Fiberglass overlaps , why so large?

[sabahcat]

Originally Posted by PAR

So if you have a 1/4" (6 mm) skin and it's overlapped 2" (51 mm), you have an 8:1 step or lapped scarf on that skin. If the laminate thickness is less then 1/4", then your scarf ratio is taller. Naturally, on thicker laminates, it would be wise to consider a taller ratio, though this may be dependent on panel loads.

So on a typical cored panel where the skin is about 1 mm thick?

[waikikin]

Originally Posted by sabahcat

I always use pencil grain timber for the core
I always treat the fore and aft grain of the timber core as strength, uni's if you like, in this direction, thats why I scarf.
Treating the timber core this way eliminates the weight of glass uni's and the resin needed for their wettout .
The uni or double bias that runs across the grain of the timber holds it together, which is why it is not scarfed in that direction

This makes for a lighter vessel.


More money on glass
More money on resin
More added weight for what benefit?

I'm all for scarfing too, with a simple jig it's pretty quick, also what your saying about the timber being a unifiber in the"layup" rings true with some comment I've read from a cluey NA/Designer(cant remember) that strip timber composite/glass can be considered as a solid layup rather than purely a cored sandwich.

[PAR]

You have to compare apples to apples. If the core is offering some longitudinal stiffness, such as that with strip plank or panel stiffness as in other cores, the skins can and typically are thinner. In these cases,

(assuming: or for less dense cores ) you need to go with the designer's specs in regard to laps, and joints.

Well staggered strips with a minimum 'glass sheathing, that's there for little more then abrasion resistance and water proofing can live with butt jointed strips, but again it means well staggered joints and good inter-strip bonding/fastening. In the case of true cores, I treat them all the same (gyrational spacer) and other then good laminate and inter-panel bonds, the monocoque skins bear the loads.

In light weight cores, the common adhesives are more then sufficient to bond the panels, so a butt joint or lapped scarf will be stronger then the core material. The skins provide the lap, which can be isolated (separate) or part of the panel laminate as a whole.

Originally Posted by waikikin

. . . I've read from a cluey NA/Designer(cant remember) that strip timber composite/glass can be considered as a solid layup rather than purely a cored sandwich.

This is true of some forms of strip planking, but not all. Strip planking runs the full range from a purely narrow planked carvel build, with a protective sheathing, to a true cored composite sandwich panel. The design scantlings will offer a clue as to the role of the core. I usually make note of the build type in the plans, though I can see where this might not be included in others.

In the end I'll steal this quote from Trevor Gundberg, an engineer at DIAB (Divinycell manufacture), taken directly from one of their technical bullitens:

"For joining foam core laminates, again any typical composite adhesive will work in a variety of joint configurations. Probably the most recommended joint is the lap, with the core components butting up together and the skins on either side creating the lap. This creates a continuous skin over the core joint, thereby not creating one weak point in the laminate. Other favorable joining methods would include scarf and stepped joints. Again, the adhesive must be stronger than the foam core, and the skins must be continuous over the core joint."