Common Causes of Vibration and Noise
Cavitation due to inadequate blade area is—
as we’ve just seen—one common cause of
vibration. Two other common culprits are
- propeller tip noise
- shaft noise from poor alignment and
from bearings
Induced Drag and
Tip Noise
Propeller tip noise comes principally from
inadequate tip clearance. Figure 1-5 shows the
minimum recommended dimensions for ade-
quate tip clearances and proper water flow.
Props don’t really screw their way through the
water as some useful, but simplified, propeller
calculations indicate. Instead, the propeller
blades are hydrofoils that generate lift like
airplane wings, keels, or sails. This means that
the blades generate vortices at each blade tip
just as any hydrofoil or airfoil does. These vor-
tices are termed induced drag. They represent
wasted energy, so it’s a shame we can’t elimi-
nate them, but they are unavoidable. The key
here is that they are energy.
If there’s enough distance between the
prop’s blade tips and the hull, the energy in
these vortices largely dissipates and is swept
aft in the wake before it can impinge on the
hull hard enough to make noise. If you have
too little clearance, though, these vortices
can create a surprising amount of noise or
vibration.Frequency Testing
You can simply listen to a boat underway as
you move about, and try to determine if an
annoying noise is propeller-tip noise, shaft
noise, or cavitation. (Cavitation often sounds
like small bits of gravel being driven throughChapter 1: Propellers and Shafts
Figure 1-5.
Propeller
clearances