derbyrm wrote:
With a GPS and a spring scale, you could get a pretty good approximation of
the thrust. Attach a drag; e.g. a triangle of plywood, to a line and
measure both the reduction of speed and the pounds of drag. Assume as a
first approximation that the thrust/drag curve is about linear for a small
increment of drag. (We know it's exponential, but you're probably not up
near hull speed where it goes vertical.) If there's any wind or current,
make runs in several directions.
Add drag in increments and you'll generate a series of slopes. Find an
exponential curve that fits and you should have a pretty good guess at the
thrust curve.
Roger (or maybe not)
http://home.insightbb.com/~derbyrm
"Bob S" wrote in message
...
Brian Whatcott wrote:
On Wed, 26 Jul 2006 08:43:23 -0700, Bob S
wrote:
I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.
I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.
The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.
Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.
Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.
These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.
I would be pleased to hear from any of you who are also interested in
electric propulsion.
Bob Swarts
Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.
V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)
Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP
Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph
This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.
Take it with a pinch
Brian Whatcott Altus OK
Basically I agree with your analysis, but you assume equal over all
efficiencies for both motors, which negates both Minnkota's published
specs and my observations. Modern small electric motors can (not saying
this is actually the case)have efficiencies well above 90%. One of the
problems is that neither Minnkota nor MotorGuide will state efficiencies
for their motors or props. But IF this is the case with the Minnkota 50 lb
model, then an over all efficiency of 80% would not be impossible. In any
case, without being able to accurately measure the current or thrust at
speed, it is difficult to wring much more information from my results.
Also, there were probably a couple hundredths of a volt more drop along
the internal and external motor cable (I measured at the input end of the
factory cable). Note, though, that your theoretical values of available hp
(.3 and .43) are not in bad agreement with my observed results of .28 and
.53 hp.
I sure would like to see some more people contribute some actual numbers.
BS
I am not sure this will work satisfactorily. The drag total drag force
is comprised of that from the plate and that from the boat itself. I
could measure the force on the plate as you suggest with a spring scale,
but unless it is significantly greater than the drag on the boat, I
can't solve for the thrust. And while it may be much greater than the
drag on the boat as its size increases and the speed decreases, that
will be at low speed, so I still don't have thrust at full speed.
BS