Calling all prop clearance gurus

My prop tip clearances seem insufficient. See the attached photo. The shaft itself and the alignment are true but yet there is some vibration felt.

It’s a 1987 downeast style Duffy 26 with a 150hp Volvo. WOT 18 kn., 4,000 max RPM, 1.93 gear, 18x13 four blade prop, 1 ½” bronze shaft. One blade is .030” forward, out of track, will fix. The most useful reference I know of is David Gerr’s Propeller Book, but it is general and not tailored to semi-displacement boats. Here are the clearances:

Upper, to hull: 2.5” (14% of prop. dia) Guideline is 2.7” (15%).
Lower, skeg 1.6” (9% of prop. dia) Guideline is 2.7” (15%?)
Fwd to deadwood 11.7” (65% of prop. dia) Guideline is 5.4” (30%)
@ half blade radius
Aft (blade to rudder) 3.5” (19% prop. dia) Guideline is 2.7” (15%)

Also, the overhang past the cutless bearing of 4.6” for a bronze 1.5” shaft is excessive. Am thinking of cutting the shaft to reduce the overhang to 2.5” to improve the overhang and bottom clearance. Barely enough for a collar style zinc (about 1 ¼” long). But that will decrease the upper clearance to hull which is already at the minimum.

So the question is: which is more important from a noise & vibration standpoint- reducing the overhang or do nothing and not further reduce the top hull clearance??

Many thanks

 

If you have vibration, the first thing to do is to establish what kind of vibration; its magnitude, amplitude and its frequency and hence its location.

Until you do that, you wont know if any of your proposals above would cure it. Especially if the boat is built as originally designed.

But for general advice 20% tip clearance is what one would aim for, below 15% you're asking for trouble. Not to say it wont work, but, "in general" terms.

 

Thanks,

The vibration is very slight, as felt by the hand on the deck, engine box and cabin bulkhead. Actually barely noticeable, except at one narrow RPM range below cruising speed. The cutless was worn and was going to re-true the prop so I wanted to seek this advice on whether the prop clearances, etc. were adding to vibration or structure borne sound.

I have a sound/frequency meter and have sound levels at the helm (94 dB at cruising) and in the aft cockpit. The engine box has 1"of 1lb/SF weighted vinyl foam. Are you suggesting I check frequency at key spots like near the prop, rudder, muffler, exhaust tube clamps, etc??

 

Yes.

Obtain the displacements at a range of varying frequencies (shaft rpm) along the drive chain (inc structure). This will enable you to build a simple picture of what is occurring and where and its magnitude. Not all vibration is problematic; although it may feel like it, hence you need to quantify the vibration.

From the results, you should be able to then identify the source/cause.

 

Ad hoc,

Thanks. Could you please explain what "displacements" are? Do you mean the frequency at the two loudest dB readings? At say 2 speeds; 3400 cruising RPM and 3000? I know very little about this subject.

 

In simple terms it is the amount of movement that is experienced at a given frequency. The complex equations of motion, for even a single degree of freedom with an external force, the solution of the equation directly gives the displacement, its movement.

All knowing the simple relation between displacement, velocity and acceleration is:

v(t)= dx/dt and a(t)=d^2x/dt^2.

Thus the displacement is found from the velocity of the item being analysed.

In metric the unit value is mm/s. Values below 10mm/s are ok, values between 10-20mm/s need investigation, values above 20mm/s are a cause for concern.

 

In addition to Ad Hoc's recommendations, I'd check the flange coupling at the gear end. Loosen the bolts half a mm all around and then check the flange gap at four positions.

 

Comments by others have been excellent. My 2 bits worth,
a) The tip clearance to hull should be 20% dia. but not less than 15%
b) The prop overhang is too high. Approx. max allowance is no more that 2 x shaft dia from end of bearing to front of prop hub.
c) Is shaft bearing spacing okay? Long unsupported lengths can 'whip' and make lots of vibration.
d) Is the shaft/prop fit and the key fit good? Poor machining, key riding, or an elliptical bore (my personal favorite), can create impressive vibration.
e) Is the prop 'true'? Balanced, blades same pitch, area, bored to the pilot hole, etc, etc.
f) Is the prop from a recognized mfr.- not a cheap knockoff.
g) Also re-check your prop/speed calculation. dia, pitch, blade area, etc.
h) check the shaft/stern bearing fit. If too loose you could get shaft orbiting and this will vibrate real good.

