Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technology Corporation
Cavitation is severe and the noise levels are high. Eventually, the "man at
the wheeln (the duty officer) reduces the power to zero and into full reverse,
which causes the propeller pitch to cycle from "full ahead" to "full
astern." We expect the noiseto diminish to a low level, although the shafts
continue to turn. As the pitch changes into reverse, the noise level will
increase again, as the ship begins to accelerate. When the ship is 50 to 100
m away from the target floe, the duty officer changes the power setting from
"full astern" to "full ahead" and the propeller pitch again rotates through
the zero power position to full power. At this time, the ship may still be
going backwards, and the noise caused by the acceleration may be considerable
as the process begins over.
There will be variations on the above scenario. There are two
propellers, and the officer conducting the icebreaking will need to change
direction at some times. Then, he is likely to use power differently on the
two propellers, even having one set for "full aheadtq while the other is set
for "full reverse".
During the recording session on JUDY ANN, we did not know how the duty
officer was handling the controls for the two propellers. We could observe
the ship motions and we could hear the sounds, and we have tried to
reconstruct how the power was being controlled. Generally, the propeller
blade rate was audible as a rapid series of impulses. At times the blade
signal disappeared; we took those to be times when the pitch changed through
zero power on both propellers. Seeing when the ship was going ahead and when
it was going astern, we could generally relate the disappearance of the blade
sound to a direction reversal. However, there were times when the ship
reversed direction and the blade sounds persisted. We will return to these
considerations, but first it will be beneficial to examine the results of the
spectrum analyses.
Figure 0.1 presents two unrepresentative spectra from the series of 53
computed. The spectrum in Figure B.1A was begun at time 13:12:24, when the
icebreaker was in the "bollard" condition of being stopped.by the ice but
having full power applied. This spectrum had the highest overall band level
(tied with four other spectra) and the highest levels for the 400 to 3150 Hz