[Rhodes22-list] Bill Effros - Electric Drive

Sat Sep 25 11:17:06 EDT 2004

Bill,

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

However, most portable generators are designed to deliver only 120 VAC or 12
VDC & this trolling motor requires 36 VDC.  For you, as an aftermarket
application, the simplest way to generate this DC voltage would be to use
the 120 VAC power output to drive a 36 VDC power supply.  This power supply
can be built with off-the-shelf solid state components, will be very
reliable, & about 95% efficient:

1.33 kw / 0.95 =  1.42 kw continuous

You don't want to run your generator at its maximum rated output for very
long because the advertised maximum load on generators is usually for an
intermittent load.   Typically, the maximum continuous load rating of a
generator is about 75% of the maximum intermittent load rating:

1.42 kw / 0.75 =  1.87 kw or 1870 watts max intermittent

A Coleman Powermate Model Pulse 1850 has a 4-cycle 3.5 hp Briggs & Stratton
engine & is rated at 1850 watts intermittent load & 1500 watts continuous
load at 12 VDC or 120 VAC.  So, this genset is sized about right.  This
genset looks like a rectangular box with a handle on top with dimensions:
8.3" Length X 11.8" Width X 16.4" High.  I don't know the weight.  The 36
VDC power supply would easily fit into a box of similar or slightly smaller
dimensions & could be permanently mounted inside the lazarette compartment.

The horsepower ratings of outboard motors are on the basis of power output
at the crankshaft.  The above Minn Kota trolling motor power can also be
expressed in terms of horsepower:

1.33 kw / 0.746 kw/hp = 1.78 hp

This horsepower is at the output of the electric motor, which is underwater
& directly coupled to the propeller.  So, the trolling motor has no
mechanical losses due to a transmission or lower unit.  Assuming the
outboard will have a full F-N-R transmission, a right angle spur gear in the
lower unit, & roller or needle bearings throughout (in other words - a high
quality, state-of-the-art unit), the mechanical efficiency will be about 72%
at the 5500 RPM transmission input speeds typical of a gasoline engine.
Note: The details of where that 72% efficiency came from are from my ME451
Machine Design class & are beyond the scope of this discussion.  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.  All of this stuff
scales up pretty linearly.  To get an electric trolling motor that performed
something like the 9.9 hp outboards that everybody seems to favor on our
Rhodes 22's, you would need one rated at about:

101 lbs thrust * [9.9 hp / 2.48 hp] = 403 lbs thrust

Assuming the voltage stays constant at 36 VDC, the current would be:

37 amps * [9.9 hp / 2.48 hp] = 148 amps

The genset required to run this bad boy would need to be rated about:

36 VDC * 148 amps * 1 watt/VDC-amp / (1000 watts/kw * 0.95 * 0.75) = 7.46 kw
or 7460 watts max intermittent

Bill, hopefully, this answers some of your questions.

Roger Pihlaja
S/V Dynamic Equilbrium

----- Original Message -----
From: "Bill Effros" <bill at effros.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Friday, September 24, 2004 11:39 PM
Subject: Re: [Rhodes22-list] Bill Effros - Electric Drive

> No, Bob,
>
> I just took a look at what others had been talking about, lifted it,
noticed that it was seriously quiet, and went back to the experts on the
list.
>
> Roger, Michael, anyone
>
> These electric motors are rated at up to 105 lbs. of thrust.  What does
that mean?  How does it relate to...anything.
>
> I bought a $100 electric motor several years ago, clamped it along side my
Tohatsu, and sure enough, it would move the boat.  But it was not what I
would call robust.  Is this an idea that has any possibility of working?
>
> Bill
>
>
> ----- Original Message -----
> From: Robert Skinner
> To: The Rhodes 22 mail list
> Sent: Friday, September 24, 2004 8:38 PM
> Subject: Re: [Rhodes22-list] Bill Effros - Electric Drive
>
>
> Bill -
>
> Are you using an hybrid electric drive too?
>
> Brad -
>
> Would you send me the pix of the Beijing boats.
>
> /Bob Skinner
> ------------------------------------------------
> brad haslett wrote:
> >
> > Bill,
> >
> > Your idea of combining an electric drive with a small
> > generator is quite good!  Yamaha now makes a competing
> > model to my Honda which is cheaper and lighter.  The
> > noise level is about the same and is certainly less
> > than the quietest outboard.  It is nice to have around
> > the house as well.
> >
> > Attached are a couple of photos from the Summer Palace
> > in Beijing.  The lake is full of electric boats.  We
> > rented an 8 passenger boat and I was suprised at how
> > fast it moved.  The other photo is of a solar powered
> > cruise boat on the same lake.  If I ever get the time
> > to build a "houseboat" this is what it would look
> > like.
> >
> > Brad Haslett
> > "CoraShen"
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