MAN B&W S65ME-C8.2-GI-TII, Project Manual

Page: 320 / 395

MAN B&W 17.02
Page 1 of 3
MAN Diesel MAN B&W G70ME-C, S70MEC/GI, L70MEC,
S65MEC/GI, G60ME-C, S60MEC/GI, L60MEC 198 42 208.8
2nd Order Moments on 4, 5 and 6-cylinder Engines
The 2nd order moment acts only in the vertical
direction. Precautions need only to be considered
for 4, 5 and 6-cylinder engines in general.
Resonance with the 2nd order moment may oc-
cur in the event of hull vibrations with more than
3 nodes. Contrary to the calculation of natural
frequency with 2 and 3 nodes, the calculation of
the 4 and 5-node natural frequencies for the hull
is a rather comprehensive procedure and often
not very accurate, despite advanced calculation
methods.
A 2nd order moment compensator comprises two
counterrotating masses running at twice the en-
gine speed.
Compensator solutions
Several solutions are available to cope with the
2nd order moment, as shown in Fig. 17.03.02, out
of which the most cost efficient one can be cho-
sen in the individual case, e.g.:
1) No compensators, if considered unnecessary
on the basis of natural frequency, nodal point
and size of the 2nd order moment.
2) A compensator mounted on the aft end of the
engine, driven by chain, option: 4 31 203.
3) A compensator mounted on the fore end,
driven from the crankshaft through a separate
chain drive, option: 4 31 213.
As standard, the compensators reduce the exter-
nal 2nd order moment to a level as for a 7-cylinder
engine or less.
Briefly speaking, solution 1) is applicable if the
node is located far from the engine, or the engine
is positioned more or less between nodes. Solu-
tion 2) or 3) should be considered where one of
the engine ends is positioned in a node or close to
it, since a compensator is inefficient in a node or
close to it and therefore superfluous.
Determine the need
A decision regarding the vibrational aspects and
the possible use of compensators must be taken
at the contract stage. If no experience is available
from sister ships, which would be the best basis
for deciding whether compensators are necessary
or not, it is advisable to make calculations to de-
termine which of the solutions should be applied.
Natural frequency
cycles/min.
250
Cycles/min. *)
S60MEC
S65MEC
150
100
50
200
*) Frequency of engine moment
M2V = 2 x engine speed
S70MEC
40,000
60,000
dwt
80,000
o
4 n
de
d
3 no e
o e
2 n d
e
5 n
od
Fig. 17.02.01: Statistics of vertical hull vibrations, an ex-
ample from tankers and bulk carriers
178 61 17-2.0