I bet I can guess the "Thrust Measurement" the trolling motor marketing
types use! :-)

"Brian Whatcott" wrote in message
...
Oh well.
I tried.
To explain that thrust varies with speed.
But home aircraft builders (and some boaters evidently)
just *love* the simplicity of measuring a tethered vehicle.

If you bought my maths, you would say:
Hey! At standstill, the thrust should be huge!
As it happens, when tethered, these vehicles see some water (or air)
flow, so you are measuring thrust at some limiting low speed,
depending on the propeller pitch etc.

This means you can make a tethered vehicle show a rather high thrust
by selecting a very low pitch prop.
This renders the prop quite useless for a vehicle of reasonable speed.
But for a vehicle that is really really slow like a trolling boat, the
tethered thrust test does give some idea of motor power.

Brian Whatcott Altus OK

On Mon, 30 Jun 2003 17:28:50 +1000, Andrew Puddifer
wrote:

For such a small motor, you could just the the baot to a jetty with a
spring scale and measure the thrust?

Andrew.

Ervin Charles wrote:

Thanks Brian,
I appreciate the in-depth analysis but to a layman like me that
didn't
clear up a lot. Lets just assume that a 1/2 hp motor 'does' give us a
hull
speed of 4.5 mph swinging the afore mentioned 11 x 12 prop at 850 rpm.
What
would the thrust in lbs be?
And yes, I am aware that prop efficiency is probably around 80%. I have
no
way of knowing what the hull drag will be...or do I?

Using your formula...I think...

Hull speed is 4.5 mph (1.5 times your 3 mph example of 1.34 m/s = 2.01
m/s)
1/2 hp = 373 watts

373 watts / 2.01m/s = 185.5 Newtons
or about 46 lbs of thrust (again assuming 100% prop efficiency)
Is this about right?
Hoping I don't sound like an idiot...
Charles E.