[Rhodes22-list] electric motor selection

[Roger Pihlaja]

Robert,

Perhaps the following data will help you:

I have a MINN KOTA Endura 30 trolling motor.  I run it off a Group 24 gel
cell battery.  Once, when the battery was about half discharged, I measured
the voltage & amp draw simultaineously with a very accurate digital VOM at
full throttle when the trolling motor was pushing my 10 foot inflatable
sport dinghy.  The trolling motor was drawing 30.1 amps @ 12.2 VDC.

    Power = Volts * Amps = 30.1 amps * 12.2 volts = 367.2 watts = 0.49 hp

Therefore, generating 30 lbs of thrust requires the expenditure of about 1/2
hp or about 60 lbs thrust/hp, assuming similar mechanical losses in the
transmission and prop between the outboard engine & the trolling motor.
Thus, to approximate an 8 hp outboard, you would need a 480 lb thrust
electric motor!  If it were a 12 VDC system, this electric motor would draw
about 481.6 amps continuously.  I leave it you to determine the size of
battery bank & wire sizes that would be required.  They will both be
impressively huge & heavy.

Roger Pihlaja
S/V Dynamic Equilibrium
----- Original Message -----
From: "Rik Sandberg" <racerrik@rea-alp.com>
To: "The Rhodes 22 mail list" <rhodes22-list@rhodes22.org>
Sent: Tuesday, November 05, 2002 4:05 PM
Subject: Re: [Rhodes22-list] electric motor selection


> Robert,
>
> Good luck getting five knots for more than about 5 minutes. :-)
>
> Don't believe that there is a trolling motor out there that will compare
to
> your 8 hp. Unless you can find one with about 250 - 300 lbs of thrust or
so.
> If you really wanted to get serious and buy a huge battery bank, etc you
> could go with a Ray electric outboard or a Lynch, but your talking some
> serious bucks (thousands) for these motors, plus the batteries will weigh
a
> ton (well not really a ton, but I bet 500lbs ) Plus major modification to
> your boat to get it all in there.
>
> You can get up to 105 lbs of thrust (about like a 2 hp gas engine) out the
> larger Minn-Kotas but even this is using 36 volts. At the least for this
you
> would need at least 3, large 12 volt batteries and if you want to run very
> fast for long, you will need 6.
>
> You can use a trolling motor pretty satisfactorily for getting in and out
of
> a marina, etc. but longer distances at a high speed with such a motor are
> not a very realistic possibility at this point IMHO.
>
>
> Rik
>
>
> ----- Original Message -----
> From: "Robert Dilk" <Robert.Dilk@trw.com>
> To: <rhodes22-list@rhodes22.org>
> Sent: Tuesday, November 05, 2002 2:21 PM
> Subject: [Rhodes22-list] electric motor selection
>
>
> I am confused. If I have a 8 hp gas outboard motor, what size THRUST
> electric motor do you need? Is an electric trolling motor a good idea?
>
> I guess any motor will push me out into the river, But....
>
> Or what size (thrust) electric motor would be good for a Rhodes 22. Assume
> I want to be able to drive the boat to 5 knots.
>
> Any help is much appreciated.
>
> Bob Dilk
>
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