15mm bottom panels with Kevlar?

[Howie]

Way back it was mentioned that 5/8" was thick enough for the hull bottom panels and this was upsized to 3/4" due to difficulty finding this ply. If one could source 15mm okoume would it make sense to go in that direction? Assuming 33 lbs/ft3 for okoume, it should be a weight savings of around 50 lbs, which is a good offset to some of the other features I plan to add.

Also, I have been reading up on increasing puncture resistance with kevlar. Interested in exploring this since I will be boating in the PNW, and it seems like a good idea if it is not too much of a PITA (it probably is, lol)

If I did it I would put a layer on the inside of the bottom panels (as suggested years ago by Brian). Sounds like kevlar is no fun to lay down with since it wants to float in the epoxy. This guy seems to have a good technique...basically putting on a tack coat and sticking the kevlar to it at the right time. Then coating and immediately covering with some glass.

https://flyfishingtraditions.blogspot.com/2014/08/kingfisher-build-laying-down-kevlar-and.html

If I ended up doing this before stitching while the panels were flat, would I be creating any problems when I went to open them up? I know I would need to sand/prep before the fairbody and chine seams since this would no longer be raw wood.

[Brian.Dixon]

Since the original bottom ply thickness was spec'd at 5/8" (16mm), I would not use plywood that is only 15mm thick.  You can, but then I'd want you to add an additional layer of 10-oz glass on the inside of the bottom panel (and chine flat) assembly ... or at least 5-oz kevlar (aramid) over the whole length.  Since I'd want this extra glass over the seams, you'd have to add the kevlar to the inside faces when they are flat and unassembled, then after assembly, go back and add an extra layer of 10-oz glass tape over the seams ... to meet or exceed my requirements.  You'd lose your weight advantage, but not all of it. 

[Howie]

If I did this I would probably put 6oz glass over the kevlar to help keep it down. From my reading, the epoxy ends up being 1.5 to 2x  the weight of the cloth. This would be about ~30-35 pounds added back.

Only seems to be worth doing if you think a 15mm panel with 5oz kevlar/6oz glass on the interior would be substantially more puncture resistant than an 18mm panel with the standard construction...the weight savings is minimal.

[Brian.Dixon]

It's definitely worth it from a puncture perspective, in my opinion, but I've known several people that cussed kevlar after trying to put it in.  I would do a couple of 2' by 2' pieces of ply first, to see if it works for you and/or to get some practice, then do the boat.  And I'd expect a few shallow bubbles here and there too, but after the kevlar cures, then drill holes on each end of the bubbles and use a syringe to inject epoxy into them. You can use masking tape over the holes to keep the epoxy from draining out.  I don't think putting kevlar on flat panels is terribly difficult, but it probably just benefits the project to have a little practice first.

One thing to keep in mind about kevlar is that it absorbs water if exposed to it, but that's one of the reasons we don't put it on the outside of the boat ... gouges may expose water to the kevlar, and freezing temperatures may start some delamination.  If you put it inside the boat, and especially with the 6-oz glass over it, then bob's yer uncle...

[Howie]

Thanks Brian-

Just to confirm the steps if I were to do this:

scarf/cut bottom panels 15mm aft, 9mm fwd
while flat, laminate 5oz kevlar with 6oz fg on top
same with 15mm chines
stitch panels and chines and fillet/reinforce with fg tape per plans
Once bottom panel assembly is set to stringers add 2nd layer of 6mm ply to fwd bottom

[Brian.Dixon]

scarf/cut bottom panels 15mm aft, 9mm fwd
while flat, laminate 5oz kevlar with 6oz fg on top
same with 15mm chines
stitch panels and chines and fillet/reinforce with fg tape per plans
Once bottom panel assembly is set to stringers add 2nd layer of 6mm ply to fwd bottom

Almost!  If you examine the original build instructions, you will see that a layer of 10-oz glass is added to the entire inside surface of the bottom panel assembly after the seams are complete.  In your build order below, you are effectively doing all of the interior glassing prior to assembly, but the seam layers are added after that ... the seam itself, in terms of glass over the seam, ends up short since you are not doing final interior glassing over the top of the finished interior seams.  I'd add one more 10-oz strip of tape (2" wider than the previous) over the fairbody and chine seams.  Like this:

scarf/cut bottom panels 15mm aft, 9mm fwd
while flat, laminate 5oz kevlar with 6oz fg on top
same with 15mm chines
stitch panels and chines and fillet/reinforce with fg tape per plans
apply 10-oz tape over the fairbody and chine seams (on top of the fillet/glass applied in the previous step)
Once bottom panel assembly is set to stringers add 2nd layer of 6mm ply to fwd bottom

[Howie]

Ah, got it. Thanks!

I wish I could figure how much "better" this would be with regards to puncture resistance to see if it is worth doing. The kevlar would be about $400 to do the entire bottom assembly. Could be relatively cheap insurance or totally unnecessary.  I should probably do some sample panels both for figuring out the application and doing some destructive testing in the backyard.

[Brian.Dixon]

Ah, got it. Thanks!

I wish I could figure how much "better" this would be with regards to puncture resistance to see if it is worth doing. The kevlar would be about $400 to do the entire bottom assembly. Could be relatively cheap insurance or totally unnecessary.  I should probably do some sample panels both for figuring out the application and doing some destructive testing in the backyard.

So far, nobody's managed to puncture a Great Alaskan.  How necessary the kevlar is, is anybody's guess.  The standard 3/4" (18+ mm) thick bottom with 10 to 20 ounces of glass sheathing on the bottom is pretty darn tough.