When running undercritical, significant varying
torque at MCR conditions of about 100150% of
the mean torque is to be expected.
This torque (propeller torsional amplitude) induces
a significant varying propeller thrust which, under
adverse conditions, might excite annoying longi-
tudinal vibrations on engine/double bottom and/or
deck house.
The yard should be aware of this and ensure that
the complete aft body structure of the ship, in-
cluding the double bottom in the engine room, is
designed to be able to cope with the described
phenomena.
Overcritical running
The natural frequency of the onenode vibration
is so adjusted that resonance with the main criti-
cal order occurs about 3070% below the engine
speed at specified MCR. Such overcritical con-
ditions can be realised by choosing an elastic
shaft system, leading to a relatively low natural
frequency.
The characteristics of overcritical conditions are:
• Tuning wheel may be necessary on crankshaft
fore end
• Turning wheel with relatively high inertia
• Shafts with relatively small diameters, requiring
shafting material with a relatively high ultimate
tensile strength
• With barred speed range, EoD: 4 07 015, of
about ±10% with respect to the critical engine
speed.
Torsional vibrations in overcritical conditions may,
in special cases, have to be eliminated by the use
of a torsional vibration damper.
Overcritical layout is normally applied for engines
with more than four cylinders.
Please note:
We do not include any tuning wheel or torsional
vibration damper in the standard scope of supply,
as the proper countermeasure has to be found af-
ter torsional vibration calculations for the specific
plant, and after the decision has been taken if and
where a barred speed range might be acceptable.
For further information about vibration aspects,
please refer to our publications:
An Introduction to Vibration Aspects
Vibration Characteristics of Two-stroke Engines
The publications are available at
www.marine.man.eu → ’Two-Stroke’ → ’Technical
Papers’.
Critical Running
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