Marine Engineering Study Materials

Vibration is defined as the motion of a particle or a body by which it is displaced from its stable equilibrium. Vibration can be ‘in and out’ or ‘to and fro’ or ‘oscillation’ of a mass. As far as marine vessels are concerned, vibration can be broadly classified into two categories.

Hull Vibrations

Hull vibrations include vibration of main hull, sub structures (such as deck house, uptakes, masts, deck, bulkheads, etc.) and local structures (such as panels, plated and minor structure members).

Machinery Vibrations

Machinery vibrations are caused by ship’s propeller, shafting, thrust block, main engine and other machinery.

THE WORST THING IS THAT HULL AND MACHINERY VIBRATION INTERACTS EACH OTHER. THAT MEANS ONE CAN CAUSE EXCITATION FOR THE OTHER.

Effects of Vibrations in Ships

Vibrations are unwanted on board ships and it must be minimized. Vibrations may lead to:

• Damages to hull structure
• Machinery failure or malfunction
• Personal discomfort
• Loosening of joints, nuts, bolts, etc.
• Fatigue fractures
• Fluctuation of stresses, etc.

How Vibrations Occurs

Consider a simple pendulum vibration as shown in figure below.

 There are 3 positions shown in the figure. The pendulum is at stable equilibrium when it is at position ‘b’. When pendulum is hit, it oscillates between ‘a’ and ‘c’, passing through ‘b’. So it can be concluded that:

• Vibration starts on a rigid body because of an external force or excitation on the body.
• Then the body is displaced from its stable equilibrium.
• Now the body tends to return to its equilibrium position under the action of a restoring force, which can be gravitational force or elastic forces.
• The body keeps moving back and forth crossing its position of equilibrium due to inertia.
• Finally amplitude of vibration reduces gradually or vibration dampens and die out and body regains its stable equilibrium, if no more excitation is given to it.

Elements for Vibration

There are 4 elements for vibration.

1. Mass [m]: Rigid body executes vibration
2. Spring [k]: Elasticity of body
3. Damper [c]: Means of energy dissipation
4. Excitation [F(t)]: System receives energy from excitation

CHARACTERISTICS OF A VIBRATION CAN BE CHANGED WHEN ANY OF THESE ELEMENTS ARE CHANGED.

Degree of Freedom (DOF) of Vibrating Mass

The degree of freedom or DOF is the number of independent co-ordinates needed to represent the motion or vibration of a body.

 

Beam is said to have an infinite degree of freedoms. Because it is not a rigid body. Deflections at different parts of the beam are different.