Thread: Plywood Kayak Scantlings B.O.A.T

G'day MIK
Getting perilously close to the point where I will have to wave money at Ted for ply & 'poxy & etc. Just a bit of varnish & paint left to do on current build. (photo below)
At Dave's suggestion, I've been doing my kayaks in 4mm ply, & being several boats familiar with the material, am thinking to do Eureka in same. 6oz glass outside, & probably on floor & chine panels inside. Have also read carefully the ultra-light build article at your web site. The idea of a canoe coming out at not much more than my kayaks (12-14kg) is seriously appealing - I'm a lazy sod at heart.

Do you see any glaring issues with using 4mm pac.maple rather than 6mm, and other scantlings as per the 3mm build ?

cheers
AJ

Just now realised I got mixed up ... it is an Eureka enquiry - ah well - I was a blonde when I was younger!

Howdy AJ,

It depends on your objective. The Pacific maple can be a variable beast. Usually Ted gets ply with decent thickness outside veneers, but I have seen some in other places where the outside veneer is paper thin.

There is no actual timber called Pacific Maple and a whole bunch of Meranti related timbers are used.

So as long as you avoid ones with paper thin external veneers... there will be no problem.

The only problems you are likely to see are maybe a little more tendency for the wiring to pull out (but only marginal) and perhaps the wire stitches will have to be around an inch closer than I specify in the plan - you will know anyhow after building your bunch of kayaks.


Pics from http://www.flickr.com/photos/boatmik...7601053377705/

The only specific problem with the Eureka and thinner plywood is if you tighten the stitching for the bottom before the sheer is spread out you will end up putting some hollow in the middle of the bottom. This is not visible except for when the boat is upside down and will have no repercussions on performance. So spread the sheer before tightening the bottom stitching too far.



There is no structural problem with using 6oz glass on the outside. It is your call ... but...I know the US people are mad about glassing everything. Somehow they have got hold of the myth that it only adds a little weight.

This is wrong. Timber is so light that glass dramatically overpowers the weight of the timber - so you move from a 44lb boat to a 60lb boat very easily - particularly when you are talking about canoe sized surface areas.

I know it is still much lighter than an average 'glass canoe - but it is good to keep as far from that territory as possible!

There is one very nice advantage in using wider glass fabric rather than tape - and that is the amount of work it saves - you avoid having two glass edges to sand down per seam.

With glass the actual advantage doesn't lie so much in the thickness.or weight, but because of geometry of what happens when you add a layer to three ply and the glass properties themselves.
___________________________

1/ Three ply plywood is very floppy across the narrow dimension of the sheet and stiff longitudinally.

Longitudinally
This is because the two veneers on the outside are set a distance apart (away from the neutral axis) so the internal veneer acts like a core. The result is that the two outer faces work together much like any sandwich structure (foam surfboard, gyprock) to increase stiffness dramatically. The further you move the timber from the middle of the plywood, the stiffer it becomes.

Transversely
Now we come to the other side of the argument. The single ply in the middle is attempting to deal with almost all the transverse loads alone. If you bend it transversely it is the thin veneer in the middle struggling to prevent the bending. It is not thick and it is right at the middle of the plywood - so it is in the wrong place to do much real work in resisting bending. This is why once you move to 6mm ply you often get 5 ply - a much better material than the 3ply in terms of dealing with bending loads. This CAN make it too stiff to build some boats though!

So the solution is quite nifty. Even a very thin layer of glass on the outside means that the core veneer and the glass can work together to deal with transverse bending loads using the outer veneer as a core. So you get a remarkable amount of transverse stiffening even with a tiny amount of glass.

Sometimes in boats built of three ply there is a tendency for the ply to get longitudinal cracks along the edges of the deck where it meets the substructure - or close to the chines or keel on the bottom. I have taken boats with severe damage of this type and simply added some very light glass to provide a dramatic stiffness increase in the area needed - another advantage is the glass is so thin that there is almost no fairing and it is completely invisible in all lights.

