Hi Kerry,
I was figuring something like that too. That means that in my case, the new propeller is 6 times more efficient than my old prop at 3kts of boat speed.
So if we assume that the electric drive and propeller is 100% efficient at that speed (just for this conversation) then Gerr was grossly overstating the power required to move my boat at 3kts. It also means that my old prop was less than 17% efficient at 3kts boat speed. Additionally, a simple propeller efficiency adjustment would put Gerr's formula back on track.
If that was true, then it also means that an electric boat conversion that doesn't change the propeller will be much less efficient (up to 6 times less efficient) than a conversion that does upgrade the prop. However, we can observe that this is not true, conversions that don't change the prop don't use significantly more energy to go 3kts than the ones with new props, if the proper reduction ratio is chosen. Remember that the gear ratio is before the propeller shaft (just like the ICE transmission), so it should not be part of the calculations.
Therefore, it is not just the prop. Partially perhaps, but that is not the whole story.
Fair winds,
Eric
Marina del Rey, CA
PS. If anyone sees any holes in my reasoning, in these or any other thread, please point them out. We're all learning this stuff together...
--- In electricboats@yahoogroups.com, "Kerry Thomas" <kjthomas@...> wrote:
>
> Hi
>
> Don't have time to go into it at the moment, but I suspect it has to do with
> the larger, high torque slow speed propeller sizing for EP compared with
> smaller faster props for ICE, giving more efficiency at slow speeds.
>
> Like running a tugboat prop at 5 knots compared with running a container
> ship prop, designed for 15 knots, at 5 knots.
>
> -----Original Message-----
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Eric
> Sent: Friday, 19 August 2011 5:33 a.m.
> To: electricboats@yahoogroups.com
> Subject: Re: [Electric Boats] power requirements - predicted vs observed
>
> Hi hanermo,
>
> Interesting point and I agree with each point, but it doesn't address the
> question. The Gerr formulas are stated to cacluate the propeller shaft HP
> required to push a hull. So it's supposed to be the HP applied to the
> propeller AFTER all of the parasitic drag and driveline efficiency losses.
> So regardless of the power source, the number is supposed to be how much
> power needs to be supplied to the prop for a given speed. Everything that
> you mentioned were losses before the power made it to the propeller shaft.
>
> So all of this is just derailing the conversation. Accurate info, but not
> what we're discussing right now.
>
> Let me try to simplify the topic to keep this conversation on track. The
> Gerr power formulas claim to quantify how much power the propeller needs to
> push a hull at a given speed. These numbers have been verified through
> years of observations with ICE engines and are accepted to be accurate.
> However, through observation, one can see that an electric drive can push a
> hull at the same speed with much less power applied to the propeller.
>
> My boat at 3kts uses about 17% of the power via electric compared to what
> Gerr predicts. That is so far off, that I think that Gerr is predicting
> something other than what I am measuring. By 5.5kts, my observations are
> 42% of what Gerr predicts. Gerr must be using some assumptions that are not
> accurate for my boat.
>
> My simple question was if Bill S. had a single data point to validate this
> question. What is the observed energy load on Barbara Ann's electric drive
> at 7.2kts?
>
> It is physics, not magic, so I know that there is an explanation.
>
> Fair winds,
> Eric
> Marina del Rey, CA
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