At one time we used both end grain balsa and foam core for building fiberglass hulls. In recent times balsa core has become dis-credited due to the number of hulls that have become damaged due to water penetration of the fiberglass and into the balsa core.  In our own defence we generally recommended balsa be installed above the waterline. In the light of current experience, we do NOT recommend balsa cores except for decks construction.

   One method we have used for attaching the PVC foam core to the mould is to sew it on.   For best results use a medium size bag needle with light string or nylon yarn.   Make the stitches about 3” [75mm] long.    Outside the hull the stitches are let into grooves, which you cut as you proceed so they will lie flat with the surface and do not interfere with the installation of the laminate.   The rows of stitching should be 9” to 12” [230mm to 310mm] apart.   We have found it best to stitch vertically up the sheet, first along one edge and then progressively working across the sheet in vertical rows.  Additional stitching may be necessary where the sheets join, using a cross stitch patterns up the edges of the sheets which should ensure that both sheets lay uniformly on the mould former.

   Another possible method, depending on the suitability of the foam, is to drill holes in the battens and screw the foam from the inside, removing the screws before the hull is turned for removal. One or two missed screws will stop the mould releasing so you have to get them all out.

   Install the foam sheeting carefully to insure there are no hips or hollows in the hull surface.   It will take a considerable amount of work to fair out any large irregularities created at this stage, so utmost care will ensure a fair hull and one that will need the least amount of finishing to provide a professional looking surface when the hull is completed.