This is all interesting but not very relevant unless you are building an electric tugboat. It's like comparing the ability to pull a plow with a 40 hp John Deere tractor vs a 400 hp Corvette. I bet the tractor wins.
A prop in a nozzle has much greater efficiency at zero speed (who cares how efficient it is at zero?) but the efficiency drops off fast as the boat gets moving through the water. I have an Epic Whisper electric outboard that uses a Vetus bow thruster prop running is a nozzle. It has enough bollard pull to power the boat up onto the trailer with ease. Take off the nozzle and it barely moves past the first roller. But moving through the water and measuring amps in vs speed (what you really care about) there is no difference with or without the nozzle over above 2 or 3 mph. And that's with a prop designed for a nozzle. If I compared the prop/nozzle to no nozzle plus a "normal" prop the nozzle difference would be even less. This is real data coming from an ammeter and a GPS on calm current free water.
This argument about how many electric hp = a gas hp is silly. Horsepower is a physical unit - it doesn't matter if it comes from horses, steam engines or nuclear reactors. In practice, a displacement boat can usually give satisfactory service with an electric motor with a continuous hp rating lower than the max rating of a gas engine that might be typically installed in the same hull.
The real speed constraint is the batteries anyway. Not many boats have enough capacity to run more than, what - maybe half an hour, at full throttle. My 6 hp boat virtually never runs at more than 2 hp. It goes practically as fast and the battery bank lasts a lot longer.
Denny Wolfe
----- Original Message -----
From: Dave Kellogg
To: electricboats@
Sent: Friday, May 29, 2009 9:45 PM
Subject: Re: [Electric Boats] RE: Electric power for over 30' [Question for Dave K]
Hey Tom,
I am running a 14 inch 13 pitch 3 blade prop inside a Rice Power/Speed nozzle, this gives me the most effecient use, my friend with the 50 hp diesel is running a 13 inch 8 pitch open 3 blade prop........
First I would never hook the 2 boats stern to stern for a pulloff, that to me is just BS. Both tests were made with the same 550 lb. pull scale. At 3000 rpm's he had slightly more pull than my electric but he never runs his diesel at that rpm. At 100 amps I was reading just a little over 300 lb pull. At the 3000 rpm's diesel he was about 20 lbs pull over the electric..
That's the no BS account..... believe it or not is your choice...
Dave K (my real name)
-- On Wed, 5/27/09, iloveamercedes@
From: iloveamercedes@
Subject: [Electric Boats] RE: Electric power for over 30' [Question for Dave K]
To: electricboats@
Date: Wednesday, May 27, 2009, 3:38 PM
Hey Dave, I am intrigued at the prospect that a rated 8 hp electric motor
can have the same pulling power as a 'rated' 50 hp Diesel motor!
Now I may not understand what "Bollard Pull" is... but I do understand
that there are a few things people s-t-r-e-t-c-
is about Gas Mileage and evidently the other one is the power electric
motors produce. lol. Ok Dave, I'm only kidding but please help me to understand
how this can happen? I mean 'Efficiency' can only do so much!
Is this '300 lb Bollard pull' at startup, full speed or someplace in
between and if this is true, why am I looking at a bigger motor? A 300 pound
pull is a lot of pull/push...
So let me recap:
If you took a 20' line and hooked your sailboat (8 hp) to your friends
sailboat (50 hp) Stern to Stern and you both opened throttle; Nothing would
move until either you ran out of electricity or your friends sailboat ran
out of Diesel.
-Tom- (not my real name)
In a message dated 5/27/2009 11:08:35 A.M. Eastern Daylight Time,
inganear1@yahoo.
Tom, there are several projects completed and many underway in
approximately your size range. 48 volt systems seem to be the normal voltage for most
but some run on 36 volts. Look through the photo section, you'll find
several to draw ideas from. There are several guys one the site that are
building a business out of electric conversions making kits/pods and other
things that may be of interest to you. My boat is a 30 ft ChrisCraft Capri
sailboat that weighs in at 12000 lbs. I use a Etek brushed motor 8 hp and
have the same bollard pull as a friend of mine on his 50 hp diesel, (a
little over 300lbs), so you see it's not just rated Hp that does the job, it's
also efficiency..
Dave K
--- On Tue, 5/26/09, Tom <_iloveamercedes@
(mailto:iloveamercedes@
From: Tom <_iloveamercedes@
Subject: [Electric Boats] RE: Electric power for over 30'
To: _electricboats@
Date: Tuesday, May 26, 2009, 8:15 PM
Hello everyone!
I know there must be a thread for powering sailboats over 30' with
electric motors. Can someone direct me to it?
My sailboat is a Cal 35 Cruising. She is of 15,000 lbs. Displacement.
I think this project would need around 35 hp. She has a 4-108 Perkins of
'Claimed' 50HP hahahaha!
Anyway, with what little I know about this, I would need a bank of about 5
Golf Cart Batteries.
I also want to utilize Solar Panels as well as a Gas Power Generator.
So let me know how screwed up my ideas are so I can get an idea of where
to start here.
Thanks
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