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MAN B&W 17.01
Vibration Aspects
The vibration character<strong>is</strong>tics of the two�stroke low
speed diesel engines can for practical purposes
be split up into four categories, and if the adequate
countermeasures are considered from the early
project stage, the influence of the excitation sources
can be minim<strong>is</strong>ed or fully compensated.
In general, the marine diesel engine may influence
the hull with the following:
• External unbalanced moments
These can be classified as unbalanced st and
2nd order external moments, which need to be
considered only for certain cylinder numbers
• Guide force moments
• Axial vibrations in the shaft system
• Torsional vibrations in the shaft system.
The external unbalanced moments and guide force
moments are illustrated in Fig. 7.0 .0 .
In the following, a brief description <strong>is</strong> given of their
origin and of the proper countermeasures needed
to render them harmless.
External unbalanced moments
The inertia forces originating from the unbalanced
rotating and reciprocating masses of the engine
create unbalanced external moments although the
external forces are zero.
Of these moments, the st order (one cycle per revolution)
and the 2nd order (two cycles per revolution)
need to be considered for engines with a low number
of cylinders. On 7�cylinder engines, also the 4th
order external moment may have to be examined.
The inertia forces on engines with more than 6 cylinders
tend, more or less, to neutral<strong>is</strong>e themselves.
Countermeasures have to be taken if hull resonance
occurs in the operating speed range, and if the vibration
level leads to higher accelerations and/or velocities
than the guidance values given by international
standards or recommendations (for instance related
to special agreement between shipowner and shipyard).
The natural frequency of the hull depends
on the hull’s rigidity and d<strong>is</strong>tribution of masses,
whereas the vibration level at resonance depends
mainly on the magnitude of the external moment
and the engine’s position in relation to the vibration
nodes of the ship.
MAN B&W MC/MC�C, ME/ME-B/ME�C/ME�GI engines
MAN Diesel
A – Combustion pressure
B – Guide force
C – Staybolt force
D – Main bearing force
Page of
C C
st order moment vertical cycle/rev.
2nd order moment, vertical 2 cycle/rev.
D
st order moment, horizontal
cycle/rev.
Guide force moment,
H transverse Z cycles/rev.
Z <strong>is</strong> or 2 times number of cylinder
Guide force moment,
X transverse Z cycles/rev.
Z = , 2... 2
Fig. 17.01.01: External unbalanced moments and guide
force moments
B
A
178 06 82�8.1
198 41 40�5.2