Ship Launching

INTRODUCTION

Launching of any structure is quite an anticipated occasion depicting the hard work paying off and victory after years of scrupulous construction and numerous man hours deployed. And when it comes to a near self-sustained vessel sailing on the notorious ocean or river waters, the celebration is unimaginable. It is one of the most important procedures in the entire chain of ship construction processes.

               But even a subtle miscalculation or error may compromise the ship’s launching and mourn the spectators’ faces as they witness something going horribly wrong. To obviate any such risk ship launching is meticulously planned and everyone directly involved is quite punctilious.

TYPES OF SHIP LAUNCHING

Various types of Ship Launching Methods are used depending upon the feasibility of ship and geographical parameters.-

Stern First

This method dates back to ancient times and is one of the most familiar methods for ship launch. The prerequisites involve arranging the slipways to be nearly perpendicular to the shoreline. Nowadays reinforced concrete mats are used as slipways to provide sufficient strength.Construction is done on temporary cribbing so as to give access to the hull’s outer bottom.As preparation for launching, a pair of standing ways with greased surfaces is erected under the hull and out into the barricades.A pair of sliding ways is placed on top, under the hull, and a launch cradle with the bow and stern poppets is erected on these sliding ways. Common mechanisms for releasing the vessel in launching ceremony include weak links designed to be cut at a signal.On launching, the vessel slides back, down the slipway until it floats by itself.

Image courtesy: Google Images.

Sideways

This type of launching is used where the water channel is not feasible for lengthwise launching. The slipways are built so that the vessel is side-on to the water and is launched sideways. However more sets of launching assisting construction are required to support the ship and this act as a downside. Rigorous calculations have to be made to check the stability of the vessel as it touches the water surface to avoid capsizing of the vessel due to lateral moment created during launching process.

 Image courtesy: Google Images.

Air Bags

This method has the upper hand of requiring less permanent infrastructure, risk and cost and is quite safe and innovative. A series of inflated tubes placed under the hull deflate to cause a downward slope into the water. The airbags made of reinforced rubber layers have high load capacity and are usually cylindrical in shape with hemispherical heads at both ends. The Xiao Qinghe shipyard was the first to manifest this type of launching in 1981.

 Image courtesy: Google Images.

Float Out

Though not technically recognised as a ship launching method, this method is most widely used one among the shipyards. Ships built in drydocks are launched simply by filling the dock with water and the vessel is 'Floated out'. Thus it is a simple, effective and safe procedure. though the initial investment is high.

 Image courtesy: Google Images.

LAUNCHING CURVES

Predictions of the movement are vital to ship’s safe control. A set of six curves is prepared to predict the behaviour of the ship during launch. They are curves plotted against the distance of travel down the slipway for end launching process

1.       Weight

2.       Buoyancy

3.       Moment of Weight about fore poppet

4.       Moment of Buoyancy about fore poppet

5.       Moment of Weight about after end of groundways

6.       Moment of Buoyancy about aft end of groundways

 Image courtesy: Basic Ship Theory (K.J. Rawson & E.C. Tupper).

The Important features of these curves are as follows-

·         At the point at which the moment of buoyancy about the fore poppet equals the moment of weight about the fore poppet, the stern lifts.

·         The difference between the weight and buoyancy curves at the position of stern lift is the maximum force on the fore poppet.

·         The curve of the moment of buoyancy about the aft end of the ways must lie wholly above the curve of the moment of weight; the least distance between the two curves of the moment about the aft end of ways; gives the least moment against tipping about the end of ways.

·         Crossing of the weight and buoyancy curves before the after end of ways, indicates that the fore poppet will not drop off the end of the ways

           Thus launching, though a celebratory and ceremonial event requires a lot of background calculations with minimal errors so as to be able to predict the vessel’s motion in advance. Each parameter needs to be assessed with utmost accuracy to prevent stability loss or any undesirable accidents on the launching day.

           Here is a video of various ship launches recorded. They are fascinating to watch, but at the same time, they involve a long calculation and thinking process and a huge effort of engineers to accomplish the task.

Video courtesy: wapindia.in

Article by: Shivansh Singh