While installing the hatch and 1/4″ marine-grade Douglas fir plywood that underlays the FRP skin on the salon roof, I cleaned the old fairing compound off of the cut edge of the original Chris Craft fiberglass. What I found was…disturbing.
What we have here is three layers of fiberglass. The top layer follows the shape of the mold, as does the bottom layer (presumably, so the boss wouldn’t notice what was going on in between).
But in between, they thickened up the layup in a high stress area at the base of the windshield by mooshing the middle fiberglass layer into a corrugated pattern. Thing is, resin rich FRP layups like this are extremely brittle. Which might explain the cracks we were seeing at the base of the windshield. The FRP layup here is almost 1/2″ thick. On the leading edge of the hatch, where they didn’t corrugate the middle layer, the FRP layup is only about 5/32″ thick.
Since the salon hatch hole measured 5’x10′, I had to scarf three sheets together to fill the space. The upper layer of plywood is oriented longitudinally, whereas the lower layer was transverse. This cross-oriented, two-layer 1/4″ plywood underlayment is reportedly the same layup Weaver Boatworks uses on their multi-million dollar sportfishermen decks.
After wetting out both layers of plywood with US Composites 635 epoxy, I coated them with a bonding agent of 635 epoxy and wood flour (wood dust and cabosil). After laying the top panel in place and squeezing out as much air as possible, I started carrying heavy things up the stairs and put them on the salon top. The whole time I’m thinking “I am getting too old for this $4!t”. 😉
The idea was to not have any nail or screw heads on the top layer, since they have a tendency to “print through” the fiberglass and top coat.
The original Chris Craft design only had longitudinal pieces around the hatch opening. Weaver Boatworks uses a similar frame structure for the boats (though in laminated plywood rather than solid mahogany), but with longitudinal pieces running down the entire length of the structure.
The panels are each cut to fit very tightly between the frames, then are edge sealed with US Composites epoxy and glued in place with wood flour-thickened epoxy. I’ll put additional longitudinal stiffeners in all the way from the aft salon bulkhead to the forward-most roof frame before I take down the supports I’ve been using to hold the roof level.
After putting a nice fillet on the wood flour-thickened epoxy glue, I used 9oz boat cloth to make the longitudinal stringers a permanent part of the roof structure.
With the path forward worked out for strengthening the salon roof substructure and framing, the next topic in our 1969 Chris Craft Roamer 46 Refit: Stripping the Cabin Top.