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, "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@yahoogroups.com]
> On Behalf Of Tom
> Sent: Tuesday, 22 November 2011 3:02 PM
> To: electricboats@yahoogroups.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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