As the ship has high output and large size with the development of
shipbuilding and steel technologies, the shaft stiffness increases, but it is
the situation that the hull is deformed much more easily than before due to
using high-strength steel plate. Therefore, deep experience and high
attention of the designer are required as the propeller shaft cannot endure
the reaction force change coming from the deformation of the hull, if the
calculation of shaft alignment is done without considering the deformation
of the hull.

It can be said that the other area that is very closely connected with
shaft alignment is the lateral vibration of propulsion shaft system. So, it
should be considered in the safety assessment of shafting. It is better that
the distance between centers of supporting bearings of the shaft system is
longer in terms of shaft system alignment, but in terms of lateral vibration,
the natural frequency becomes lower, so there’s a chance that resonance
occurs in the range of engine operating speed.

The research related to lateral vibration still remains as a problem to be
solved due to unclear elements such as supporting bearing’s stiffness in
shaft system, oil film’s stiffness, propeller’s exciting force, etc.
Until now, it only ensures whether there’s a sufficient margin to avoid
the natural frequency of 1st order propeller blades to be within ±20% of
the engine nominal speed in Classification Society, international standards,
etc. Therefore, when considering such a situation, it is necessary to verify
the calculation result of the natural frequency in the lateral vibration with
the actual measurement.

In the shaft system of a ship, the increase of local load in the stern tube
bearing which supports a propeller shaft occurs prominently due to the
influence of the propeller weight at the shaft end, similar to the case of
the cantilever beam. Especially, the after stern tube bearing is likely to
have a concentrated load in the bottom of aft side while the forward stern
tube bearing does on the bottom of forward side. While such magnitude
and distribution of local load are determined by the relative inclination
angle between the shaft and bearing, the bottom of aft stern tube bearing is
most affected among them. Such local load can deflect significantly toward
the aft end of aft stern tube bearing in case that the shaft sags down,
when the eccentric thrust force acts downward due to the propeller force in
the hydrodynamic transient status.

Case studies by some authors have presented the real-time dynamic
behavior analysis of the shaft system in going-straight and turning by using
the telemetry system. While the impact analysis of the shaft system in
going-straight and turning of ship was carried out by domestic researchers
recently, it was difficult to find the case which analyzed real-time dynamic
behavior so far, so that it was considered meaningful to review the shaft
behavior’s impact on the shaft system through this study.

50,000 DWT oil/chemical tanker is a type of ship emerging recently as a
highly efficient eco-friendly ship and it lowered the engine speed by
applying de-rating technology. It reduced fuel consumption significantly
compared to similar ships and its feature is to maximize propulsion
efficiency through applying the propeller of increased diameter.
Therefore, some negative changes in terms of shaft alignment should be
compared to similar ships, as the change in aft structure and increased
weight of the propeller affect the deformation of the hull. Also, as the
forward stern tube bearing is skipped, the natural frequency of lateral
vibration becomes lower, so that the possibility of resonance in the
operating speed range is expected to be slightly increased.

After a review of previous researches, it is considered that there’s no
comprehensive case study reported yet, which is related to the hull
deformation, the lateral vibration and acceleration of the vessel and the
shaft behavior in going-straight for 50,000 DWT oil/chemical tanker.
Therefore, a theoretical review and analysis of measured data were
performed in this study by using finite element analysis, strain gage method
and reverse calculation method. And then the results are reported as follows
after reviewing in detail the stability of the propeller shaft system of the
target vessel.

The finite element analysis result expects that the shaft is placed right
down compared to the design value when it moves from light loading to
full loading due to the hull deformation, and the reaction force of each
bearing satisfying allowable values even under deformation. Also, the effect
of hull deformation acts as a little positive factor increasing stability of the
shaft system by relieving the relative angle of inclination of the aft stern
tube bearing.

While the hull deformation which is analyzed by using the strain gage
method, is expected -2mm from the intermediate shaft bearing and about
-4mm from the main engine bearing and it is a little bigger compared to
the existing 47,000 DWT class, the increased weight of the propeller and
main engine and the aft change due to the increase in propeller diameter
are considered as main causes.

The reaction force of the bearing supporting shaft system met allowable
value like in the finite element analysis result, also in the full deformation
and the cross validation result of bearing force obtained by the strain gage
method, jack up method, and the shaft alignment program showed good
correlations in most conditions so that the reliability of the analysis was
able to be confirmed.

The calculation result of lateral vibration’s natural frequency showed that
resonant revolution speed was located in the area of more than 163.8%
compared to MCR, so that it was above the limit value(±20%) and it was
confirmed that there was no notable resonance point also i