Kiwiprops™

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Vibration

Occasionally we have reports of vibration or noise apparently emanating from a Kiwiprop™ unit

Analysis of any vibration issues is always complex

Every propeller will have it's own unique vibration and noise pattern as the US Navy has long known and used in identifying distant vessels.

There are always numerous possibilities that can manifest as vibration or harmonic "beating" where this is received as the difference between independent sources of noise or vibration operating at different frequencies.

The beat frequency is always calculated as: F(Beat) = F1 - F2

Typically with a reduction gear between the engine and the shaft - there is always an opportunity for cyclic or harmonic vibration or "beating" between these two sources.

Other sources may be much simpler and can be caused by any number of other issues:

  • Blade replacements a different size to the original blades
  • Pitch differential between individual blades
  • Damage to a blade(s) - particularly the tips area
  • Paint runs or damage on the leading edge of a blade(s)
  • The misalignment of the shaft
  • Engine mountings moving under thrust and causing subsequent shaft mis-alignment
  • Engine / Geabox drive pplate damper failure
  • Wear in the shaft bearings
  • Loose couplings in the drive train
  • Loose rudder bearings
  • Engine accessories loose etc

 

Shaft angle is always a significant possible course of "excitation" for any noise or vibration. It is important to realise that the propeller is operating in the streamlines which due to the underbody of the vessel may be very different from the shaft angle alone.

Say a propeller has 20 deg of pitch at the blade tip and the shaft angle is 20 deg. Assume in this simple case the streamlines are parallel to the waterline - not always true.

One side of the propeller blades will see a pitch of 20 + 20 = 40 deg while the other side will see a pitch of 20 - 20 = 0 deg. This immediately will generate a substantial side force that can translate to a vibration or noise in the shaft - exacerbated if the shaft bearings have more play than recommended.

Other causes of noise or vibration can be as simple as paint runs - particularly on the leading edge of a propeller blade. Damaged leading edges can also cause vibration. It is always best to fair any dings out with wet and dry to at least a smooth and hopefully symmetric edge.

As the propeller blades sweep past the hull - they displace water which is incompressible which can translate to vibrations being transmitted through the hull.

Slower speeds, higher clearances and finer blade tips all assist in minimizing this potential area of vibration. Hull thickness and stiffness will also be important.

In attempting to diagnose any vibration or noise the first test will be to try and see at what frequency the noise or vibration is being perceived.

Is it a "beat" frequency which is a combination of two independent frequencies ?

Is it of a frequency equal to the shaft speed ? [ Divide engine rpm by reduction ratio ]

Is it of a frequency equal to 3 x the shaft speed which would immediately indicate it is originating off the blades not the shaft or drive train ?

A very simple trick which can be used to assist in locating the source of any noise consists of placing a long wooden stick (or large screw driver) to one ear - then touching the hull in various places with the other end much like a stethoscope.

This a simple but very effective way of trying to track down any noises and where they seem to be originating from. Touching the engine or drive train bearings or Saildrive gearbox in turn will quickly isolate the source of most noise and vibration.

Applying the above tests over a range of engine rpm can assist in attempting to identify the source or sources in the case of beat frequencies

Once located as to source - possible actions to minimize can be undertaken.

Often a simple solution is simply to cruise at a slightly different rpm setting.

All rotating machinery will have it's own harmonics and there will be rpm ranges where little can be done other than alter the rpm settings. Always begin by comparing over the full engine rpm range to assist in deciphering any noise or vibration issues.