Thanks to everyone who has chimed in on my questions and although I hope
we aren't finished with this issue yet, I especially want to thank:
Mark Stafford who has been working tirelessly trying to bring me up to
speed while working his full time job as a Air Traffic Controller.
Also, Dave Kellogg
Denny Wolfe
Paul Smith
Thanks everyone. Did I forget anybody?
OK, my real name IS Tom.
I just don't take myself very seriously and hope I haven't offended anyone
as a result.
I think this question got a lot of you thinking/talking about issues we
all would like to hear about so; Thanks again.
Now I'm thinking up my next question.
Respectfully, Tom
In a message dated 5/30/2009 8:05:01 A.M. Eastern Daylight Time,
dwolfe@dropsheet.
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
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