MAN B&W
17.04
Page of
MAN Diesel
MAN B&W S80MC-C7/ME-C7
198 46 79-8.2
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-
cific engine.
Based on service experience from a great number
of large ships with engines of different types and
cylinder numbers, the PRUvalues have been
classified 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
S80MC-C7/ME-C7 – 3,880 kW/cyl at 76 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
N.a.
0,0
9,3
27,3
N.a.
N.a.
N.a.
N.a.
N.a.
PRU acc. to 2nd order, Nm/kW
N.a.
46,4
36,4
0,0
N.a.
N.a.
N.a.
N.a.
N.a.
Based on external moments in layout point L
N.a.
Not applicable
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