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
From:
Sent: Wednesday, 23 November 2011 5:19 AM
To:
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
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
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
>
>
>
> _____
>
> 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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