Multiple layers of glass?

[disenmann]

I'm planning on building a stripper in the 16'-17' range with a beam around 34". The canoe will get used some light use, mainly on flat water, but for now I am treating it primarily as a wood working project. Weight is not an immediate concern of mine as I don't plan on portaging, but strength and durability are high on the list.

Most opinions I have read are that one layer of 6oz glass in and out is adequate. Some say an extra layer of 6oz over the football is necessary overall strength and impact resistance. To save some wieght I had thought of using a layer 4oz in and out with an extra layer over the football. This would make the bottom stronger than just one layer of 6oz while keeping the weight down. Would the 4oz be adequate for the over all structure of the canoe?

Also is there any advantage/disadvantage of laying the full sheet of glass over the small piece covering the football? Seems to me that the full sheet may protect the smaller piece from damage/delamination if there was a heavy impact.

Thanks, Dave

 

[pklonowski]

Don't do the 4oz glass on the inside... if you do, you'll end up like a lot of folks, who find cracks on the inside of their boats every time the boat kisses a solid surface just a little too hard. Been there, done that, and the constant repair work is not fun.

There are numerous posts here from Todd Bradshaw, detailing the engineering aspect of this design flaw. Search for "4oz" and they should pop up.

Go with 6oz inside & out, and an extra 6oz football, inside & out. You'll save a ton of work in the long run.

 

[Dan Lindberg]

For a 1st boat, I'd agree with the 6+6 layup, it will get a bit heavy though.

But, think of your 1st boat as a learning experience that you will make mistakes on and get practice on layup/wetout methods.

And do a search on Todd's posts, as he both explains it very well and offers a
lot of hard earned advice.

Dan

 

[Todd Bradshaw]

Good, because I'm too lazy to re-write all that stuff... The other thing to consider is that six-ounce usually takes considerably more filler coats to bury the weave enough for a final smooth sanding that isn't going to cut into the cloth. Granted, this does add weight, but from a practical durability standpoint, it's often well worth it. This thicker cushion of filler coats will tend to limit damage from most rock/canoe encounters to just a scratch in the filler coats - rather than ugly fractures in the glass. Filler coat scratches can be resin-filled, re-varnished and pretty much go away, glass fractures are white-ish and stay that way despite efforts to re-coat and fill them. Unless you're running a lot of shallow rivers or nailing a lot of rocks pretty hard, the boat may be able to have a long, useful life and never get any scratches that aren't just in the filler and pretty simple to re-fill and hide. The lighter the cloth and finer the weave, the thinner your filler coat cushion will be and the more prone the canoe will be to visible, non-hide-able damage.

I always lay the football half-layer down first, then the full layer over the top and saturate them together. It uses less resin for saturation (saving weight) and automatically goes a long way toward smoothing out the small step-down where you transition from two layers to one. By the time the fillers go over the top, there is no transition left. The only tricky part is that you need to be a bit careful not to over-agitate the resin as you're saturating the two-layered sections. If you roll or squeegee too fast or too hard, you can trap tiny air bubbles down in the cloth layers that will show. It's not a big deal, but something to keep in mind as you do your initial cloth saturation.

Some folks like to apply filler coats to both the outside and the inside, to hide all the weave. I generally don't fill the inside (or do just a partial fill, leaving some texture). Inside filling adds weight which isn't doing much that's not purely cosmetic and I also like the non-skid interior that the cloth texture provides. The tool I use for a squeegee to "comb" the inside glass to a neat, very uniform weave texture is a slab of ethafoam about the thickness of a slice of bread. We always saved the odd chunks of ethafoam packing foam that came in the boxes when we bought things. In the shop, we would have a few of these chunks and a Rapala fillet knife handy. If you needed a squeegee, you just walked over and cut one off the block. The "soft pressure" that they yield will allow you to get the cloth down tight to the surface and comb it out in just a few minutes to a really uniform texture that may still show the weave, but looks professionally done. They beat the pants off of any other squeegees I've ever used, and they're free and disposable.