The first of these spooks is corrosion which plagues us in three forms; oxidation, galvanic action and corrosion by impressed current. I will not digress more deeply here as explanation of the chemistry involved may be readily obtained elsewhere. Any person contemplating steel construction would be well advised to understand it thoroughly before building, as neglect will almost certainly guarantee failure.
Oxidation or simple rusting may be overcome by thorough plate preparation which means sandblasting, and the application of suitable protective coatÂings. Galvanic action, corrosion due to potential differential between immersed materials is best contended by ensuring that there is no mixture of metals underwater. This means ideally a mild steel propeller, with all bearing surfaces nylon or rubber. All steel used in the hull must be of low carbon content (not more than 003%) and desirably of the one rolling mill batch. Sample welding rods must be welded to steel test pieces and subjected to analysis to obtain welding seams having the least potential disparity between the parent metal and the weld metal.
Sacrificial zinc base anodes of adequate size must be fitted to the underwater area by welding the backing bar to the hull. Bolting is inadequate.These anodes are now being increasingly used by fishing boats in N.Z. and are obtainable here. The small anodes available for yachts at ships chandlers are quite unsuitable for prolonged protection and should be avoided. Magnesium anodes usually prove too active and cause rejection of the surrounding paint film and antifouling; while only providing short-lived protection.
Both edges of the light key plates were to have the correct welding gap. To ensure this hardboard templates were G clamped to the hull, corrections being noted with pencil opposite each rib. The corrected template was then laid over a suitable plate like a pattern and transcribed. The chain block by which means the 30ft hull plates and heavy 1/4" keel sidings were hoisted up can be seen above.