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BUILDING AN ALLOY SEA BOAT by Troy

 Making the frames. The frame shape is drawn onto a flat board or steel sheet using a grid and dimensions from a simplified table of offsets. The table of offsets takes about three hours to draw up following clear instructions.  Each frame is fabricated from 80 x 6mm flatbar and 5mm plate brackets using information on detailed drawings. Photograph shows a typical frame. Note the truss like structure of the frame and the temporary mild steel crossbeam to support the frame on the building jig.


Photograph shows the centreline bracket fabricated from 5mm plate with the flat bar frames and the cockpit beam.

This contributes significantly to the strength of the boat




The stem is fabricated from 60 x 6 mm flat bar and 6mm plate. There are several methods of making the stem depending on facilities available. The preferred method is to profile the curvature of the forefoot from 150 x 6mm plate and butt weld.
Some builders prefer to notch the stem, set to shape and weld up the notched. Other with access to equipment simply set the stem to shape pressing out any tendency to cripple. Whatever way the fabrication has plenty of strength when the hull plating is welded to  the stem. The stem shape is drawn from a simplified table of dimensions. Most builders then make a template from 18-gauge sheet or particleboard.



The building jig is fully detailed on the drawing and made from
50-x 50-x 3mm mild steel angle.
Any available straight sections can be used.





Each fabricated frame is set up inverted on the jig supported by a temporary mild steel cross support. The shape comes in automatically. Longitudinal members such as keel, stem, chines and intercostal stringers are then fitted.

 










The plating shape is template directed from the frame set up on the building jig. Photograph shows a bottom plate being sprung into position and secured by construction welds.





The transom is cut to shape given in simplified table of offsets and set up at correct angle supported on temporary mild steel angle bar supports






Hull plating is continually welded down hand while still on building jig.










Hull plating shown completed and running strakes fitted.

NOTE: Transverse weld om side plate. This is a 6.5 Metre version and side plating was from 6 m length plates requiring the front section to be welded. This is done on the shop floor before offering the plate uo to the side of the jig








Internal structure. Note full transom plate. This boat was fitted with an external pod. 

 



The internal welding can be completed. Light sealing welds on all watertight connections and intermittent staggered welds internally taking care to return all end welds around mouse holes.

 




External pod with walk through door and boarding platform.

All options are shown in plan set.








Above floor fuel tank to comply with Australian commercial survey requirements the standard Alloy Sea Boat has above floor fuel tanks. Other options are available.
Option also available for below floor tanks in self contained cofferdam.





Above deck fitting out
Photograph shows a standard cabin shape with three forward facing windows. Note hatch folds back with out "wooding " window and the anchor can be comfortably hauled in from the hatch position without having to stand on the deck.




A raised sheer forward walk a round version after first coat of paint.




This article was published on Wednesday 29 July, 2009.
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