Downloadable Sailboat Plans by George Whisstock

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Aluminium Alloy Construction (Page 5)

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The on-going development of welding machines and torches, means that it is now possible to MiG weld aluminium down to about 2mm thickness without excessive distortion. This has allowed all-welded construction for the majority of marine aluminium alloy structures. Previously, thin material had to be riveted or adhesive bonded.

A MiG welding torch is still quite bulky and it is important that the torch is presented to the work at the correct angle. These limitations need to be taken into account when the design is carried out and also when extrusions are selected, to ensure adequate access can be obtained to allow proper welding of the structures.

Three concepts of structure can be identified, although the boundaries between them are not necessarily very clear:

1.	Transverse framing consists of close frames, typically perhaps 400mm apart. Longitudinals, apart from engine beds, keel girder and similar major structural items, are usually limited to panel stiffeners in areas of high slamming load and flat shape. The frames are generally formed from extrusion, usually angle but sometimes Tee. The structure is more reminiscent of steel construction.

2.	Longitudinal framing usually consists of relatively widely spaced ring frames through which pass relatively closely spaced longitudinals. Ring fram spacing is typically about 900mm but can vary considerably, often becoming greater towards the ends. Ring frames are often fabricated Tee sections with varying depth of web to suit structural requirements. Longitudinal spacing is typically 300mm amidships, growing naturally closer as the longitudinals approach the bow. Extruded Tee section is most commonly used but flat bar, bulb section and angle are also seen.

3.	Heavy skin construction disposes of much of the sub-structure of the two previous systems and concentrates the majority of the structural strength in the skin of the vessel, which is typically two to four times thicker tan that in either transversely or longitudinally frames structures. This type of structure suggests that it is perhaps best suited to rather specialized vessels, perhaps to venture into ice-bound waters, although it is sometimes used for conventional sail yachts.

Of the three, longitudinal framing would seem to provide the best structure for aluminium alloy, both for round bilge vessels and chine hulls. It can utilize the large range of extrusions that there are available. Skin panel areas can be kept to an optimum and therefore skin thicknesses can be kept to a minimum. This will have an advantage as much of the weight of the structures will lie in the skin itself.

The longitudinals naturally get closer towards the bow and thus further reinforce the structure in an area of high slamming loads. Many of the other structures in a vessel – engine beds, tank margins, genoa track reinforcings and hatch carlings, for example – run longitudinally and they can more easily combine with hull and deck longitudinals, or quite often replace them.

The run of the longitudinals themselves can usually be designed so that they can be pulled around the frames without mechanical shaping – in a way reminiscient of battens on a classical carvel hull prior to and during planking. This makes for an intrinsically fair hull. Setting up is easier as there are fewer frames to position and more working space between them. The plating is well supported, which again helps to retain fairness.

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