 

One thing you might want to check. With the boat in the water , so everything is settled in place. Unbolt the shaft flange from the engine flange. Check for close alignment of the flange plates. If need be, adjust engine mounts accordingly to get best alignment. When you take the boat out for repairs Unbolt the flanges so not to spring the shaft.

 

When vibration is a hassle sometimes getting an extra 1/2 inch or more of glass over where the prop is located and vibrating the hull will quiet things down.

Plan B is a super expensive 5 blade prop , of smaller diameter. .

 

Are you sure your vibration is drive line related? Structure borne noise in insidious and if your engine mounts are original they may be suspect given their age. Especially check the one under the injection pump, they fail first from fuel contamination, the rubber will be sticky and gooey.

If you are only feeling the vibration in gear only it could still be mounts. They may only transmit vibration under thrust and be OK in static.

The first this then that approach is a good way to go as suggested.

 

The biggest problem I see is the overhang, that will cause the shaft to sag or even bend when left ashore with a heavy prop. We aim to get the prop less than shaft diameter from the last bearing support, ideally we work on half an inch and put the shaft anode on the prop nut.

 

You stated that the shaft alignment is true.
There are two alignments to check, the first and most easily done is by pulling the shaft apart a bit at the coupling and checking the space with feeler gauges. This will confirm
that the shafts are parallel. Ie the concentric axis are PARALLEL

That does not mean that they are concentric and intersect. When the boat is in the water, and you are putting the bolts through the coupling flanges, they should slide through without only slight resistance. Often the bolts are threaded where they go through the flanges and often the outside diameter of the threaded area is smaller than the shoulder. You can make up the joint but you can build in some misalignment. A very quick way to check this is to take either some longer bolts that have a shoulder long enough to go through both flanges and or say if the hole size is 3/8 inch, slide a couple of drill bits through a few holes at the same time.

You stated that the vibration is more evident at a narrow rpm band width.

There are a couple of things that can be happening here.

The shaft speed has reached its first critical speed and its inherent vibrational effects are at a maximum for this speed and then diminishes with increased rpm. While there are a few things that can be done, ie relocate cutlass bearings, increase the diameter of the shaft ,reduce the cantilever etc. most are not really practical.
If the vibration is slight as you mentioned and then goes away, it might not be worth any effort to try to get rid of this small problem.

The other situation that can occur, and I am assuming that your bearings and alignment are proper, is sympathetic vibration or natural frequency of vibration.
In a rotating shaft there is always some vibration as small misalignments or even the slight deflection from the weight of the shaft will cause the mass not to be concentric with the center of the output, the engine, and the center of the input, the propeller.

This swinging unconcentric mass will cause a vibration and this vibration might hit the natural frequency of vibration of perhaps the motor on the motor mounts. At this point, the vibration will be at a maximum. As the engine/prop rpm is increased the vibration may disappear and it sounds like perhaps it does in your case.

In rotating shaft design, some of the elements are the diameter of the shaft, alignment, the location of the nodes, (stabilizing points of the shaft, engine, cutlass bearing) and of course the operating rpm.

This sympathetic vibration effect can exhibit itself in day to day occurrences. If you have a ballpoint pen sitting in a tray in your car and at a certain speed, it will begin to rattle and then disappear with increased speed, the pen has gone through its natural frequency of vibration caused by vibration of the car and surrounding conditions.

I am not really offering a fix except to suggest that as others have stated, check both shaft alignments, ensure that the bearing is tight, check the condition of the motor mounts for rigidity.
But if the vibration is slight, as stated, it could be just a result of the geometry and dynamics of your system.



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