THE SHIP’S MOTIONS AT SEA

Source

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

Synchronous Rolling

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.

Parametric Rolling

Parametric Rolling

During rough seas while moderately pitching the vessel rolls to one side simultaneously because it is thrown up & down on water.The bow is down inside water & ship has rolled to one side the sudden immersion of large flare (this specially takes place in container vessels since they have very large flares) causes the restoring buoyancy force to push the bow upwards & to roll to other side. The opposite happens to the other side & with in few cycles the angle of roll will reach the large angles. The maximum angle of roll would occur at the maximum dip of the bow during pitching.

If you want to explain in simple words the Parametric rolling is a phenomenon where by a ship which is pitching moderately in bad weather suddenly experiences very heavy rolling without warning.

If you experience Parametric rolling change the heading & bring the wind broad on the bow. Steer zigzag course for few hours until weather abates.

Experiment- The ship specimen from normal Rolling to Parametric Rolling

Reference

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

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Comments 14 comments

KULLU THAKUR 4 years ago

NICE EXPLANATION,UR PRACTICAL EXPLATION HELP'S STUDENT'S TO UNDERSTAND THE CONCEPT....


Alex 4 years ago

Thanks for the explanation


DANIELPRASANNA profile image

DANIELPRASANNA 3 years ago Author

U r welcome


Park Jin Sang 3 years ago

Could I use your explanation as above for training of vessel crews?

Also Could I put the contents in company's manual for vessel crews?


DANIELPRASANNA profile image

DANIELPRASANNA 3 years ago Author

Dear Mr. Park Jin Sang

You can use my explanations & use the contents.


Park Jin Sang 3 years ago

Thank you very much for your kind understanding.


GERRIN LACIERDA BUMAYA 3 years ago

THANK YOU SO MUCH FOR THAT VERY NICE EXPLANATION


Tepua PulataloManuella 3 years ago

Could I use your explanations and the contents for teaching?


DANIELPRASANNA profile image

DANIELPRASANNA 3 years ago Author

Dear Mr.Tepua

Yes go ahead & use.


Behrad 2 years ago

The explanation for parametric roll is missing, which is why I came here... which is disappointing! Otherwise, good job!


DANIELPRASANNA profile image

DANIELPRASANNA 2 years ago Author

Dear Mr.Behrad

Thank-you so much for pointing out my mistake. I have accidentally repeated Synchronous rolling twice. Now I have corrected it.


anshul 2 years ago

thanks a lot for ur effort which is helping students a lot


DANIELPRASANNA profile image

DANIELPRASANNA 2 years ago Author

Thank-you


Batch 51-deck 17 months ago

Sir,

thank you so much for the explanations of six degree of freedom of a vessel. This help us a lot in our report! thumbs up!

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