THE SHIP’S MOTIONS AT SEA

A ship which is floating in water will always have a motion, even if the engine is stopped. The reasons for this are Ocean currents, Winds, Tidal change, Tidal flow etc. The formation of wave is explained by Trochoidal theory.

When the ship is moving through water under propulsion various motions can be observed. Mainly Rolling & Pitching can be felt by the personnel onboard. But there are six kinds of motions. They are Rolling, Surging, Pitching, Swaying, Yawing & Heaving. Special cases are Synchronous rolling & Parametric rolling.

Let us see about all of these motions. First of all we have to imagine X,Y & Z axis through the ship. Please refer the picture below.

 

Rolling

Rolling is the rotary movement of the ship around the X axis. If the ship is floating in calm water, during that time the centre of gravity & centre of buoyancy are in a vertical line. If a wave crest strike the ship on starboard side, the underwater volume of that particular side will increase. The centre of buoyancy, being the centre of the underwater volume it will shift to starboard side.  But centre of gravity will not change because there is no change in weight so at this particular instant centre of buoyancy & centre of gravity are not in a vertical line thus creating a lever. This will cause the ship to incline to port. When the wave crest passes to port side vice versa will happen. It is an involuntary rotary movement.

 

 

Surging

Surging is the liner movement through the X axis. When a crest of wave strikes the one end of ship, ship will be pushed bodily in the direction of wave. When a trough of wave forms at one end of the ship, ship will be pulled bodily towards the trough. The ship will oscillate in liner direction. It is an involuntary non-rotary movement.

 

Pitching

Pitching is the rotary movement of the ship around the Y axis. If the ship is floating in calm water, during that time the centre of gravity & centre of buoyancy are in a vertical line. If a wave crest strike the ship on bow area, the underwater volume of that particular side will increase. The centre of buoyancy, being the centre of the underwater volume it will shift to forward side.  But centre of gravity will not change because there is no change in weight so at this particular instant centre of buoyancy & centre of gravity are not in a vertical line thus creating a lever. This will cause the ship to incline to stern. When the wave crest passes to stern side vice versa will happen. It is an involuntary rotary movement.

 

Swaying

Swaying is the liner movement through the Y axis. When a crest of wave strikes the starboard side of ship, ship will be pushed bodily in the direction of wave to port side. When a trough of wave forms at starboard side, the ship will be pulled bodily towards starboard side. Vice versa will happen if the crest strikes the port side. The ship will oscillate in liner direction. It is an involuntary non-rotary movement.

 

 

Yawing

Yawing is the rotary movement through the Z axis. When a crest of wave strikes the bow on starboard side, bow of the ship will be pushed in the direction of wave to port side. When a trough of wave forms at bow on starboard side, the ship will be pulled bodily towards starboard side. Vice versa will happen if the crest strikes the port side. The ship will rotate around Z axis. It is an involuntary rotary movement.

 

Heaving

Heaving is the liner movement of the ship through the Z axis. It can be observed during rough weather. When large wave comes under the bottom of the ship, the whole ship will lifted up bodily, when the crest strikes & the whole ship will be lowered down bodily, when the trough forms. The ship will oscillate in liner direction. It is an involuntary non-rotary movement.

 

Synchronous rolling takes place because of resonance between, the natural period of roll of the ship & the natural period of the oscillation of the waves. The rolling will gradually increase to high capsizing values.

If the synchronous rolling is encountered immediately alter the course of the ship, if synchronism was occurring in on the original course it will not occur at new course.

Synchronous rolling takes place because of resonance between, the natural period of roll of the ship & the natural period of the oscillation of the waves. The rolling will gradually increase to high capsizing values.

If the synchronous rolling is encountered immediately alter the course of the ship, if synchronism was occurring in on the original course it will not occur at new course.

Reference

Ship stability-III Nutshell Series Book 6, author Capt.H.Subramaniam