Question

A new vessel exhibits severe after end vibration.

(a) As Chief Engineer, outline a procedure to investigate and; identify the source of the vibration

(b) Suggest possible remedies to obviate/reduce aft end vibration

SOLUTION.

(a) Since it is a new ship, deterioration of the propeller or worn bearings/machinery is unlikely. However, damage can occur at any time in a ship's life. Discounting damage, there may be temporary conditions affecting the vessel such as: .load conditions (but usually this is not confined to the aft end).

Shallow water - pressure reflections.

Resonance at a particular shaft revolutions when manoeuvring.

Shallow draught - large variation in blade loading.

These would only occur for particular conditions which would help to identify them. If the vibration was more persistent at around service speed, the investigation would be carried out during occurrence of the worst examples.

The machinery would have been fully tested/proven. The propeller would be the most likely cause, either directly (may be cavitating or resonating with the structure) or indirectly through propeller-hull interactions. To check for resonance with the hull girder or local structure, the need is to identify the frequency [Vibrograph- measures frequency and amplitude) and check if it matches with the fundamental blade frequency of the propeller (revs X number of blades). If this proves to be the case, then it may indicate an unfortunate choice of propeller.

(b) If resonance is due to the propeller blade frequency matching the natural frequency of the structure, it will probably require a change of propeller, increasing the number of blades.

It may be due to propeller-hull interactions if the minimum clearances are used. The shipowners/designers have tended to fit the largest propellers the ship can accommodate for maximum efficiency. The minimum aperture clearances may be insufficient to dampen pressure pulses into the hull. In some cases, this has resulted in the propeller being cropped. The propeller may be cavitating badly, but modern design/testing techniques tend to overcome this. However, the variable wake around the propeller disc could also be the cause and this is notoriously difficult to check with model tests. It may be necessary to fit some flow correction device (ducts, fins, vanes, roofs etc) to improve flow distribution into the propeller. A highly skewed propeller could also be used to alleviate the problem and this also may allow a smaller propeller, increasing hull clearances.