In April, aboard Marty Raymond's and Jay Sterling's Cruz-In #74, I saw core samples of their salon and pilothouse roofs. These cores go a long way in explaining how these surfaces were laid up. We already know that a single mold was used to form the inner bulwark, deck, cabin side, and roof. The core samples (see photos ) are 1-1/8" thick and composed of (1) a layer of gel coat, (2) 3/8" of fiberglass, (3) 9/16" of 7-ply plywood, and (4) another 3/16" of fiberglass. Tommy Chen, whose yards built our tugs, told me that the plywood in the ceilings is broken into squares and that each square is separated from its adjoining squares by a waterproof material. This is clearly visible in the core samples as each has a 1-1/4" wide channel, of what appears to be polyester filler, separating the plywood pieces. The reason for including the polyester separator is two fold: it makes the decks stiffer i.e. feel more solid underfoot; and should any water find it's way into a wood square, via a leaking screw hole for example, the water will be contained within that square.
Knowing how these surfaces are made informs how we can safely attach things to them. If a mounting screw is 5/16" or less then water could never get into the core via the screw hole. If the penetration is greater than 5/16", the fasteners should be bedded to prevent water getting into the core. Finally, limiting a fastener's penetration to 1" or less will keep the fastener from penetrating the lower layer of fiberglass and into the space above the headliner.
In summary, the salon and pilothouse ceilings are a plywood checkerboard sandwiched between layers of fiberglass. Each square of the checkerboard is isolated from its adjoining squares by a waterproof material. A little forethought before screwing into these structures will keep the roofs robust for a long time.
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