How do different propeller shaft angles impact the performance of marine vessels?

[Furkan]

Hello All,

Yes, zero-angle, parallel to the waterline sounds great, yet It does not applied most of the time. I even witnessed an addition of extra weight at the end of the shaft to submerge propeller better with an angle. But, what's the logic behind it? How does the steep or inclined position of the propeller in the water effect speed and performance? Any chance for propeller to partly submerged to the water? Does my question have to do with speed, propeller's diameter and if the hull is planing or not? I'm eager to listen to your ideas and read the discussion here.

Thanks.

 

[C. Dog]

You may need to be more specific here because all propellers and hulls
are designed for a particular purpose, and generally become inefficient outside the scope of that purpose.

Propeller shafts are angled for trim reasons, to lower the bow and improve hull efficiency. In part the angle is to counteract the mass of the engine and transmission.

 

[philSweet]

Moderate shaft angles relative to the inflow angle, say less than 15 degrees, have a very small effect on performance, but there is some loss of versatility, and the prop and shafting may be tweaked a tiny bit to better handle some vibrations. The vibrations are there, but they are manageable. The thrust generally follows the flow through the prop and not the shaft angle. At zero speed, the thrust vector on the shaft is in line with the prop axis (but there are pressure effects on the hull and appendages that change the total thrust vector). As the boat starts moving and the prop slip drops, the prop starts to paddle it's way through the water and the thrust vector begins to tip more in the direction of the flow direction.

The trade-offs between shaft length, prop size, prop clearance, shaft skegs and bearings, shaft and hub resistance, vibration issues, and maneuvering versatility all come in to play to get best efficiency, and each varies boat to boat. Auxiliary sail boats often ran 20 degrees shaft relative to the waterline, and had a 10 degree bottom angle as well. This worked fine for them since efficiency wasn't really the issue here. While sailing with a folding prop, this gave low hydro resistance due to the short exposed shaft, and it saved weight.

 

[DogCavalry]

And definitely partly submerged props @Furkan . That's a surface piercing prop, pattent 1911. Used in the fastest boats.

 

[rangebowdrie]

For an example, let's consider a right-hand prop, and we're looking at the boat from astern.
As the shaft angle increases the pitch of the descending blade on the right hemisphere relative to water flow is increased, where the ascending blade in the left hemisphere the pitch is decreasing.
Over a complete shaft revolution each blade goes thru a cycle of relative pitch change.
Among the things this causes is asymmetric blade thrust, and noise.

 

[philSweet]

Except it doesn't, to the first approximation. The side with the greater apparent pitch is traveling forward because of the shaft tilt, and the side with the flatter pitch is traveling backwards. If the geometric pitch angle of the fat part of the blade relative to the shaft axis is about the same as the shaft drop angle, it all balances out. Fifteen degrees of shaft drop is normally pretty close to the blade pitch angle at the fat part of the blade. This is why shaft angles from 0 to 15 degrees are rarely a problem. But a big, flat, highly loaded prop at more than 15 degrees can become a problem rather quickly. For instance, a prop with a pitch ratio of 1.1 has a geometric pitch angle of 15 degrees at 0.75R. At 90 degrees and 270 degrees rotation, the blade will have the same angle of attack on the water with a shaft drop of fifteen degrees.