The important of the work. Usually radiography is sufficient for thin hulls. Where thicker materials are used for important fabrications and full penetration welds employed in both butts and tee butts, then ultrasonics would be used firs, followed by radiography. This is because ultrasonics will detect almost all of the defects (except those which lie in the same plane as the path of the sound beam) and give the location of them.

                Radiography is complementary to ultrasonic testing, indicating those defects which would not be picked up by ultrasonics and giving a permanent record on film of the area examined. For welds such as tee butt joints radiography is not possible. In such a case, ultrasonic testing is solely employed and then technique devised to cover the whole weld and heat affected zone area.

 

How much non-destructive testing is necessary ?

The degree of inspection will be specified by the designer and may be above the minimum required by the classification societies. The builder will also want to ensure weld quality. Unsatisfactory workmanship can be corrected in the early stages of construction, thus avoiding repairs to welds at a later date which cannot be made without undoing work already done.

 

Where would magnetic or dye penetrant testing be used ?

These two process reveal surface defects or cracks that have come to the surface. Dye penetrant testing requires a reasonably smooth surface because of the dye which, on a rough surface, would give spurious results. Magnetic crack detection would therefore be used for normal undressed welded surfaces, and applied to compartment boundary welds, return welds around stiffener endings, cope holes, corner welds and any other important structural welds that cannot be radiographed or ultrasonically tested.

Because there are two types of inspection, destructive and non-destructive. Destructive testing applies only to those tests such as tensile tests, etc, used to establish material properties. Non-destructive testing is, as the name implies testing done without destroying the material.

 

When is non-destructive testing done ?

All the time. Visual examination of all details during the building of a ship is non-destructive testing. This applies in particular to welding. Constant visual examination is necessary if welding standards are to be maintained. This includes the fit-up, as well as the welding.

 

What methods are there ?

Several : visual ; magnetic testing ; dye penetrant ; X-and gamma rays (radiology) ; ultrasonic testing.

 

What are these processes used ?

Visual, magnetic and dye penetrants are used to reveal surface defects whereas radiography and ultrasonic testing reveal internal defects.

This depends upon the size and type of ship. Certainly the hull must be checked for watertightness. All hatches not required to be open should be closed. Emergency lighting should be available, a portable pump or pumps placed on board, the rudder ‘gagged’ to prevent movement if it is fitted, and the propeller free to move independently of the propulsion machinery. Some method of arresting the vessel after launch will also be required. Where there is sufficient depth of water then release of an anchor may be sufficient. Where the conditions are such that the river width requires the arrest of the vessel’s movement in a specified distance, then drag chains would be used.

 

How is the launch cradle removed after launch ?

Usually the cradle is made so that after launch it breaks up and floats clear of the ship. The parts are then dismantled as required. Where poppet housing brackets are attached to the hull these are removed when the vessel is dry docked. Another method of removing the launch cradle is to place the vessel in dock, lower the water until the cradle is just off the dock bottom or on specially prepared blocks, secure the cradle to the dock bottom, release it from the ship, float the ship up and remove it from the dock. The dock is then pumped dry and the cradle can be dismantled.

The launch cradle is held in the final moments by a series of levers called the ‘trigger’. The levers reduce by simple moments the forces acting upon the holding wire or rope to manageable proportions. When the wire or rope is cut, the trigger is released and the vessel is free to move. An older method is to allow a weight to fall knocking the restraining shores away. This knock the shores away. The weight is called the dog weight and the shores the dog shores.

 

Are any special arrangements required inside the hull ?

The internal structure is fortified to prevent damage on ‘setting up’, and when the weight comes on the fore poppet when the sterns lifts. Fig. 76 indicates the method employed. The internal shoring is fitted behind the poppets or stopping up, onto solid structure. Such structure should be checked to make sure it is of sufficient strength to take the loading. The wedging of these shores should be overdone or structural deformation may occur.

{Fig. 76} ‘Poppets’ and ‘stopping up’ that are not backed by main ship’s structure must have reinforcement fitted to prevent structure distortion

The greased surface is protected by the use of grease irons. These are tapered lengths of steel about three inches wide with the end at the thickest part of the taper bent at a right angle, to which is welded an eye for withdrawal. The sliding ways are then turned in or pulled onto the grease irons (for greasing, the sliding way is turned in or out, simply pulled from off the ground ways) which are then removed.

How is the launch cradle kept on the ways ?

A stop is fitted along the length of the ground ways. This called the ribband. It may be fitted on top of the ways or along the side. Fig. 75 shows both methods. The ribband is shored for added strength in case of sideways pressure during the launch.

{Fig. 75} Ribbands – two methods. Shoring of the ribbands is essential especially from the position of the forepoppet at ‘stern lift’ position to the completely waterborne position down the ways

What greases are used for launching ?

