John,
Right about hard spots against the hull sides. Where ever a hard spot is created along the hull, especially from a vertical bulkhead, normal flexing of the hull will stopped at that point. All forces will be concentrated to the line of the intersection and things are going to eventually break. In extreme cases, the bulkhead can act like a knife and cause a complete cut through the hull. The same thing holds true for stringers, but here, when the stringers
DO have a gap between the stringer and hull.
As a general rule of thumb, 0/90º plain weaves have less strength than 45/45º plain biaxle weaves. Mat backed fabrics have better bonding capability than non-backed fabrics and create a filler layer between multiple layers of fabric, (this is more true as the weight of the weave increases). Heavier weaves have more strength than lighter weaves. You can search through manufacture's sites and get really more information than folks like us need...
For those of us involved here, just break it down to two categories: Structural & Non-structural. Fabrication, Tabbing and bonding fall into the structural realm, and Covering, coating, and encapsulation are non-structural items.
For Structural, stick to biaxle weaves, (nothing lighter than 1208 or heavier than 1708). for Non-structural stick to 0/90º weaves. For non-structural surface treatment I range from gossamer like veil to 1 1/2 oz. mat to 6 oz. 0/90º plain weave. I rarely go heavier than 6 oz. cloth, rather, just add more layers where needed. I use the veil primarily as the outer most skin layer between the gelcoat and structural layers of molded parts. I use it sometimes as the final layer over a mat surface to further hide the chopped strands from printing through and to obtain less surface porosity that would have to be filled otherwise before spray coating.
Mat normally comes in only two weights: 3/4oz. and 1 /1/2 oz. 1 /1/2 oz. mat is actually just two layers of 3/4 oz. that are sized together at the factory. Note that all chopped strand mat and some veils are sized with a binder that dissolves when subjected to styrene. Epoxy will not cause the sizing to dissolve, so the wet out of the mat is not as complete when using epoxy, like it is when using polyester or vinylester. Mat still works OK with epoxy, it's just not quite as strong and will have a cloudy appearance from trapped microscopic air.
Main bulkhead tabbing, where tight to the hull bottom, and perpendicular to & between stringers can be done nicely with one layer of 1708 or, the most, 2 layers of 1208. Tabbing to hull sides can be as light a one layer of 1208. Be sure that your tabbing over the gap remains in a nice flowing fillet pattern and doesn't get tucked into the gap. The larger the radius of the fillet, the more forces will be distributed evenly over a broader srea. A tucked tab will always eventually fail...
Thickness of face coverings will depend on the amount of abuse the surface has to endure. Bulkhead facings can be quite thin. The necessity for a fiberglass fabric or mat to be included in the facing over plywood is to prevent checking and cracking of the resin as the wood expands and contracts with temperature changes. Cracks in the resin are open doors to moisture.
Things like cockpit panels are more prone to impact damage from gaffs, rod butts, and the like. A heavier covering here will decrease minor dents and abrasion. Heavy traffic areas like decks need to be protected even more, not so much from foot traffic, but from gravity related damage from falling objects such as downrigger weights or your buddy that forgets to change out of his golf spikes before boarding.
JohnF wrote:I know I'm asking alot.
No big deal this time of the year, I stay holled up in the morning, at least until the temperature reaches 50ºF or so. Opps, it's now up to 64º, so I guess it's time to get back to the outside grindstone...
Br,
Patrick