Optimizing core/skin thicknesses......for a deck

[Roly]

For my project (layup for the deck of a 34' sloop disp. 10000lbs) Dave Gerr's
methology specifies the following: (calculated)
Solid FRP hull thkness = 7.42mm
Basic Hull core thkness = 16.32mm (18mm)
Deck-core thkness = 24.48mm (25mm)
Outer, deck skin Thkness = 2.968mm (3mm)
Inner, deck skin thickness = 2.23mm
Gerr favours a deck core thickness of 1.5 times the hull core thickness.

MacNaughton, on the otherhand, for the same boat dimension recommends the following:
Solid FRP hull thkness = Not applicable.
Basic Hull core thkness = 20mm
Deck-core thkness = 20mm
Outer, deck skin Thkness = 4.2mm
Inner, deck skin thickness = 4.2mm

I can follow that a thinner core needs a thicker skin,but none of these figures
stack up with what I know has been used as a layup on some 25yr old regularly "raced around the cans" boats.
ie Basic hull core thkness = 1/2" (12.5mm) balsa
Deck-core thkness =3/8 (9.5mm) balsa.....thinner than hull!
Outer,deck skin thkness =3.2mm
Inner, deck skin thkness =4.2mm
These are all polyester with the bulk of the skin thickness made up of csm.

OK, you might say just use the tried and proven but I would like to think we, ( not me), have a better understanding of composite mechanics and can use stronger fabrics with thinner skins and less weight to achieve a lighter, stronger end result. I suspect what you are going to tell me next is that there is a relationship between core an thickness of skin,and a minimum practicle skin thickness........?
My proposed layup: Epoxy resin-
Deck core thkness =3/4" 20mm balsa
Outerskin (2.5mm) =225gm csm +320gm Kevlar+800gm db+225gm csm
Innerskin (2.5mm) =225gm csm +800gm triax+ 800gm triax.

Any thoughts on this? Redundancies, insufficiencies? Overkill?
All laid up in a female mould,with vinylester gelcoat and the first 225gm in clear same,peelplyed, and then on with epoxy and rest of layup.

I would really appreciate some comment from someone who knows.
Even if its just, "you are dreaming".

[catmando2]

Why would you use chopped strand for anything these days?

Seems it just sucks excesive amounts of resin with no real strength.

Have done the Kevlar and 600gsm D/B on 40 foot racing cats on 20mm foam, on much larger roof spans than a mono,so that layup may be OK.

Dave

[waikikin]

I think the choppy is to mash into the back of the paterned non skid in the mold? & the next chop with the epoxy for core bonding, thats the one I'd leave out & use an adhesive instead. Overall the layup doesn't sound over the top,the cat (12.4m) I'm doing uses a layer of 850 gm Quad with 225/300gm chop each side of 19/25mm balsa & 40mm foam on underwing & deckhouse lid. I reckon you want a nice layer of pva on that old mold & be careful of aggressive removal of the peelply so you dont pop it early. Maybe you can do a nice deckhead liner off the top of your molding cos they're great for hiding the source of leaks from deckgear & guiding them onto quarter & settee berths Personaly I prefer not to use(but have done so) balsa in decks 'cos I reckon dodgy deckgear sealing is the undoing of it, to fix to it nice & proper you want to form like a filler annulus around where bolts come through it & do a nice little countersink on top to form a nice little grommet of sealant- same goes for foam but the consequences of leaks isn't so tragic with foam in the deck.Jeff Ps: I was joking about the liner.

[Tim B]

Stick the non-skid surface on afterwards. Stippled GRP finish is worse than useless when it gets wet. A thin layer of CSM might be used to stop the weave of the cloth showing up on the gel-coat. CSM has the advantage that it's properties are roughly the same in any direction. Woven rovings and cloths don't have the same properties in all directions, so you need to be a bit clever about how you do your layup.

Tim B.

[marshmat]

I've started to develop a very strong preference for veil fabrics over CSM as far as print-through goes. They wet out a lot nicer and are much easier to work, and don't waste nearly as much material.
(Veils are very light fabrics, only a few ounces, generally long, randomly oriented strands instead of the short strands of CSM, but with no regular weave pattern.)

I would like to think we, ( not me), have a better understanding of composite mechanics and can use stronger fabrics with thinner skins and less weight to achieve a lighter, stronger end result. I suspect what you are going to tell me next is that there is a relationship between core an thickness of skin,and a minimum practicle skin thickness........?

The conservative route involves sticking with tried and true materials, and tried and true scantling guidelines like those of Gerr's book.
The more adventurous route, if you want to try to optimize the structure a bit more, cut out some weight, etc. involves doing a full accounting of all the stresses on the deck, picking a starting point for the laminate schedule, mathematically predicting how it will perform under those stresses, and adapting the laminate schedule accordingly. This takes a lot more time and effort, but is necessary if you're going to stray far from known territory.

[Roly]

Thanks guys, Dave, Waik,Tim , Matt.
Guess I will stick with the tried an proven.
Just a, "tad over the top", for the project anyway!
Time is running out to assemble all the materials for the xmas project.

Much appreciated. Best wishes to you all. Hope the "silly season" isn't treating
you too bad in your respective careers.
Roly

[marshmat]

Oh, the 'silly season' is absolute hell right now on campus.... but that's another story.
Good luck with the project Roly. Make sure to have fun with it- that's what really counts!