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= Hull Material =
<tr>
<th>Material</th>
<th>Specific Weight<br>lb/cu.ft<sup>3</sup></th>
<th>Tensile Strength<br>kN/m<sup>2</sup>x10</th>
<th>Compressive Strength<br>kN/m<sup>2</sup>x10</th>
To make a mould, you start with a plug in the exact shape of the hull or hull component. The plug is disposable, and can be made from a variety of materials. The exterior of the plug is highly finished.
Next, a fibreglass mould is built up on the plug, using alternate layers of cloth and matt, a felt-like material. Each layer is impregnated with a synthetic polymer resin that cures and hardens. The exterior of the mould is reinforced, and the plug is removed and discarded. The best range for working temperatures in the mould shop are is 18-21 C °C with humidity less than 80%. Direct sunlight should be avoided.
Finally, the inside of the mould is coated with a wax-like release agent, and a fibreglass hull or other component is built up inside, starting with the coloured gel coat, a layer of mat and then woven roving. Putting mat, a sort of filler, next to the gel coat prevents print-through of the roving. Anecdotally, putting mat next to the gel coat increases the propensity for blistering. After the piece is finished and cured, it is removed from the mould for finishing. The mould is then ready for re-use. Gel coats should be >30 mm and incorporate protection from ultraviolet (UV) light, which damages plastics.
Sandwich construction was introduced to reduce material cost. It uses a foam-core material, plywood or balsa wood between two layers of fibreglass. Wood of any kind is not recommended because it readily absorbs water, which will delaminate the sandwich.
Although introduced for cost reasons, a sandwich is stiffer than a single skin, and lighter for the same stiffness. Interestingly, the stiffness is not imparted by the foam-core but by the equal separation of the two skins, and the effect this has on bending forces. Another advantage of a foam sandwich is no extra insulation is required inside the hull. <ref>http://triloboats.blogspot.ca/2012/01/sea-going-sips-toward-creating-market.html</ref> <ref>http://www.nauticexpo.com/boat-manufacturer/thermal-insulation-panel-22312.html</ref> <ref>http://duflex.com.au/duflex2/</ref> Usually the final paint is applied in the gel coat.
A sandwich is not as durable as solid 'plastic', partly because more skill is required to ensure the skins bond solidly to the core without voids. Incomplete bonding and voids cause delaminating and failure of the component. These risks can be overcome with vacuum infusion. Biaxial fibreglass material is recommended with sandwich construction. Some experts advise against any kind of core below the waterline; although reasons have not been well articulated. Perhaps this was based on experience with early foam-cores.
== References ==
[[Category:HullGeneral]]