Vertical and horizontal forces generated by a propeller

[Ricky Larsen]

Dear Sirs

I need somebody to explain in an easy way the vertical and horizontal forces acting perpendicular on thrust generated by a propeller. Thank you in advance.

 

[jehardiman]

For the shaft or the blade? More than just the weight and balance? Realistically, the thrust moment is the big fatigue driver.

 

[Ricky Larsen]

I need to explain the general forces generated by a propeller (moment, thrust, normal force and side force) I have a little problem explain (and understand) the normal force acting vertical and the side forces.

 

[jehardiman]

Open propeller or wake adapted? You need the forces in the propeller? In the shaft? Or on the stern tube bearing? What forces you get depend on where you take the forces in the FBD.
Suffice to say, it is the differential wake that causes an uneven distribution of blade forces as the propeller revolves. This results in a net force vector in the plane of the disk as well as a oscillating moment and thrust in the shaft normal to the plane of the disk. The plane forces oscillate in proportion with the number of blades was well as the singular rotary in plane force caused by prop imbalance and the fixed down force caused by overhanging. Additionally, there is a torsional moment in shaft caused by the uneven blade forces as well as an axial moment caused by the thrust distribution over the disk. These are added to the whirling shaft bending moment due to propeller imbalance.
It's fairly simple if you look at one blade at a time and just superimpose all the forces and moments through one propeller revolution.

 

[DogCavalry]

As you presumably did, during your education.

 

[Ricky Larsen]

I have a case where a vessel in a drydock started up the engine and sucked up an object from the bottom of the dock hitting and damaged the propeller. I think it was due to thrust generating a flow of water under the keel generating low pressure. It was in this context I was asked to explain the vertical and horizontal forces generated by the propeller and if it could be possible that a vertical force generated be the propeller could have sucked up the object.

 

[jehardiman]

I have a case where a vessel in a drydock started up the engine and sucked up an object from the bottom of the dock hitting and damaged the propeller. I think it was due to thrust generating a flow of water under the keel generating low pressure. It was in this context I was asked to explain the vertical and horizontal forces generated by the propeller and if it could be possible that a vertical force generated be the propeller could have sucked up the object.

Do you understand disk theory? Start from there.

 

[Ricky Larsen]

I will start from there, thanks. It is a little out of my field, and I have not worked with it since I left school, so I must catch up

 

[baeckmo]

In this context, it is not so much a question of vertical or lateral forces per se, as the effect of inflow velocity field into the prop disc. The situation you described emulates a bollard pull with restricted inflow area. The screw disc will act as a flow accelerating sink. With an open prop working at low or zero advance ratio, you will see inflow from all directions, even from behind. When the inflow routes are restricted, as with a dock bottom, there will be high velocities along the restriction walls; any loose object will be pulled into the "eye of the sink".

If the interest is about the resulting forces upon blades or shaft support, as JHM has noticed, you have to analyze the effects of the inflow variations, and how they generate a non-axial vector sum representing the total impulse on the shaft and its support. Is your focus on blade forces, you have to follow the individual blade as it travels through the inflow field with its variations in direction and intensity, not to forget small scale vortices from upstreams disturbances.

 

[Ricky Larsen]

Thanks a lot. All your inputs are highly appreciated.

 

[jehardiman]

As you presumably did, during your education.

Just as a public service announcement; I think it should be pointed out that Naval Architect does not mean Marine Engineer or vice versa. While my degree was in Naval Architecture & Marine Engineering (NA&ME) many schools, especially the Maritime Academies and schools outside of the US offer the two as separate degrees/certificates. The Chief Engineer doesn't have to be able to calculate the as loaded KM and the 2nd Officer doesn't need to memorize the care and feeding of a MAN B&W S90ME-C9.2. Different schooling for different positions.
Carry On.