material discussion and placement in a boat hull.

General Composites Discussions Engineering Talk
#1

Hi guys,
im building an old school wood boat for fresh water, and would be trailered. 16’ long 6’ beam v8 inboard 2 seater. 1935 garwood speedster design. I am using a cold molded hull building technic. I will use okoume 1088 marine grade plywood 5/32"thick x 6"wide x needed length planks to make up the layers, of which will be layed on opposing 45-degree angles from one another. all the planks will be encapsulated in epoxy such as west systems or other compatible, and also bonded to each other with epoxy. To this i would like to add interlaminate layers of composite for strenght and long life. I have come up with the idea of layers and materials as follows.

from most interior of hull bottom to the exterior: 4 layers of wood totaling .62"
layer of okoume 1088, carbon fiber 2x2 twill, okoume 1088, okoume 1088, okoume 1088, carbon fiber/ kevlar hybrid fabric 3K 2x2 dual twill OR carbon fiber/ Spectra hybrid fabric 3K 2x2 twill, outer s-glass sheathing layer.

i took the idea for the placement of the layers and laminate from the book “The Gougeon Brothers on Boat Construction” only they used kevlar on the inner layer and kevlar/ e-glass on the outer layer.

which idea would provide a stiff and long living hull?
any and all information would be most helpful thanks!

#2

Define for each layer why it is there. Then figure out the loads per layer. You will quickly find out that wood, sheathed with carbon fiber, will act as a sandwich, in which case the skins (the carbon) should take ALL load, and thus be thick enough to do so.

I would limit myself to using some manmade fiber to protect the wood from chafe and abrasion, and provide enough thickness in the epoxy layer.

Some other random remarks:

-west system epoxy has a relatively low Tg. (heat resistance). For coating the exterior I would suggest something with more heat resistance. Especially with dark colours.
-if you leave the boat natural finished, consider something else than okoume. Okoume yellows quite rapidly, not making the boat prettier. Use a nice mahogany instead. Select your veneer for colour and grain. This is what makes a boat stand out from others.
-okoume is not the stongest of wood species. Although OK for sailboats, making a V8 powered boat which should have longevity, is a bit of a long stretch for the wood. My choice would be Core-Cell, probably M130 for the bottom, and M100 for the sides, and multiaxial glass all around. This you can finish with mahogany veneers, making a low maintenance, strong boat looking like a wood boat.

#3

the loads per layer is far beyond what i know how to calculate thus i would not be able to calculate the needed thickness of the sandwich composite, hence why im here…lol

the 4 layers of 4mm thick planks of wood actually form a laminated hull in cold molded construction. thus i figured i could slap some composite layers inbetween the wood layers as i was building and beef up the final strenght of the laminate without adding more wood/hull thickness/or much weight. the additional strenght i am searching for is more for peace of mind and to also allow higher speeds without worry of the hull.

with the type of construction i am doing i have read that i dont even need any composite just the wood and epoxy for this boat i even bumped up the hull thickness by nearly .20" from what my references told me i would need. I wanted to use the composite so as to bolster stiffness. I was thinking the hybrid fabric for the outer skin because of its increased abrasion and puncture resistance as aposed to the standard outer skin of e-glass.

all the wood (except the hull) in the boat gets three coats of epoxy. The hull planks get coated all sides and allowed to begin to gel, then the underlayer gets a thickened epoxy and the plank is fitted in place. My plan is that after i get the first layer on the frames, i will vacum bag the additional layers of composite/wood to ensure adequate pressure and resin content in all areas.

for the finish, an epoxy primer/filler and two part black marine polyurethane on the sides, a epoxy primer/filler and multi season bottom coat below the water line, and a mahogany deck epoxied down, and clear coated with a two part high gloss clear marine polyurethane. this boat will be covered and trailered.

#4

Black paint on the sides can (will probabily do) cause problems by heating the epoxy laminates over its Tg.

Kevlar is a pain to work. Bad adherence, absorbes wáter if punched, bad to finish and for your saw and drills… you can better choose another fibers (I would do so).
Carbon is weak for abrassion. You’ll need a few plies for an effective protection.

To calculate your sándwich you can use ISO standards, both composites and Wood construction, single skin or sandwich.