Glass weights ... I have used 2oz (75gsm) and sometimes similar glass weights to the balsa canoe (0.75oz) weight cloth - the difference in stiffness - whatever the glass thickness is dramatic.
___________________________

2/ Design for screwdrivers.

Some people on the PDRacer forum in the USA said that the 4mm ply we used for our boats would be easy to puncture. We (Midge and I) disagreed - most wooden raceboats in OZ are built of the stuff!

One of the guys on that forum worked out some tests using a screwdriver and a scale of different ply weights to test different ply and glass combinations and proved that our 4mm ply sides and decks were much more vulnerable.

Midge pointed out that he had never seen a screwdriver on a beach.

In all my years of working with boats I have seen remarkably few cases of structural damage to ply boats. The only cases I can recall have been because of collisions between boats and other boats or boats and hard objects while trailer borne. One particularly dramatic one was when a beautiful Strueur racing canoe had two forklift blades driven through it.

Eliminate those and .... um ... one cracked fillet on the inside of a ply canoe that was dropped from roofracks. Do you have any idea of the number of boats I've been involved with? Many of them not glassed except on the outside seams. The point is that if you hold the ply together at the seams it is strong enough to sustain almost any sort of load.

Also another interesting thing happens with glass protecting a surface. Instead of digging into wood (because it is relatively soft) many sources of potential damage will skid off a glassed surface with little or no damage. A canoe that David Wilson built years ago came back into the duck flat workshop a few weeks ago. He built it when I worked there a decade and a half ago and gave it to a friend. On their first trip on the Murray they were paddling nicely and the boat suddenly stopped - mid river. They tried to paddle but the boat only swivelled around the one spot. It was sitting on top of a star dropper (a vertical steel post with sharp corners). They got it off and rushed to shore - a tiny amount of visible damage. So the boat turns up at Duck Flat again after 15 years and we look at the bottom - and yep - a tiny little graze in the glass.

My feeling is that this effect is almost independent of the thickness of the glass. That a very thin glass layer will protect the timber structure almost as well as a very thick one. Remember my assumption is that the timber structure is tough enough to deal with most things of this type unassisted.
___________________________



So - rather long-winded all this - but my suggestion would be...

The 4mm Maple will add some weight - it is probably about the same weight as 6mm Gaboon but then you have to add glass to provide the stiffness as above.

Ted has some 2oz glass - maybe that would be good for the outside without adding too much weight. Cover the bottom panel and the bilge panel but only come an inch onto the topside panel.

You will enjoy using the thinner glass - it is wonderful to use and also is easy to fair up later. You have to be careful not to go through it at the chines when sanding of course.

Glass tape the inside normally - there is no need for extra glass in the floor area - the glass on the outside will do its trick and give you similar or maybe even more stiffness than using 6mm ply.

But your call... People always build the boats they like!

Best wishes
Michael

Thankyou for such a full & wide ranging reply Michael. You have given me much to mull over. I have been using 6oz glass primarily as an abrasion / puncture resistor, and in lieu of glass taped seams rather than as a panel stiffener, although that has been a bonus. Even the interior glass mentioned is purely to reduce the likelihood of things like stones in sandshoe treads from breaking through the paint film. I had no idea a significantly lighter glass would be an option, although I should probably have twigged given the reading-up I did on glass tensile strength while knocking out slalom boats in the mid '80s.

Aiming for a boat of roughly similar weight to my kayaks. As far as I can see they are within a flea-bite of using the same area of ply despite Eureka's extra length. 6oz glass resists punctures on limestone snags around Clayton Bay very well. Far better than the paint over the top of it. And certainly better than the 2oz stuff on my daughter's surf board. So despite the advantages of 6mm 5-ply and/or weight savings of 2oz cloth, it is very tempting to stick with what I know. Will need to think about that...
cheers
AJ

G'day Michael,

Not my question, but thank you very much for such a well thought out reply.

You have answered a lot of things that I have been wondering about as well.

Oh dear, I am feeling the "need" for a eureka. Got a couple of other boats to build first but will be talking soon.