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MAN B&W 17.04
Power Related Unbalance
To evaluate if there <strong>is</strong> a r<strong>is</strong>k that 1st and 2nd order
external moments will excite d<strong>is</strong>turbing hull
vibrations, the concept Power Related Unbalance
(PRU) can be used as a guidance, see Table
17.04.01 below.
PRU =
External ___________ moment
Engine power Nm/kW
With the PRU�value, stating the external moment
relative to the engine power, it <strong>is</strong> possible to give
an estimate of the r<strong>is</strong>k of hull vibrations for a specific
engine.
Table 17.04.01: Power Related Unbalance (PRU) values in Nm/kW
Calculation of External Moments
In the table at the end of th<strong>is</strong> chapter, the external
moments (M 1 ) are stated at the speed (n 1 ) and
MCR rating in point L 1 of the layout diagram. For
other speeds (n A ), the corresponding external moments
(M A ) are calculated by means of the formula:
M = M x A 1 { n __ A
n } 1
2
kNm
(The tolerance on the calculated values <strong>is</strong> 2.5%).
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Based on service experience from a great number
of large ships with engines of different types and
cylinder numbers, the PRU�values have been
classified in four groups as follows:
PRU Nm/kW Need for compensator
0 - 60 Not relevant
60 - 120 Unlikely
120 - 220 Likely
220 - Most likely
S65ME-C8/-GI – 2,870 kW/cyl at 95 r/min
5 cyl. 6 cyl. 7 cyl. 8 cyl. 9 cyl. 10 cyl. 11 cyl. 12 cyl. 14 cyl.
PRU acc. to 1st order, Nm/kW 15.8 0.0 6.7 19.8 N.a. N.a. N.a. N.a. N.a.
PRU acc. to 2nd order, Nm/kW 188.2 109.1 27.1 0.0 N.a. N.a. N.a. N.a. N.a.
Based on external moments in layout point L 1
N.a. Not applicable
MAN B&W S65ME-C8/-GI 198 69 85-2.1
MAN Diesel