MAN B&W 17.04
Page of
MAN Diesel
MAN B&W L35MC6 198 58 79-3.0
Power Related Unbalance
To evaluate if there is a risk that st and 2nd or-
der external moments will excite disturbing hull
vibrations, the concept Power Related Unbal-
ance (PRU) can be used as a guidance, see Table
7.04.0 below.
PRU =
External moment
___________
Engine power
Nm/kW
With the PRUvalue, stating the external moment
relative to the engine power, it is possible to give
an estimate of the risk of hull vibrations for a spe-
cic engine.
Based on service experience from a great number
of large ships with engines of different types and
cylinder numbers, the PRUvalues have been
classied in four groups as follows:
Table 17.04.01: Power Related Unbalance (PRU) values in Nm/kW
Calculation of External Moments
In the table at the end of this chapter, the exter-
nal moments (M
) are stated at the speed (n
) and
MCR rating in point L
of the layout diagram. For
other speeds (n
A
), the corresponding external mo-
ments (M
A
) are calculated by means of the formula:
M
A
= M
x
{
n
A
__
n
}
2
kNm
(The tolerance on the calculated values is 2.5%).
PRU Nm/kW Need for compensator
0 - 60 Not relevant
60 - 20 Unlikely
20
- 220 Likely
220 - Most likely
L35MC6 – 650 kW/cyl at 210 r/min
5 cyl. 6 cyl. 7 cyl. 8 cyl. 9 cyl. 10 cyl. 11 cyl. 12 cyl. 14 cyl.
PRU acc. to st order, Nm/kW 9. 0.0 3.9 5.7 0.3 2.3 .4 0.6 N.a.
PRU acc. to 2nd order, Nm/kW 88.0 5.0 2.6 0.0 .2 0.5 .8 0.3 N.a.
Based on external moments in layout point L
N.a. Not applicable
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