This section is available on request - MAN Diesel & Turbo

MAN B&W 17.06
Critical Running
When running undercritical, significant varying
torque at MCR conditions of about 100�150% of
the mean torque <strong>is</strong> to be expected.
<strong>Th<strong>is</strong></strong> torque (propeller torsional amplitude) induces
a significant varying propeller thrust which, under
adverse conditions, might excite annoying longitudinal
vibrations on engine/double bottom and/or
deck house.
The yard should be aware of th<strong>is</strong> and ensure that
the complete aft body structure of the ship, including
the double bottom in the engine room, <strong>is</strong>
designed to be able to cope with the described
phenomena.
Overcritical running
The natural frequency of the one�node vibration
<strong>is</strong> so adjusted that resonance with the main critical
order occurs about 30�70% below the engine
speed at specified MCR. Such overcritical conditions
can be real<strong>is</strong>ed by choosing an elastic
shaft system, leading to a relatively low natural
frequency.
The character<strong>is</strong>tics 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.
MAN B&W MC/MC-C/ME/ME-B/ME�C/ME�GI engines
MAN Diesel
Page 2 of 2
Torsional vibrations in overcritical conditions may,
in special cases, have to be eliminated by the use
of a torsional vibration damper.
Overcritical layout <strong>is</strong> 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 after
torsional vibration calculations for the specific
plant, and after the dec<strong>is</strong>ion 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 Character<strong>is</strong>tics of Two-stroke Engines
The publications are <strong>available</strong> at
www.mandiesel.com under
‘Quicklinks’ → ‘Technical Papers’
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