2nd Order Moments on 6cylinder Engines
The 2nd order moment acts only in the vertical
direction. Precautions need only to be considered
for 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. 2nd order moment compensators are
not included in the basic extent of delivery.
Several solutions are available to cope with the
2nd order moment, as shown in Fig. 7.03.02, out
of which the most cost efcient one can be cho-
sen in the individual case, e.g.:
) 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 3 233.
3) A compensator mounted on the fore end,
driven from the crankshaft through a separate
chain drive, option: 4 3 243.
As standard, the compensators reduce the exter-
nal 2nd order moment to a level as for a 7-cylinder
engine or less.
Briey speaking, solution ) 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 inefcient in a node
or close to it and therefore superuous.
Fig. 17.02.01: Statistics of vertical hull vibrations in tankers and bulk carriers
178 06 924.1
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