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.
Compensator solutions
On engines where engine-driven moment com-
pensators cannot be installed aft nor fore, two so-
lutions remain to cope with the 2nd order moment
as shown in Fig. 17.03.02:
1) No compensators, if considered unnecessary
on the basis of natural frequency, nodal point
and size of the 2nd order moment.
2) An electrically driven moment compensator
placed in the steering gear room, as explained
in Section 17.03, option: 4 31 253 or 255.
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. Due to
its position in the steering gear room, solution 2)
is not particularly sensitive to the position of the
node.
Determine the need
A decision regarding the vibrational aspects and
the possible use of compensators should prefer-
ably be taken at the contract stage. If no experi-
ence is available from sister ships, which would
be the best basis for deciding whether compensa-
tors are necessary or not, it is advisable to make
calculations to determine this.
If the compensator is initially omitted, measure-
ments taken during the sea trial, or later in service
and with fully loaded ship, will be able to show if a
compensator has to be fitted at all.
Preparation for compensators
If no calculations are available at the contract
stage, we advise to make preparations for the fit-
ting of an electrically driven moment compensator
in the steering compartment, see Section 17.03.
Natural frequency
cycles/min.
4
n
o
d
e
3
n
o
d
e
2
n
o
d
e
20,000
40,000
60,000
dwt
80,000
5
n
o
de
Cycles/min. *)
*) Frequency of engine moment
M2V = 2 x engine speed
S50ME-C/ME-B8
150
100
50
200
250
300
S50ME-B9
S40ME-B9
Fig. 17.02.01: Statistics of vertical hull vibrations, an ex-
ample from tankers and bulk carriers
178 61 16-0.1
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