balsa core, endgrain or not?

[sigurd]

When considering a cheap canoe hull, I am thinking of using balsa core, with longitudal hardwood and glass sheath. With the cost of all the epoxy I think pure glass will be as expensive as the hardwood veneer, for the same puncture and abrasion resistance? With the veneer I'd put 50gsm glass between it and the core, on the outside. The inside could be veneer, but light glass, 200gsm or so would probably be cheaper.

You can get endgrain balsa or planks. If I use planks, I'd lay them in the transverse direction. If I get endgrain panels, I'd have to consider strengthening the skin in the transverse direction.

But which one will be more impact resistant?

[TeddyDiver]

Endgrain..
Just curiosity, but how thick panels you are talking about??

MVH Teddy

[sigurd]

balsa: if planks, 4mm, endgrain is 6mm.

[TeddyDiver]

Have you a mold ?? You need one for endgrain sandwich. Otherwise it's easier to make stripplanked and then you get better results with spruce..

[waikikin]

Have a look for Michael Storer or Known here as boatmik , he did a nice longtitudinal balsa planked canoe. Regards from Jeff.

[sigurd]

Waikikin, I saw one on the net - could be the same canoe.
Less than 6kg it was, 7mm core with 75 alternating 150 gsm glass.
Teddy, I have been considering stripping birch, spruce or similar, as well. I think I would need a total of 3mm laminate, then. It would be much less stiff, but puncture and shock resistance should be easier to get, I think? I've never seen endgrain balsa. I'd build a mold from stringers on a frame - I assume you don't think it is necessary with a complete mold?

[robherc]

sigurd:

For abrasion resistance, glass is OUT...try Kevlar, or better yet (if you're prepared to do some vac-bagging) Dynel for the outermost layer(s) if you're wanting a surface that'll stand up to a good bit of bumping & grinding + dragging & scraping. The Kevlar adds strength similarly to e-glass, but is pretty expensive; the Dynel doesn't add much strength, but is the BEST for abrasion resistance.

As far as the balsa, end-grain is a good bit stiffer & more impact resistant, but it's also harder to work with in tight curves. You'd almost be better off cutting the endgrain into strips & strip-planking with it than anything else, IMO.

Just my $0.02(US, unfortunately), take it for what it is/isn't worth

[PAR]

Dynel or for that matter Xynole (which is 2 times better then Dynel at abrasion resistance) will suck up huge amounts of resin and make for a heavy laminate. Kevlar is the fabric of choice if the ultimate in both regards is required.

I don't think a cored structure is the cheapest way to build a light canoe. Even if the core is inexpensive, the goo and fabric cost will be higher then other material choices.

Impact resistance is a pretty relative term in regard to a small, light weight canoe. A good light weight species plywood and a thin sheath is fairly hard to beat, pound for pound, dollar for dollar.

[sigurd]

PAR, I agree with your assessment of thin plywood, but to get the shapes I want, it would have to be cut up a lot I think. So in that case it might make more sense to me to laminate the plywood myself. But the stiffness of the balsa laminate is alluring, so i'm trying to find a cheap way to make it abrasion and impact resistant. To that end I was thinking of using ipe or another hard, strong wood as the outer 0.6mm sheath, 50gsm glass under it, and a thin glass sheath on the inside of the hull, the glass sheaths could be reinforced with carbon tow (also cost effective for me now, in small amounts). Not sure what to expect from such a laminate though, and using a much thinner and denser core like spruce or birch at least in the bottom seems the safer way but needs more supports.

Kevlar vs hardwood as sheathing, it would be interesting to know the differences.

[TeddyDiver]

Originally Posted by sigurd

Teddy, I have been considering stripping birch, spruce or similar, as well...

Birch is too heavy and has other issues too.. Stringer mold (for endgrain) is allmost a ready strip planked body so it's excess work.. but possible. In thin cores endgrain balsa sucks relatively lot of resin and there's hardly none if any weight benefit compared to stripplanked spruce.

[sigurd]

How much resin?

[sigurd]

http://www.powerandsailmarinesurveyo...balsa_core.htm

Pre-coat one side of the end grain balsa sheet with laminating resin just prior to installation. The average amount of resin to wet out a LamPrep® surface is 1.5oz (40-grams) per sq.ft. An AL600/10® coated surface uses 0.70-oz (20-grams) per sq.ft. The resin is generally hand applied with a short nap length or foam covered roller.

That is a lot of epoxy. How much does spruce need then?

[TeddyDiver]

Originally Posted by sigurd

That is a lot of epoxy. How much does spruce need then?

To make a quess maybe 1/4 compared to endgrain balsa but it greatly depends of the quality, density and humidity% of the timber (balsa and spruce or any timber) and the viscosity and ambient temperature among other things so only a test will tell.

[robherc]

Originally Posted by sigurd

Kevlar vs hardwood as sheathing, it would be interesting to know the differences.

Kevlar:
Strengthening is encapsulated in resin, and is a waterproof fiber...so as long as the Kevlar is still at all in-tact, there is no danger of rot.

Hardwood:
Covered in Epoxy for waterproofing, and (depending on the wood) can be water & rot resistant, though not to the degree of Kevlar. Thicker than Kevlar fabric laminate, so can last through more long-term abuse before "wearing through" and needing replaced (probably easier replacing the whole boat).

This is by no means an exhaustive comparison, but I think it'll give you an idea of the balance here. Waterproof/rotproof vs. Longer-life, lower-cost.

(You can, however, extend the life of the Kevlar with very little weight added by using 2-3 layers of Kevlar...drawback: higher cost)

[sigurd]

Wonder if norway spruce is a good strip material?