Cases

Techno Fysica BV is a Dutch-based engineering company, highly specialised in solving all kinds of problems related to the dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of the damages observed.

In a series of 10 general cargo ships, cracks developed in the hull at the location where the stern tube penetrates the aft ship. As is common practice nowadays, the engine room is a s compact as possible to obtain a maximum of cargo space. This results in a short, stiff shaft line, which makes certain engineering aspects, such as shaft alignment and thermal expansion, extra critical. 

Aft ship of a compact, high powered general cargo ship with a short, stiff shaft line.

In this particular design, in order to achieve an equal additional load due to the weight of the propeller shaft on both gearbox bearings, the gearbox was placed above the centreline of the stern tube, i.e. 1.27 mm on the propeller side and 2.33 mm on the engine side.

However, for this installation it was impossible with this alignment condition to ensure a positive load on the forward stern tube bearing under cold- and warm operating conditions and under zero pitch as well as under full pitch conditions.

The pitch setting has a big influence on the bending moment, and consequently the deflection, of the shaft due to the eccentricity of the thrust.

This means that under certain operational conditions, contact between the forward stern tube bearing and the propeller shaft was lost and bending moments were exerted on the aft stern tube bearing and the hull.

For the last ships of this series of 10, the manufacturer of the propeller shaft advised an alignment condition where the gear box is placed below the centreline of the stern tube, i.e. –1.46 mm at the propeller side and – 1.98 mm at the engine side. This was determined on the basis of a sag and gap alignment calculation.

In practice, this means that  the gearbox is placed approximately 4 mm lower than in the original configuration.

The additional load on the gearbox bearings is not the same for both bearings, however it is still well within allowable limits and the forward stern tube bearing carries a positive load under all possible operating conditions.

After this modification, no more cracks occurred in the stern tube connection to the hull.

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