Oh damn Dave Geer is where I got my figure for - it was for high speed boats - idiot I am!
From:
Sent: Tuesday, 22 November 2011 3:02 PM
To:
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
--- In electricboats@yahoogroups.com, "Andrew Gilchrist" <andrew@...> wrote:
>
> For most props the most efficient pitch ratio is 1.5 times the prop diameter
>
>
> The biggest diam prop with that ratio will give you the best option.
>
> A gearbox lets you optimize that further and two pulleys and a toothed belt
> are usually cheaper than a bunch of cells
>
>
>
> Andrew Gilchrist
>
> www.fastelectrics.com
>
> 612 4982 5481
>
>
>
> _____
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Chris Guldi
> Sent: Tuesday, 22 November 2011 6:40 AM
> To: electricboats@yahoogroups.com
> Subject: Re: [Electric Boats] Re: stupid question about props
>
>
>
>
>
> It gives me a starting point. I had noticed in looking at props that
> "oversquare" seems to be common. What is on the boat now is an 18" two
> blade, not sure of the pitch. The
> said he used for cruising.
>
>
>
> What's a good source for props?
>
>
>
> On Mon, Nov 21, 2011 at 2:31 PM, Eric <ewdysar@...> wrote:
>
>
>
> Hi Chris,
>
> The pitch question is harder to answer, it completely depends on your boat,
> drive and reduction ratio.
>
> That said, I think that some people here, myself included, have switched to
> "oversquare" props where the pitch is greater than the diameter with great
> success. I went from a 2 blade 13 x 10 to a 4 blade 13.5 x 15.5 prop. You
> can see pictures in this Yahoo group in the photo folder "Eric's Serenity".
> You can see that 13.5" is really pushing the limit for my boat's aperture.
>
> http://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list
>
>
> Here's a brief excerpt from sometihing that I posted on Oct 1 this year that
> discusses the differences in slip for my old and new propellors.
>
> "When you're dealing with displacement auxiliary sailboats, even the good
> propellers run at slip rates above 35%. If you look at the propeller
> calculators from most of the propeller manufacturers, the typical figures
> for displacement sailboats are around 50-55% slip. Here's the figures that
> came from my boat, before and after:
> "Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at
> 5.5kts. The new prop observed performance calcs to 27% slip at 3kts and 37%
> slip at 6kts...."
>
> It's not a direct answer to your question, but I hope that this data helps.
>
> Fair winds,
> Eric
>
>
> --- In electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> , "Chris" <cxguldi@> wrote:
> >
> > I keep reading "get the biggest prop you can find" Wouldn't pitch be more
> important?
> >
> > I can fit up to 21" if the prop is 21 or 24 inches and the pitch is the
> same wouldn't I be getting the same from them? What part of this equation am
> I missing?
> >
> > I don't think I have read anything about prop pitch so how do I know what
> pitch to get?
> >
>
>
>
>
>
>
>
> --
>
> Chris Guldi
>
> cxguldi@...
>
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