balsa core, endgrain or not?

For sure.. I'm using it.. or actually finnish spruce but..anyways most of strip planked constructions in Scandinavia are local spuce..

 

The basic problem with balsa or most core materials for that mater, is they are selected for their compressive qualities and typically by their nature, have limited if any useful longitudinal stiffness. This requires the laminate or additional structure to be employed to carry these longitudinal loads, which can be fairly high in a canoe. The thicker laminate thickness and/or additional structure make for a more costly build, that also requires more effort to produce. This is one of the reasons inherently stiff materials like plywood, solid lumber and to some degree, metals are so popular.

If you want complex shapes, then molding or strip planking will be the more cost effective methods. If you employ the Lord method strip planking, you'll have higher laminate costs, but a lighter and stiffer hull then just about another technique.

 

If you decide on plywood, I'd look into CM ("cylinder" molding). It limits your shapes a little, but it's a quick/easy way to throw a boat together, and it turns out nice & strong by overlapping the ply panels.

 

You could conically or cylindrically develop some pretty fancy shapes with plywood panels. If you introduce some torture into the panels, even more "shape" can be admitted. I've bent 3mm (1/8") ply into some fairly hard curves.

You also could use a strip planked turn of the bilge and transition to a conically developed plywood panel for the remainder of the hull.

 

Thanks for the tips. What is special about the Lord method?

Here is what I'm thinking now: I have costed some super dense and some normal density 0.6mm ply for the outer skin - cheaper and I think probably better than 400gsm or so glass. Because of the abrasion resistance on the bottom I would like them to have longitudal fiber orientation there, at least.

The 3mm spruce I would then like to lay diagonally, as shown.
veneer: 0.6mm thickness x 1.2 density is 720 gsm
3mm x 0.36 is 1080 gsm

I know it is usual to use the core in the lengthwise direction, but I am not sure it is the best way, always. The way I would do, is to use a strong keel plank, and a floor with reinforced edges, about where the hull sides starts becoming vertical. This becomes a stiff triangular beam, when held apart by the diagonal strips and longi veneer. It is also stiffened by the gunwale.
So, contrary to the normal philosophy as I understand it, the lengthwise strength is concentrated in 4 or 5 logs instead of along the skin, which, being stronger vertically than normal, is a perfect shear web between them.
With the logs being placed as far away from the neutral axes as possible, this looks to me like an efficient structure. I also think the panels will be locally stiffer than it would have been with purely longitudal core.

 

Sigurd:

With ply, the grain is alternated at 90* angles each layer. That means that when you turn your 3mm ply at a 45* angle, you're creating a double-bias effect (2 veneer plies at 45*, and 1 at 135*). There's nothing wrong with this approach to my knowledge, just wanted you to be aware of it. When someone builds in ply with the facing layers longitudinal-grained, they're counting on the middle layer (grain facing athwartships) to hold everything together (well, that AND the glass/epoxy shell).
Your way might be better, or the more-standard way might be better. I'm guessing that either will works, but it'll be interesting to see if you find any special properties your way.

 

Now I ordered the ply! 0.6mm, ipe for the bottom, makore for the rest of the outside, and kingwood for the cockpit!

 

I would like to try preservatives like tar and linseed oil, instead of coating with epoxy (Then I will be certain it can not collect water and rot). How could I glue the veneer to the 50gsm glass without getting a lot of glue on the outside? If I make thick epoxy bog, can I vacuum clamp the veneer?

Anybody tried beeswax as wood preservative?

 

I read the thread, but it doesn't seem to answer if you are making white water canoe or something for calm waters instead. A major difference in suitable materials and how to dimension them.
Are you asking impact resistance for a given panel weight or for a given material cost or given thickness or ???
The correct answer depend a lot on what of those question is assumed here.
None of the discussion so far seem to give any hints what so ever on this.

 

You are now making your project a bit more complicated than it's necessary. Just put the glass (roven) and epoxy on top of the veneer and save tar and linseed oil for lapstrake hulls..

 

If you're worried about penetrating the wood with waterproofing, try using an ultra-low viscosity (with [red]NO[/red] thinners) like MAS Infusion 3-part @ 100-150cps. The thinner epoxy will absorb deeply into the wood. this will increase the weight of the wood (and decrease its flotation) somewhat, but it'll give you the long-lasting waterproofing you're looking for from you linseed/flax oils! (just a thought)

NOTE: If you use a super-low-viscosity epoxy, you need/have to vac-bag or vac-infuse the parts to keep from having runny epoxy get all over everywhere & leave terrible runs on your project that would require extreme amounts of sanding.

 

sailor2, It is a kite boat, so pretty severe use. Difficult to answer your question - has to be as strong as possible for as little as possible money and weight. The puncture and abrasion resistance is necessary for ice and beaching and a long life in general.

 

Hmmm...how about 3 layers of 11.1g/m^2 satin-stitch kevlar 49 then (or maybe you should actually look at K29 (ballistics grade Kevlar)...rofl

 

If the low viscosity epoxy is applied while the plywood is flat on the table, the issues you mention should not cause problems. The panels can be bent into shape to form the canoe after that process. And then join the pieces by laminating over. Would make it easier.

 

So you are making a sailing canoe with free flying kite. That would be very far from severy use conserning impact strength compared to whitewater use where you would be smashed against rocks while going down "river" Therefore all mentioned methods can easily give you enough impact strength for such a small vessel.

For the abrasion, only the outer surface matters, not what's below it. You could have used even the balsa with grain not perpendicular to surface.
What matters for surface is the thickness of laminate. For best abrasion resistance go for a solid thick aramid laminate with no core or wood for light & strong. No other way gives you same or beter abrasion resistance with less cost than that except using metals. Including wood core gives you more weight & cost for same outside laminate, or with less outside laminate you get less abrasion resistance and somewhat reduced puncture resistance ; less so with high density woods and more so with light density woods or foam core.