Hi Bill,
Q2 - Finding the proper gear ratio is often trial and error. If you hit max RPM without hitting max amps, then you are underpropped or your gear ratio is too high. If you hit max amps without hitting max rpm, then you are overpropped or the gear ratio is too low. With my new prop at full throttle and a 2.55:1 gear ratio, I'm hitting my target RPM and 5.3kW (106A) at 95% of hull speed. That's pretty close to hitting all of my marks and the next gear ratio, higher or lower, would move the balance away from this sweet spot. My performance numbers are pretty good for a 10,200 lb sailboat.
Q3 - There are a number of vendors that sell props online, you just pick the one that will fit your boat. Under the advice of James at Propulsion Marine, we chose a 13 x 15.5 four blade prop. You can see a picture of my old prop and new prop side by side as well as some shots of how I had to open the aperture a bit to clear the new prop in my folder "Eric's Serenity" on this site.
http://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list
Q4 - Generally speaking, larger diameter props are more efficient. But typically we're limited in size by the hull design. So many members here have looked for heavy pitch to go with their max diameter prop. In my case, the pitch is greater than the diameter and is considered "oversquare". Oversquare props are considered to trade acceleration for speed, for a sailing auxiliary, acceleration isn't as critical. This prop has given me good "watts to knots" specs and the slippage is much lower than the old prop that was on my boat.
But the optimum prop specs and gear ratios will be somewhat different for each boat. By going with an experienced drive vendor, they have a greater chance of having first hand experience with a boat similar to yours and can get very close to perfect on the first try. That said, your boat will go fine with the stock prop and a reasonable gear ratio. But higher efficiency (greater speed and/or range) is available by matching the prop and ratio to your motor and your boat.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Bill Heintz <bheintz@...> wrote:
>
> Sorry to dredge up an old thread . . .
>
> Q2: Currently I am using a reduction ratio of 2.22. Should I go more and decrease the RPM, can I over torque my motor? It seems that torque should be a factor in propeller selection for an electric boat.
>
> Q3: Who do I call, what information do I need? Can I just say I want a 3 Bladed RH propeller with 8" pitch and a diameter of 20" 33% blade area? Or will they just want me to start from scratch and I give them all the vessel particulars and they use a spreadsheet designed for ICE to select a propeller for me?
>
> Q4: Are their rules of thumb for electric boat propeller selection?
>
> Thanks.
>
> Bill
>
> On Nov 22, 2011, at 18:07, Eric wrote:
>
> > 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
> >
> > >
> > > 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.
> > >
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