Fishtales, thanks for the info. I'd love to see the pics.
Others: there is no evidence at all that GW through bolts or uses any backing material. There is no evidence that they glass the joint either inside or outside.
I agree that the self-tapping screws are nearly useless in thin, brittle material like fibreglas. The threads have very little to bite into. Also, any working of the screws will break the hardened resin and fray the glass strands at the edges of the holes.
On quality sailboats, different approaches are used:
1. Both hull and deck have horizontal flanges that are inside the deck perimeter. The flanges are glued with 3M 5200 and bolted on 4", 6", or 8" centers with washers and nylock bolts.
2. Glassing the entire perimeter of the joint both inside and outside with multiple layers of glass cloth. This seals the joint. It is much stronger than adhesive because the load is taken by the cured fibreglas.
3. On horizontal flanges as described in no. 1, an aluminum backing strip is bonded. Holes are drilled through both flanges and the backing strip. The backing trip has threads cut into the holes. Bolts are used to fasten the three layers together.
Nos. 1 and 2 are far superior to 3.
Lower quality sailboats (Catalina, Hunter) use powerboat approaches:
- a shoebox top deck goes over the shoebox hull, bonded with adhesive, and screwed with self-tapping screws. This is inferior to using horizontal flanges because the hull and deck can both move vertically and either the adhesive or the screws are really taking most of the load. With horizontal flanges, the surfaces meet and the glass of the hull sides and the deck take the load and the fasteners keep everything together.
A sailboat hull works because of the many forces on it---especially the rigging loads. A sailboat works partly on its side when heeling in a stiff breeze. Sailboats are expected to sustain conditions that drive most powerboaters to the dock and bar. Also, specific sailboats are meant to operate offshore for days at a time. So, these sailboats use construction methods far superior to most powerboats, including GW.
But, powerboats experience severe pounding when on a plane running in chop more than 2'--over and over for hours at a time. So, I am not implying for a moment that powerboats don't warrant use of the best construction techniques. But, it seems most powerboats operate close to shore and see marinas or calm waters after a few hours of running. Powerboat buyers seem to be not as discriminating as sailboat buyers. Powerboats are sold in MUCH higher volume than sailboats so there is likely more price competition and powerboat mfgrs need to produce the styling and amenities that attract buyers at very attractive prices.
When a planing powerboat is pounding much of the vertical load is taking by the hull sides. If the hull sides are securely attached to the deck, then the overall structure is quite stiff. If the hull sides loosen from the deck (either vertically or horizontally) then there is risk of leaks and the entire boat is structurally much weaker.