In Britain usually Tallene or tallow compounds, or a proprietory product such as Basekote petroleum grease (Esso) for the ground ways and a slip coat mixture of Tallene and sperm oil, or the proprietory equivalent, Slidekote (Esso).

 

How are they used ?

The ground or standing ways are coated by pouring molten grease onto the surface and allowing it to harden. The depth will depend upon the weight of the vessel or past experience, usually from 3.2 to 12.7 mm for Tallene grease and from 3.2 to 6.4 mm for the Basekote grease. A coating is also applied to the underside of the sliding ways. The ‘slipcoat’ is applied liberally to both the ground and sliding ways. This is the lubricant which effects the launch.

When constructing the cradle, allowance is made between the poppets and stopping up for wedges to be fitted. A day or so before launch (in the case of large vessels this may be weeks) these wedges are driven home in unison, forcing the poppets and stopping up under the hull to lift it. This operation is called ‘setting up’. The middle line blocks can then be removed.

 

Is setting up necessary ?

For small vessels it may be sufficient to harden up the wedges and then ‘split’ out the capping block. An alternative to this is the use of a specially designed steel capping block which is filled with sand. For release, a plug is removed and the sand allowed to run out.

 

What governs the height of the launching ways above the slip floor ?

When the stern lifts, the hull tilts or pivots about the fore poppets. Fig. 74 shows the effect of this, which is to cause the forefoot to move closer to the slip floor. The launching ways must be of sufficient height for the forefoot in the worst condition to clear the slip floor.

{Fig. 74} Clearance of forefoot. The minimum clearance occurs when the vessel is just fully waterborne, hence the launch way height ‘h’ is critical and must be more than ‘C’

 

How are the ways positioned and aligned ?

Methods will vary form yard to yard. Chapter 9 refers to the level boards set up for leveling the building blocks and later for checking the USK. In a similar manner level boards are erected over the launch ways (once they have been put in an approximate position). These boards can be fitted to small ‘shores’ which are wedged up under the hull. The ways are then lined up, levelled, and make up timber fitted underneath and secured.

During the launch, when the stern lifts, the fore poppets are subjected to the maximum pressure of up to 25% of the launch weight. To take this pressure safely and maintain stability (because at this position down the ways, the launch cradle provides the stabilising force until the vessel is safely water-borne) the fore poppets are either securely housed in brackets integral with the ship structure, or they are connected together by a ‘sling plate’, which passes around the hull under the keel and up to the top of the poppets which are housed in brackets. The sling plate is cushioned on the hull with soft wood packing.

 

Are all poppets housed in brackets at the hull ?

No. There is only need to contain the poppet if the hull angle is such that the poppet would not support its weight without slipping.

The ship is supported in a cradle, which, on release, slides down the fixed launching ways, which have been previously greased, into the water.

 

What is the launch cradle ?

The launch cradle comprises the ‘sliding way’ which is a continuous length of timber, made up of short lengths scraped or connected together by link plates and through bolts, positioned either side of the centre line upon which the supports, called poppets and stopping up are fitted. The whole is connected together to prevent any relative movement. Fig. 73 shows the usual arrangements.

 

What are the poppets and stopping up ?

They are the supports fitted between the hull and sliding way. The poppets are vertical timbers, the stopping up is the horizontally fitted support. These are necessary where the distance between the hull and the sliding ways is small, over the amidships of the hull.

 

What prevents the poppets from movement in the fore and aft direction ?

They are braced with steel stripes of plate, called dagger plates, secured with coach screws.

The launching of the ship sideways on to the water. It is necessary where the water is not wide enough to allow lengthways launch.

 

How is the slope of inclination of the slipway determined ?

The slope is related to the weight of the vessel and the sliding friction of the launch cradle and the grease. When the vessel is on an inclined plane, the weight acts vertically and a component of that weight acts down the slipway (Fig. 72).

{Fig. 72} Components of weight on slipway

That component must be sufficient to overcome the initial friction. Over movement has started then the effect of the friction is reduced and the vessel accelerates as she slides down the ways. Thus the angle of inclination must be sufficient to allow sliding to occur.

 

How is the angle calculated ?

By knowing the starting acceleration of the ship which is determined by experiments with a model which stimulates the launch conditions. From the data obtained and using the laws of dynamic friction, the coefficient of friction is determined. When the acceleration is zero, at the start of the launch, the coefficient is equal to the tangent of the angle of the inclined plane, so if the coefficient of friction is known then the angle is easily determined.

 

What other calculations are involved ?

The mains ones are :

1.        Launch weight, position of longitudinal centre of gravity (LCG) and height of centre of gravity (CG) above the keel

2.        Stability of the ship when afloat

3.        Position of the ship down the ways when the stern lifts

4.        Pressure on the fore poppet when the stern lifts

5.        Variation of pressure on the grease during the launch

6.        Tipping moment about the after end of the ways

7.        Distance between the forefoot of the ship and the slip floor at the worst position.