Man B&W S50ME-B9.3-TII, Project Manual

Page: 278 / 337

MAN B&W 17.06
Page 2 of 2
MAN Diesel
198 42 269.3 MAN B&W MC/MC-C, ME/ME-C/MEB/GI engines
When running undercritical, significant varying
torque at MCR conditions of about 100150% 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 onenode vibration
is so adjusted that resonance with the main criti-
cal order occurs about 3070% 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