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Aluminium Alloy Construction (Page 7) Page 1 2 3 4 5 6 |7| 8

Now the majority of the other structural elements can be fitted. These will include the engine beds, any intermediate floors, tank ends and margins, angine room partitions and any other aluminium partitions and structural pillars. In a sailing boat, if the workshops permit, the whole of the keel structure and envelope can be fitted at this stage and it is usually advantageous to fit the cockpit structures, seats and wells. The rudder tube is usually fitted at this time, but the stern tube(s) are often left until the hull plating is complete and welded.

The fitting of the longitudinal stringers will have been continuing as the other elements are fitted so finally the whole structural 'cage' is complete, leaving only the plating to be carried out and minor items to be fitted

The plates are fitted in as large panels as practical, their width being controlled by the amount of double curvature to be introduced into them. This is carried out by cold forming on the rolls and wheels. Inareas of tight curvature and considerable twist, it is sometimes necessary to re-anneal the plate to obtain the necessary shape. The alloys give no colour indication of temperature so it is necessary to use indicator crayons so as to be able to judge the correct temperature of the metal.

The plate seams and butts are perpared on the inside for welding. They are tacked on the outside and the inside welds are fully completed. The step welding of the longitudinals to the skin, the frames to the skin, and the welding of the longitudinals to the frames, can all be carried as the plating progresses. The tacks on the outside of the hull plates are finally gouged out and the seams and butts fully welded externally.

A coherent scheme of weld preparation, plating and welding is necessary to achieve a satisfactory result, both structurally and to control and minimize distortion. Many yards will have their preferences in this respect. But design offices should preferably set out in their drawings and specifications an entire and detailed scheme of their requirements to ensure that the structure and quality that they want is consistently achieved.

It is also important to have a coherent scheme of weld inspection and non-destructive testing and to ensure that any necessary repairs or corrections are carried out properly at this time. Again, design offices should preferably set this out in detail and ensure that it is adhered to.

Stern tubes, drain tubes, seacock stubs and other sundry items can conveniently be fitted at this stage. Seacocks are a constant source of frustration for builders and designers of aluminium alloy vessels as the classification societies do not usually allow non-metallic valves and prefer the flanged type of valve to threaded. Both these requirements involve the prescence of dissimilar metals in places where this is best avoided. The societies will sometimes agree to threaded non-metallic seasocks up to 3" size being threaded on to thick-walled well supported stubs, which extend a minimum above 150mm above the designed waterline, and this is a very satisfactory solution.

While maybe not structurally so significant, the superstructure needs to be built with the same attention to proper construction and control of distortion as the hull. It is a very visible part of the vessel and considerable care needs to be taken that the lines run a designed so that the intent and styling of the design are not lost.

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© George Whisstock.