Help, Thrust Generated By Outboard

[Guest]

IN ORDER TO DESIGN A MOTOR MOUNT I NEED TO KNOW THE MAXIMUM THRUST GENERATED BY AN OUTBOARD OF A GIVEN HORSEPOWER. THE APPROACH I HAVE BEEN USING TO COMPUTE THE THRUST APPEARS TO YIELD VERY LOW THRUST VALUES. MY METHOD IS AS FOLLOWS:
GIVEN: 130 HP OUTBOARD WITH A PROP GOOD FOR A TOP SPEED OF 40MPH
FIND: THRUST IN POUNDS

1 HP = 550 FT LBS/SEC; AT 40MPH A DISTANCE OF 587 FT WILL BE TRAVERSED IN 1 SEC.

WORK = POWER OVER A GIVEN TIME
THRUST X DISTANCE COVERED IN A SEC = POWER GENERATED IN A SEC

THRUST X 587FT = 130 X 550 FT LBS/SEC X 1 SEC YIELDS A THRUST OF 122 LBS
THIS IS OBVIOUSLY INCORRECT, PLEASE HELP

[BrettM]

Not to comment on your method but 40 mph is not 587 ft/sec. More like 58 ft/s

[SailDesign]

Most engines produce maximum thrust at low speeds. I know this doesn't make your job easier ;-P
However, if you figure out the drag on the boat at, say, 5knots, and then guesstimate the acceleration, you should be able to figure out something close to the thrust.

[dougfrolich]

Try (326*Shp*e)/Va

Shp=Hp at the Shaft
e= Propeller efficiency
Va=Speed of water at prop

Given your problem; 130hp I assume is the net hp at the shaft.
I assume the propeller is adequately sized
I assume your boat is fair, and water fed to the prop is clean.

(326 x 130 x .70) / 37 = 802 lbs. of Thrust