Blueprinting is a colloquial term for bringing any piece of machinery as manufactured to its specification as designed. The term is well known when applied to engines but less understood as applied to things like RIB hulls.
A designer specifies hull shape by drawing the lines of the hull on a ‘Lines Plan’; traditionally on a drawing board but nowadays usually by computer. The builder builds the boat to this drawing as accurately as possible.
During construction, dimensions will usually stray. Modern FRP (Fibre Reinforced Plastic) RIBs come from FRP moulds which, in turn, have been moulded over ‘plugs’.
A plug is the hull shape made from relatively cheap materials ( wood, MDF, foam ). Build accuracy, material movement due to moisture and heat changes, transportation or inadequate supporting structure all contribute. Sometimes, overheating during cure due to too many layers laid in one hit, can distort the laminate and pull it away from the mould. It then cures the wrong shape. Another reason is that production craft builders hate sharp corners like those on spray rail and transom edges.
They are more likely to stick when separating hull and mould and are hard to laminate; fibres are reluctant to get into sharp corners leaving resin-rich edges. These chip easily in service and builders, understandably, don’t like customer complaints. Hull Blueprinting is regaining the desirable features where they have deviated from the specification. RIB bottoms aft should be straight. Transom, step and spray/chine rail edges should be sharp.
Rarely, in certain, usually specialist, craft some intentional rocker or hook (convexity or concavity in the fore and aft bottom lines) may be incorporated.
Hook lifts the stern, makes the boat run-flat, increases wetted surface thus reducing speed, although getting on the plane and low-speed cruising can be easier. Rocker sucks the stern down and lifts the bow which can cause porpoising and a wild ride. Things are worse if one side has more hook/rocker than the other. Careful eyeing of the aft bottom surface with the help of a straight edge will show up any problem – check both sides. Getting the bottom straight is usually a specialist job involving filling, grinding and constant rechecking. Depending on thickness required, either fibre laminations or the correct filler must be added, after proper surface preparation. The job must be properly done. The problem can be worse than at the start if half a modification removes itself.
Rounded transom, step, chine and spray rail corners can benefit from being sharpened. These are areas where water flow should separate from the hull cleanly. Water will try to climb around a radiused edge using energy, increasing drag and losing lift, in proportion to the radius. Ease of sharpening depends on construction. Much may be achieved with a file, depending on gel coat thickness. If the radius is large then the surface must be built up and then ground back. This is only worth doing if maximum performance is desired, unlike rocker and hook, craft handling with these rounded edges is usually OK assuming corner radii are consistent. Handling will probably be less crisp, however, if the edges are not sharp.