > Let's look at what was suggested. I installed a 5.5kW drive in my 5.1 ton boat. Perhaps installing two 2.5kW drives would increase efficiency at low speeds because the motor will be operating at a more efficient power level.
> > >
> > > So, my boat uses about 500W (about 10% power) to cruise at 3kts (50% hull speed). My ME0913 motor operates at about 70% efficiency at such low power ratings, so the shaft output should be around 350W, if the manufacturer's motor performance charts are correct. So we now know that my boat takes about 350W of shaft power to motor at 3kts...
> > >
> > > Fair winds,
> > > Eric
> > > Marina del Rey, CA
> > >
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