On Fri, 30 Oct 2009 22:41:50 -0500, cavelamb
wrote:

Joe wrote:
On Oct 30, 7:12 pm, Bruce wrote:
On Fri, 30 Oct 2009 17:13:13 -0500, cavelamb
wrote:





Joe wrote:
I'm hoping someone might have a refference site for free wheeling prop
torque produced by a prop around the size of a 20X20 sailing at hull
speed. (9kts)
One goal on the boat we are designing and building will be an
electric drive system and we are leaning towards a power phase 150
drive.
http://www.uqm.com/propulsion_specs.php
It's a 200 HP perm magnet motor, so as you sail and the prop turns it
will generate power for the battery banks. These motors are now being
used in electric hummers in the Army. They need 420 volt DC using
two banks of batterys (35 batteries each bank) and will require 75 LBS
or torque to generate electricity.
Now I'm pretty sure the torque on a prop around the 22" size will be
way over 75 lbs but do not know exactly. Im hoping it's twice that at
least for gear reduction.
So I'm hoping someone might have, or know where I can get data on prop
torque. If not.... I guess I'm going to have to get a monkey wrench
and a scale and find a boat of the proper size to test.
Any suggestions?
Joe
As a starting point, it will probably make about the same amount of
torque as it uses under power - for the same prop rpm.
Probably a good starting place.

I've seen at least one 40 ft. sailboat with an auto alternator belted
to the prop shaft, the theory was that one could generator electricity
while sailing.

Another point is that there isn't a free lunch and I suspect that the
force necessary to turn a propeller generating X amount of power has
to come from somewhere. Probably in drag on the boat which will have
to be overcome by larger sails.

By the way, the statement "will require 75 LBS
or torque to generate electricity" is quite simply wrong as RPM of the
generator also enters into the equation. First because torque is not
measured in pounds, it is measured in force at some specified arm
length - Foot Pounds, Inch Ounces, etc.

Hello Bruce,

Did you check out the spec sheet?
Yes

And you are right, drag will be produced but overcomed by larger
sails. You have seen alt drives on shafts ect this is the same deal
except this generator is a state of the art high temp NdFeB permanent
magnet motor generator. Might need a big ass variable pitch prop to
max the input but overcoming it's drag with canvas can be done.

The type of fields is really immaterial other then that permanent
magnet fields do not require exciting current and are therefore more
efficient, within limits.

To run your motor at maximum torque output you will need to run it at
about 1,000. If you design a fixed pitch prop for that RPM then you
have no flexibility as the torque required to turn it at a higher RPM
would be more then your motor can provide at increased RPM.Thus as you
say, you will need a variable pitch propeller.

However, as the motor provided maximum torque at low RPM (max at
stalled speed) you have to design for some intermediate RPM. and I'm
not too shore how efficient this is going to be as normal propeller
design is that a large diameter prop with low pitch produces high
pulling forces and lower speeds while a smaller diameter and higher
pitch produces high speed but lower bollard pull.

This is not necessarily reason to scrap the idea rather you will need
to look into propeller RPMs versus Torque and probably end up using
only part of the total range.

From the chart you appear to be able to get between 400 and 350 Nm
torque (296 - 260 Ft. Lbs.) in a range of 1,000 - 3,000 rpm, or about
56 HP to 148 HP.

However, to actually generator that torque you are going to have to
use a really high pitch at low RPM, or have an over large diameter
prop at high speed. I suggest that you probably will never actually
generate maximum torque. even with a variable propeller.

According to the chart you will generate about 40 KW at 1,700 RPM and
120 KW at 3,000 RPM which is 53 HP to 160 HP of input power. It would
appear that there is going to be considerable drag created here.

I have use "rule of thumb" for these calcs as they are only intended
to show approximate values but they do show the general conditions
that you will be operating under.

The pitch required for driving the boat verse driving the generator
are nearly identical opposites. A variable pitch prop - providing it
can "feather" (or turn to zero pitch relative to the free stream) would
indeed be better.


Answering Cavelamb the motor produces 475 lbs of torque at full power.
After strapping down a few shafts on crewboats, I know damn well a
32X32 will produce well over 475 lbs torque at the end of a 36 "
monkey with little speed.

That's full power, though, and way higher RPM than the prop will turn the
shaft when not under power.

If possible, drag the prop and not the RPM. Then refer to the engine's
documentation (hopefully) to read how much power the engine WOULD BE
producing at that RPM.

You have me thinking that a cool setup would be an "outboard generator".
A modified long shaft outboard leg, with the correct pitch prop installed
and a big alternator in place of the motor...

Again Cavelamb, sorry to high jack your post but I never saw his
original reply.


Cheers,

Bruce
(bruceinbangkokatgmaildotcom)