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No. 6/95 Edited by DSO-245
SHIP TYPE:General Cargo SIZE(GRT):4000 BUILT YEAR:1991
Broken tailshaft
Course of eventsThe vessel in question was under way to Taiwan
loaded with containers when problems arose with the controllable
pitch propeller. The vessel wasdisabled and tug assistance had to
be called. The voyage continued under tow to the discharge port
where survey ascertained that the tailshaft wasbroken. After
discharging the cargo the vessel was dry-docked.
Extent of damageOn withdrawing the tailshaft it was discovered
that the shaft had broken between the forward and aft stern tube
bearings (see diagram). The shaft ran inwater-lubricated rubber
bearings and was not fitted with a continuous liner. Between the
forward and aft bronze liners, the shaft was covered by a GRP(Glass
Reinforced Polyester) lining. When the remaining pieces of GRP were
removed, it was found that the shaft was heavily corroded in the
area ofthe fracture. The aft rubber bearing was missing and the
forward one completely destroyed.
Probable causeThe GRP shaft lining had probably failed and
allowed water to reach the steel surface beneath the cover. With
corrosion arising from the penetration ofwater, the fatigue life of
the steel was reduced and lead to a fatigue failure of the
tailshaft. The incident probably began as a torsional corrosion
fatiguecracking and culminated in a total torsional rupture of the
shaft. Apparently the GRP lining protection between the two liners
had not been fittedcorrectly.
Lessons to be learnedExperience has shown that it is very
difficult to obtain a completely watertight connection between a
bronze liner and the protective coating if eitherGRP or rubber is
used. Furthermore, if the coating is damaged during mounting or
dismantling of the shaft, water may enter between the coating
andthe steel and give rise to corrosion, even if the damage appears
negligible. Ordinary inspection will not reveal any corrosion of
the steel shaft. Thecoating must be removed completely, and
subsequent refitting must include complete recoating if future
shaft failure is to be avoided.
Conclusion
Water-lubricated stern tube arrangements involving shafts
without continuous liners are vulnerable to damage as described
above and should thereforebe avoided.
Casualty Informationis published by Det Norske Veritas,
ClassificationSupport.
Det Norske VeritasNO-1322 Hvik, NorwayTel: +47 67 57 99 00Fax:
+47 67 57 99 11
The purpose of Casualty Informationis to provide the maritime
industry with'lessons to be learned' from incidents of ship damage
and more serious
accidents. In this way, Det Norske Veritas AS hopes to
contribute to the
prevention of similar occurrences in the future. The information
included is notnecessarily restricted to cover ships classed with
DNV and is presented,without obligation, for information purposes
only.
Queries may be directed toDet Norske Veritas, Classification
Support, NO-1322 Hvik, Norway.Fax: +47 67 57 99 11, e-mail:
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