Splined Props... Okay... Splined Couplings?

Just talked with my prop guy. He says there is a bending moment on surface props, because of the unequal loading and this is why they are normally splined. That being said he said it is not required for the coupling end because the bending moment is not present at the other end of the shaft because of the loading on the strut.

 

Your prop guy is correct, there is a bending moment at the prop side, that's why the shafts are larger than for submerged props in the same power range.
But if your shafts must be splined at one side, they can be splined at the other side as well, without much extra costs, unless you already have a tapered flange and don't want to change that.

 

torsional vibration with a surface prop/shaft is significant

 

The load on a surface prop is not in the prop it is in the shaft approx 2 inches forward of the prop hence the thicker shafts required. There is stress at the root of each blade which is why SP props have a larger hub not only to take a thicker root and shaft.

Splines are loose, if you fit one dry you will feel a slight slop. This is normal for conventional props as the whole disc gives thrust and the slop is not felt.

A surface prop on the other hand say a 2 blade has 180 degrees for the sake or argument of thrust on each revolution, the spline slop would be felt twice per revolution meaning splines would not be as good as taper, where as the taper is tight.

A splined SP prop would rattle itself loose eventually.

 

I think Arneson, Don Smith, Simplicity Marine, Mercury, ZF, France Helices, MSA, and Howard Harley would disagree with you. IMHO.

 

Im sure they would, unless they are some tapered spline.

I would like to hear an explanation of how they survive spline when they are constantly on off thrust from the one blade and one side of the prop.

 

Well spotted Frosty
straight versus tapered spines....another subject
the load goes on and off, thats different than reversing the load

 

Rik might be able to help on this one. Otherwise, as they say "You can't fix what aint broke"

 

Ive often given this some thought,-- and I know Levis don't do spline.

They obviously survive but I guess they use multiple blades which must help a lot.

 

Your Prop only has one blade? haha I'm just messing with you. But I know there are some boats that run 3 blades on Arnesons with no problem.

 

Splined shafts

Well, our drives are manufactured with splines on both ends, both prop and coupling. On the coupling end there is also a slight taper where the flange is connected. Where there has been shaft failure on the coupling end, in the few instances we've seen it (The stresses are at the prop end, primarily), it has usually been when a customer or yard has chosen a cheap coupling, rather than from Centa or other manufacturers with high quality standards. This is one area where it pays to spend a little more (We only very rarely will not send the coupling, usually only in cases of retrofit where someone has made up their mind already or with something extremely different)

As to the vibration, there are several answers. First off, on our drives there are two gaskets that make a very tight seal. We warranty for 3000 hours, so 600-700 hours of annual use has never seen a single warranty replacement due to props vibrating loose (at least that we are aware of) in over 20 years. Secondly, we test for vibration on every new design, and we also test once onboard at commissioning. We are aware of the limits and any anomaly becomes apparent immediately.

On the rare occasion, we will have a customer request that the prop end is not splined. We generally don't recommend that since the splines effectively ensure that the propeller stays balanced and mounted correctly, but it is a "customer is always right" situation and we will customize it for them.

This is one of the reasons why there is generally a weight limit with surface drives: If the propellers get too big, the shaft diameter becomes too great, and the drive length grows to the point that you start running into both torque issues and LCG issues.

 

We use tapers on both ends. The key on the propeller side needs to have a spoon ending not sharp. The fitting between the prop and the shaft taper needs to be blued 80 % (a very good fit). The keys are done to ISO.
If I want that my competitor does not uses his propeller on my dives, I would use splines very complex ones. Have you ever seen a bigger ship with splines?