BİLGE AKUSTİK
Noise and Vibration Control Engineering
“I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
Major Substructure Vibration
Propeller Cavitation Induced Stern Vibration and Noise
The crew of a new 720-foot container vessel complained of excessive noise In their
quarters. The crew's quarter were aft. The ship was propelled by a 32,000 SHP steam
turbine turning a 6 bladed propeller at 110 rpm.
Symptoms
A noise and vibration survey was conducted on the vessel during a full load
transatlantic voyage. A vlbrograph equipped with a 5:1 magnification probe was used to
measure vibration levels in the steering gear room, engine room and in the aft
deckhouse. A sound level meter was used to measure noise levels In these same areas.
The results of the survey were as follows:
a. The noise which was causing crew complaints was in
the after section of the ship. The noise consisted of
crackling and occasional violent "hammer-like" blows
on the hull In the vicinity of the propeller.
Noise level readings in the quarters generally showed
dBA readings above the comfort range.
b. Blade frequency vibratory displacement amplitudes
were found to be moderate at full power and there was
no indication of any resonances in the upper speed
range.
c. Vibration measurements showed strong displacements
at multiples (2,3,4,5, and 6) of blade frequency.
Problem
The crackling noise and "hammer-like" blows detected In the vicinity of the
propeller are generally associated with propeller cavitation. Likewise, the strong
vibratory displacements at multiples of blade frequency are known to be generally
caused by propeller cavitation. While the vibration levels In the crew living spaces at
multiples of blade frequency did not fall In the uncomfortable range, the noise created
by the vibrating decks and bulkheads created noise levels In excess of recommended
comfort levels.
Based on these findings, a cavitation study was performed In a model basin to
determine the amount and cause of the cavitation. The amount of cavitation was found
to be excessive when a propeller blade passed the vertical or 12 o’clock position. This
condition was caused by very poor water Inflow to the upper area of the propeller disc.
Solution
The model basin recommended that flow directing fins forward and above the
propeller be considered as a solution. Several configurations were tested in the tank
and one selected which best improved the flow Into the top of the propeller disc, thereby
reducing propeller cavitation.
These fins were installed on the vessel and a new vibration and noise survey
conducted with the following results:
a. Noise measurements in the crew quarters
showed a general 2 to 7 dBA decrease. Furthermore,
the crackling and "hammer-like" blows were no longer
evident In the vicinity of the propeller.
b. Blade frequency vibration displacements which
were never considered excessive remained about the
same.
c. The amplitudes of multiples of blade frequency
(2,3,4, and 5) were generally lower in the vicinity of
the propeller and most importantly In the living
spaces.
The installation of the fins improved flow to the propeller, reduced blade
cavitation and its characteristic crackling and hammering noise, decreased the vibratory
displacements at multiples of blade frequency and reduced noise levels in the living
quarters resulting in improved habitability.