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MAN B&W 17.02
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 occur
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
<strong>is</strong> a rather comprehensive procedure and often
not very accurate, despite advanced calculation
methods.
A 2nd order moment compensator compr<strong>is</strong>es two
counter�rotating masses running at twice the engine
speed.
S50MC
S60MC
S70MC
S80MC
S90MC
Cycles/min. *)
300
250
200
150
100
50
*) Frequency of engine moment
M2V = 2 x engine speed
Natural frequency
cycles/min.
MAN B&W S70MC/MC-C, L70MC-C, S65MC-C,
S60MC/MC-C, L60MC-C, S50MC/MC-C, S50ME-B8,
S46MC-C, S46ME-B, S42MC, S35MC, L35MC, S26MC
2 node
5 node
3 node
4 node
20,000 40,000 60,000
80,000
dwt
178 06 92�4.2
Fig. 17.02.01: Stat<strong>is</strong>tics of vertical hull vibrations in tankers
and bulk carriers
MAN Diesel
Compensator solutions
Page 1 of 3
Several solutions are <strong>available</strong> to cope with the
2nd order moment, as shown in Fig. 17.03.02, out
of which the most cost efficient one can be chosen
in the individual case, e.g.:
1) No compensators, if considered unnecessary
on the bas<strong>is</strong> 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 the main chain drive, 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.
4) Compensators on both aft and fore ends,
driven from the crankshaft by the main chain
drive and a separate chain drive respectively,
options: 4 31 203 and 4 31 213.
As standard, the compensators reduce the external
2nd order moment to a level as for a 7-cylinder
engine or less.
Briefly speaking, solution 1) <strong>is</strong> applicable if the
node <strong>is</strong> located far from the engine, or the engine
<strong>is</strong> positioned more or less between nodes. Solution
2) or 3) should be considered where one of
the engine ends <strong>is</strong> positioned in a node or close
to it, since a compensator <strong>is</strong> inefficient in a node
or close to it and therefore superfluous. Solution
4) should be considered if the engine <strong>is</strong> positioned
over the node.
A dec<strong>is</strong>ion regarding the vibrational aspects and
the possible use of compensators must be taken
at the contract stage. If no experience <strong>is</strong> <strong>available</strong>
from s<strong>is</strong>ter ships, which would be the best bas<strong>is</strong>
for deciding whether compensators are necessary
or not, it <strong>is</strong> adv<strong>is</strong>able to make calculations to determine
which of the solutions should be applied.
198 69 76-8.1