VIBRATION

Like all things, ships are elastic structures able to vibration when they summit to periodic forces. All periodic forces have a certain frequency, if this matches with the natural frequency of the ship, it causes a phenomenon called resonance which it’s know as vibration or tremor.

From the desktop of

Gustavo Silva

The main subject of this issue is the determination of the ship’s natural frequency and avoid all the disturbance forces either internal like externals.

Besides the ship’s structure and its natural frequency there are vibrations produced by the main engines, auxiliary machinery and the propellers.

The main engines and auxiliary machinery vibrations are usually produced by three situations:

Ø   When the main propulsion of the ship its by a diesel engine or an alternative movement engine, the vibration depends of the value of the non equilibrated inertia moment, in other words, the balance of the engine or well the alignment of it. If the alignment of the engine is incorrect or suffers rough movement it could disaligned and cause vibration. This same result will happen if the propeller gets hit during installation.

Ø   When the propulsion engine is a turbine although the transmission par of the engine is regular, if the engine isn’t dynamic balance it will produce a disturbance par, causing vibration.

Ø   If the auxiliary devices aren’t perfectly balanced, the skids won’t be able of restrict the disturbance and it will cause a hull’s vibration.

VIBRATIONS PRODUCE BY PROPELLERS

The vibration produced by a propeller are by far the most damaging of all the vibration, because that zone is a high amplitude zone so this is superior than the central zone (engine zone).

The vibrations cause by a propeller has their origin in three different scenarios:

The effects of the blades over the water: the influence of the displaced water causes a hull’s vibration in relation to the propeller’s rpm and the number of blades.
Unequal forces over the hub: if the blades are no aligned over the hub or the separation of between the blades isn’t proper.
Non equal pitch.
Unbalance: currently the tolerances in this matter are too small so it’s really strange to find but it’s the most blame.

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