Deck re-inforcement

longy

Overlord of Anarchy
6,512
1,005
San Diego
Want to stiffen the aft deck sections on a cat where everyone steps aboard, and wrap around hull/deck joint to strengther this area and transom corner. Deck ( entire boat) is lightly built. 3/4" foam core, unknown lam schedule. Have sent inquiry to builder on this. Thinking of sanding off fairing, checking glass surface for cracks, adding 2 (?) layers of carbon cloth that wrap 2" around H/D joint & transom corner. Have about 3 mm of fairing thru this area (thicker fairing around H/D joint)
What cloth weights should I be looking at? 2 layers of 5.7 oz twill? Suppliers?
 

longy

Overlord of Anarchy
6,512
1,005
San Diego
If possible, I'd like to keep the added lams within the present thickness of fairing. The stern area in question is about 4' x 2', only want to work on the aft half. If the added lams get too thick we'll have to re-fair the whole area. With two layers & stepping the layers, we could get by with a few mm of added thickness
 

Zonker

Super Anarchist
9,284
5,213
Canada
Stiffness = thickness. Or maybe the other way around is better to understand.

It's much easier to add a little beam/stiffener to the underside of a deck to stiffen it than extra laminations. If you can build the beam on a bench and then glue it to the underside even better. Even a piece of 1"x2" wood glued and filleted to the deck will do. You're reducing the span from 2' to 1'. Deflection is proportional to span ^3 so cutting span in half reduces deflection to 1/8 of original amount.
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Easy answer on thickness. 100 gm laminated whatever (well squeegeed or vacuum bagged) = 0.1mm thickness

In horrible american units 5.7 oz ~~ 200 gm so 0.2mm thickness. 12 oz = 400 gm so 0.4mm

If the under deck beam isn't viable, then use double bias instead of twill.
 

longy

Overlord of Anarchy
6,512
1,005
San Diego
Got a reply from the builder: top is 800 g , 15 mm foam, bottom is 400 g. No access to underside of panel. Cracks are within 4" of hull joint, so I don't think panel flex is an issue. Boat's first life was as a sail/dive charter operation, so tanks/gear loaded/unloaded a lot.

Zonker - why do you recommend changing the thread alignment? Do you want the threads to be 45 to the deck edge?
 

Zonker

Super Anarchist
9,284
5,213
Canada
Oh, if you use 0/90 orientation then 50% of the fiber is in the lengthwise direction, not the shorter span direction and thus doesn't do anything to limit deflection.

If you can't get to the bottom you're thinking of applying extra laminates to the top only?? If so, then 2 x 12 oz CDB will reduce deflection roughly 35%

If you could get to the bottom a single 1 x 12 oz CDB on each side would reduce deflection by about 300% !

Balanced laminates kids, balanced laminates...
 
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