Compsite Deck VS ply

That would simply be a waste of some good kevlar.

 

Stength comes more from a core being flanked with a stiffer material, not a stiffer material being flanked by a softer core. I agree with Ducknut, it would be a waste.

Plus by the time you wet out the kevlar/carbon and put enough thickened resin to get a decent solid lamination it will end up weighing as much as or more then the 1/2" ply you are trying to avoid. Look up the wieght differences between the plys, From what I remember 3/16"(4mm) that I used was only like 19lbs per sheet, but you are using 2 of them so it would be 38lbs per sheet, add in weight for resin and material wet out and you would be looking at mid 40lbs range. The half inch ply was I believe 47lbs so you are really only saving maybe 5lbs a sheet. Why not go foam core instead and save 10-15lbs over that?

 

No need to be embarrassed, it was an honest question as far as I can see that just needed to be talked out. PIB was talking about Polyumac sheets in another thread, at a bit less then marine ply I might go by composite one in the future if I do another boat.

 

There are advantages and disadvantages of composite material use. But you (the user) have to understand what they are and the capabilities/capacities of these materials. For the amature more times than not, they will end up with a heavier and weaker end result than by using wood, and certainly end with a much lighter bank account.

 

You can get away with thinner plywood by adding camber to your deck.
2 advantages by applying camber, strength obtained due to shape (eggshell effect)
and positive drainage from the centerline of the hull to the sheer.
Combine the drainage advantage with non-skid finish and the result is a safer deck.

 

All you need is standard 1.4 in plywood. Fiberglass both sides, run 2 stringers 1/4 in. 2 inch. works great, did both front and back with that material after checking with boat manufacture. Most folks want to put 1/2 nad 3/4 in wood and thats just not necessary. The fiberglass gives you the strength you need.

 

wood rots - ends up looking like this





using wood requires epoxy based resins - cheap polyester resins give a "surface bond" only,cheap polyester resins are also pourous -not good !wood also requires a complete sealing of any holes drilled into it - using cheap silicone caulk/sealers will prove costly down the road
wood's heavy...

composites are allways a better choice...

 

Use 3/4" PVC foam board and your kevlar, or some glass for that matter.  Glass both sides.  It will be way stronger and lighter than plywood, and it will never rot.

 

^ ;D

4lb Air cell board, with 2 layers of Bi-axle on each side! Light as a feather, and strong as an ox!

I know I'm going to get beat up over this comment but wood sucks in a boat! Just my .02

 

Structural foam is a useful tool in boat construction, but is not the best material for all purposes.
It has it's weaknesses also. Lacks tensile strength, lacks compressive strength, fasteners must
be installed with extreme care using backing blocks and fillers to prevent crushing and shearing.
If you're a hobbyist, building for fun and to learn, wood works well. If you want to build commercially
and want to offer a warranty with your finished product, you use the material that causes the fewest long term troubles.
Me? I like wood. Easy to obtain, easy to work, seal with epoxy...4 years later and still having a blast.

 

Ha I told ya I was going to get beat up over it, truth be told I just wanted to see a pic of your boat Brett!!! LOL!!!

Wood does have it's place, Wood isn't the problem it's the 99.9% of people who shouldn't own a wood boat, heck own a boat at all!!!  ;D You sir are one of the rare exceptions.

That sure would make a sexy flouder gigging boat Brett!

 

wood isn't a problem - it's protecting the wood that is...
inexperienced/unknowing people working with incorrect materials - such as a polyester based resin,improper sealing techniques,improper glassing techniques - all these cause problems,which really turn into somethin down the road...

there's good and bad in everything - be it wood or composites - example,nidacore - a terrible product,i wouldn' use it on a bet,others will and boast of it's qualities...

properly sealed wood,laminated in an epoxy will last - first hole improperly sealed,and it's all down hill from there...

composites are lighter,and in the case with a dense structual foam,such as coosa and penske board,it's stronger,and the best part - it will not rot...yes,it costs more,however,it will last indefinatley - to me,that's worth it...

 

I've used 3/8" marine grade wood for my deck sandwiched between two layers of 9 ounce cloth. The cloth doesn't add much stiffness strength to 3/8" plywood due to the thickness of the plywood material. Majority of the stiffness somes from the distance between the layers of laminant.

If you go with anything less than 1/2" plywood I'd put stringers at least 1.5' apart. But after you go with thinner plywood and add all the stringers you might end up with the same weight deck. Something to think about.

My current synthetic skiff has 1/2" 5lb core foam sandwiched between layers of chopped matt (yes, I didn't use structural mat, and for a reason, it wasn't needed due to my distances from supports and weight savings). My front deck has no stringers at all but is stiffened by the front deck lid gutters, so those really are my stiffiner/stringers. I have no bulkheads forward of my front cockpit bulkhead.

I built a skiff once with 1" thick core foam (two .5" sheets epoxied together). It had a front deck that was approx. 5' wide and approx. 4 foot long with no stiffiners and a hole cut out of it for a drop in plastic hatch and it didn't flex at all.

If I wanted to save weight and money for the core material wasn't an issue I'd either go with .5" thick minimum depending on my deck supports or 1" maximum. But 1" is really too expensive when you can go with .5" and use some bulkheads or stiffiners to aid in support. Just don't forget to install high density core foam where you plan to install a trolling motor, hinges, etc.

Always remember this - A good poling skiff designer will use every structural part of their skiff as structure and function to save weight (rod holders = structure, structural bulkheads = rod tub supports, etc). If it's not used for both it's wasted weight.

 

Oyster - I just went and looked at your skiff build thread.

The way you have your deck stringers laid out you might be able to go with 1/4" plywood with a 6 - 9 oz cloth or chopped mat (cloth/mat only used for rot protection with minimum strength added).

You might want to try a piece laid on top of your stringers first.

Especially if you plan to fillet glass the deck piece to each of your stringers which will create a fixed connection and help with deflection.  Versus just bonding the deck board on with an epoxy putty.