Hi Andrew,
Yes, he was saying that the information in the propeller handbook is focused more on the energy intensive operation that most boaters deal with 90% of the time. We all know that reducing our speeds greatly reduces our power demands. But ICE drivelines are notoriously inefficient at slower speeds and Gerr's calculations can turn out grossly inaccurate at our normal electric operating speeds per my ealier example.
Since we're talking props in this thread, not motor types, my point was that even Gerr recognizes that his book and propeller guidelines may not be as applicable to our typical conversions. I would also believe that Gerr's calculations don't really apply to the R/C electric boats in your signature link, the scale is drastically different.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Andrew Gilchrist" <andrew@...> wrote:
>
> If he says its close to hull speed he is operating boats in a part of the
> energy demand curve that is starting to rocket up
>
> Are you talking about 3 knots which is near hull speed at 75% of hull speed
> the power demand is going to be far less than 90-95%
>
>
>
> And electrics simply produce torque required by the low, that doesn't mean
> more horsepower, it's the amps that will vary more. The motor Kv and its
> design are the big determinants of that. Some motors which appear great are
> poor. Brushed motors don't cut it after one has uses a brushless motor which
> can make more power and torque, and seemingly freakish short term power and
> torque peaks ( I am speaking on .1 second time scale) in a much lighter unit
>
>
>
>
> Andrew Gilchrist
>
> www.fastelectrics.com
>
> 612 4982 5481
>
> Australia
>
> _____
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Eric
> Sent: Wednesday, 23 November 2011 5:19 AM
> To: electricboats@yahoogroups.com
> Subject: [Electric Boats] Re: stupid question about props
>
>
>
>
>
> Hi Andrew,
>
> Ealier this summer, there was considerable discussion about the Dave Gerr
> calculations and how applicable they are to our boats. Because we are
> operating at such slow speeds and electric motors have very different power
> curves compared to ICE, much of the information in Gerr's book that has been
> validated many times over in the ICE world doesn't always hold true for
> electric boats. For example, according to Gerr's calculations, my hull would
> require 3.98hp (3kW) at the propeller to drive it at 3kts. I know that I
> only draw 500W from my battery bank to power my boat at 3kts and that hasn't
> included any losses for the controller, wiring, motor or gearbox yet.
>
> In an email from Gerr, he stated that his work doesn't really cover our
> specific use cases, his focus has been on hulls close to hull speed and
> beyond.
>
> So while Gerr's book is treated as gospel for mainstream boaters, and
> rightly so, we should remain open to different answers for our boats.
>
> We can see that just about any prop wil make a boat go, but some props will
> do the job better.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> PS. if you're interested in the earlier thread concerning the Gerr
> calculations, do an advanced search for "Gerr" in this Yahoo group.
>
> --- In electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> , "Andrew Gilchrist" <andrew@>
> wrote:
> >
> > Oh damn Dave Geer is where I got my figure for - it was for high speed
> > boats - idiot I am!
> >
> >
> >
> > Andrew Gilchrist
> >
> > www.fastelectrics.com
> >
> > 612 4982 5481
> >
> > Australia
> >
> > _____
> >
> > From: electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com>
> [mailto:electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> ]
> > On Behalf Of Tom
> > Sent: Tuesday, 22 November 2011 3:02 PM
> > To: electricboats@yahoogroups.com <mailto:electricboats%40yahoogroups.com>
>
> > Subject: [Electric Boats] Re: stupid question about props
> >
> > Dave Gerr's Propeller Handbook gives a p/d ratio of about .57 to .82 as
> > being suitable for a single screw, five knot boat. My launch has the prop
> > which was spec'd by the designer, a 15" x 12" three blade, which is a p/d
> of
> > .80
> >
> > -Tom
>
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