This was a discussion about how to build a boat in terms of impact.
Such as beach landings in sea kayaks or hitting rocks in rapids in strip canoes or kayaks.
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In Nick Shades “The strip-built Sea kayak” there is a great analogy of the timber/glass construction with a metal I-beam and the wood is really only holding the two skins apart. In this analogy to “dent” (as distinct to puncturing) the canoe the inside glass would have to stretch and the outside layer would have to compress (this is where glass has it’s strength).
Denting is very localised on the outside skin of the boat. Like bruising a peach by squeezing it with fingertips. The bending loads are insignificant (the I beam analogy) – but the cross grain compression loads where the stone is pressing on the hull are very high
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Where the “strength” of the timber comes in … in my way of thinking … is it has to have a good compression strength ie how crushable is it. This is different from the tensile strength etc that is easier to come by. I can not find anywhere the compression strength of Paulownia so I’m just winging it but I would expect it to sit somewhere between Balsa and WRC
Exactly right – and if my balsa canoe above can handle all sorts of nasties then the Paulownia will be fine. Strengths of timbers are quite proportional to their densities as you suggest. There are some exceptions but not by a huge amount
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Therefore if the timber crushes then the two layers of glass are sitting side by side and can therefore be “folded” quite easily. (see attachment)
Sea kayaks are quite different creatures from canoes – I have a “word” for you … it is … surf landings.
The sort of damage that Nick is talking about is from wrapping a fully laden seakayak against a rock when surfing in. I have never seen damage of this type. In fact I haven’t ever seen damage that has gone through to the inside skin. Most of the denting that I have seen has even been without cracking the epoxy. Just like denting a peach skin.
Isn’t it risky to reduce the density and strength of the timber if doing surf landings.
For a surf landing that goes OK there is no difference in timbers of course!
But if something goes wrong and you wrap the boat around a rock there will probably be little difference between timbers – if it is hard enough to damage one of them it will be hard and heavy enough to do significant damage to all.
I haven’t got enough time at the moment to dig out an old spreadsheet I wrote as part of my engineering course to get a quantitative answer about strength and impact absorbtion, but I can suggest a ballpark solution.
Normally strip boats use 6mm Cedar. If you go to 7mm Paulownia the strips will have pretty much the same impact absorption and tensile strength.
Paulownia needs some effort to prevent denting at this stage. But any dents can be fixed by spraying a bit of water over the affected part. But it is excellent once glassed. Thick light timber gives a very significant “I Beam effect” because thickness is not reduced. Reducing glass weights reduces the heaviest component of the build.
Trimming Paulownia strips along the centreline.
Glassing proceeds as per normal. We often use very light glass but double it where there will be contact on inside and outside of the boat. Do the reinforcing layer before doing the final all over glass.
You can get some idea of what you can get away with in the Balsa Canoe Project page. And you can get away with more because the paulownia is tougher than the balsa I used :)
Another Paulownia boat with contrasting cedar edge. I quite like the sheer edge strip to be cedar as the edge is exposed and can get damaged. The darker colour of the cedar will be hidden by the gunwale and inwale.
There seems to be a tendency in internet discussions to cut weight by reducing the timber thickness. If reducing weight then lower density timbers can be selected and timber quickly becomes much lighter than the glass.
And because Paulownia is lighter than WRC you won’t have added any weight.
Another way is to do some of the inside or outside of the hull with kevlar by substituting for glass. But be aware that Kevlar is a pain to cut and a pain to sand after laminating. It likes to ball up.
I don’t know what the sea kayaking hotshots are using for glass. For normal use 6oz (200gsm) is fine, but there might be some info about boats that are going to be surf landed on some specialist sites on the net.
Or in the plans!? The info should be there.
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Just did a fair bit of fishing through Nick Schade’s site. It looks like the general run of kayaks are glassed with either 6oz or 4oz.
If you are likely to be wrapping it around rocks I would take the heavier!
If not wrapping boats around rocks in flatwater use then we have found it is possible to go very light with glass. This is a better structural option than making the timber thin. Timber is light. Glass is heavy.