The Steamboating Forum

Hello all,

Does anyone know how to calculate the output thrust (in lbs) from a propeller of a given size & pitch at a given rpm? I know that I would be ignoring the engine power but I'm doing a back check based on the thrust needed to move the hull at speed and will be assuming (at this point) that the engine is capable of providing the needed power.

Thanks

Ken

David Gerr's covers this in some detail in his Propeller Handbook....

For motion at speed:

Thrust = 326 x (shaft hp) x (propeller efficiency) / (speed of water at prop in knots)

Propeller efficiency can prob. be taken at .7 to .75, for a steam vessel w/ a adequate
diameter prop. The higher the pitch ratio (pitch/diameter) the lower the efficiency.
(For a more accurate value, see the book)

Speed of water at the prop is given by:

(boat speed in knots) * (1.1 - (.6 * Displacement / ((water line length in feet) * (water line beam in feet) * (hull draft excluding keel) x 64)))

For static thrust (bollard pull)

Thrust in pounds = 62.72 x ((horse power) * (prop diameter in feet))^.67

Gerr's book is excellent and is well worth the $12 it costs from Amazon.

- Bart