Thrust of a propeller

[rob768]

Is there anyone who can make some sensible remarks on the amount of thrust generated by a propeller (i.e. thrustforce per kW), and the influence of speed (bollard pull conditions vs. nominal hull speed or any other influencing aspects?

thanks

 

[GregLocock]

It's pretty complex, as the efficiency of a given prop largely depends on the slip ratio.

Things that have a big effect - blade area ratio, pitch, diameter.

Things that have a smaller effect - no. of blades, shape of blades.

I've only ever sized a prop based on the work done in Froude's tank, which was a rather more complex task than using the modern charts.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JRMacGregor]

As stated above it is rather complex.

If you want to do it right, there are many textbooks.

If you want quick and dirty, an old rule of thumb for the thrust produced at bollard pull (zero speed) was 1 tonne of thrust for every 100hp. Thus a 745kW thruster (1000hp) would generate 10t thrust.

 

[JRMacGregor]

Should also have stated that

1) a prop designed for bollard pull is very different from one designed for speed
2) a fixed pitch prop designed for bollard pull will be very poor at high speed and vice versa (hence controllable pitch propellers for vessels working at two duty points - like trawlers)
3) generally, thrust per kW will fall off with increasing speed
4) in general, faster ships have LESS heavily loaded propellers in terms of kW per unit disc area
5) this is becase POWER = SHIP SPEED x THRUST DEVELOPED BY PROP
6) if you obtain data on speed and power for a ship of similar dimensions to the one you are interested in you will be able to get a good idea of the thrust produced in that case, and then the ratio of thrust to power for that type of application. You can then use that norm for your case. This does not scale well across different speeds, sizes and type of ship.

For example
10 knot fishing boat with 350kW engine
thrust in kN = 350/(10 x 0.5144) = 68kN thrust (7 tonnes) = 0.2 kN per kW

33 knot aircraft carrier with 290,000 hp (216,000 kW on 4 props)
thrust in kN = 216000/(33 x 0.5144) = 12724 kN thrust (1300 tonnes) = 0.06 kN per kW