[Rhodes22-list] Bob's Electric Drive

[ed kroposki]

Bob:
	The owner/operator of the company that built that boat died shortly
after the show.  Company ceased operations, Nimble Boats.

Ed K
	

-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Robert Skinner
Sent: Saturday, September 25, 2004 2:08 PM
To: The Rhodes 22 mail list
Subject: Re: [Rhodes22-list] Bill Effros - Electric Drive

Roger Pihlaja wrote:
> ...
> I looked in the 2004 issue of the West Marine Master Catalog on pp. 223.
> The most powerful transom-mount electric trolling motor listed is a Minn
> Kota M/N RT101/S-3X...
  This trolling motor is rated to deliver 101 lbs of
> thrust at 36 VDC and 37 amps.  To power this load, you would need:
> P = V * A = 36 VDC * 37 amps * 1 watt/VDC-amp / 1000 watts/kw =  1.33 kw
> ...
> The horsepower ratings of outboard motors are on the basis of power output
> at the crankshaft...  Assuming
> you are willing to trust my estimated power transmission efficiency, you
> would need an outboard rated at:
> 1.33 kw / (0.746 kw/hp * 0.72) =  2.48 hp
> So, in big round number, this 101 lb thrust trolling motor would be
roughly
> equivalent to a 2-1/2 hp gasoline powered outboard...

Roger -

Your calculations produce results roughly in line 
with my experience.

My electric outboard rated at (perhaps a bit hopeful) 
1.5 hp draws 28 Amps at 12 VDC and produces 28 lbs of 
thrust.  In actual use, this cross-checks with my 
4-hour endurance at full power from a fully charged 
105 AH 12 VDC deep cycle gel cell marine battery.  

The Honda 1000 watt generator 125 VAC drives a 20 Amp 
battery charger, not quite able to keep up with a 
constant WFO motor output, but quite adequate for 
1/2-3/4 throttle gunkholing all day long.

I usually don't have to use the generator, as the 
4-hour battery life is more than adequate to get me 
out of the marina and assist if the wind dies down.  

One problem:  My smaller sailboat is only 15' long 
with a tiny cabin, but the 28 lb thrust is not enough 
to push it directly upwind against more than 12-15 
MPH winds.  In those conditions, I use a 3 hp Tanaka
air-cooled 2-stroke that weighs only 24 lbs, but
wails like a banshee.  However - it will push the boat
at hull speed against a 15 MPH wind.

Hope this actual experience will convince people that
a Rhodes 22 is too big to push with a trolling motor.

At the last Annapolis sailboat show, I saw a $50,000
25' double-ender pilot-house sailboat with an inboard
electric drive.  There were 10 big 12 VDC batteries 
in the bilges - power and ballast.  The motor was 
about 10" in diameter with a 1" shaft.  Looked as if 
it would do the job.  But it had no generator, and 
would therefore be tied to shore power.

Retrofitting such propulsion would be all but 
impossible in an R22, and is they made such a rig in 
outboard format, I'd be terrified of the huge 
flexible 125 VDC cables that would be required to 
connect with the battery farm.

The 9.9 hp outboard is cheaper, more effective, 
easier to keep repaired anywhere in the world, and 
simpler.  It is also quickly replaceable if ripped
off.

Keep the pointy end up,
/Bob Skinner
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