paulownia timber instead of western red ceder

Paulownia or Kiri as it is known has been used by Rob Denney in his strip plank Harryproas. He has spoken favourably about it. Why not try sending him an e-mail trough: http://www.harryproa.com/contact.htm

Rob is usually very forthcoming sharing information/knowledge.

Hi Greymatter

I've investigated the same question myself, at the end of the day for our type of construction, ie gunnels, stringers, etc... the answer was you can use it but it could break or you could be on a winner.

Paulownia doesn't like getting knocked around, so the interior of the cat, yes, but, Paulownia is weaker than WRC, WRC I'm told is approx 1.20 times the strength and stiffness of Paulownia. So when using, its been suggested to increase the size, which would I suppose cancel out any weight savings.

But heres the website of a fella I think in Queensland that grows and mills it himself and seems a helpful bloke and sends it all over the place, both small and big lots, it is www.paulownia-timber-sales.com.au also have a read at www.woodworkforums.ubeaut.com.au scroll down to the boatbuilding section and look through I think either the resources section or misc section, I asked the same questions on there a while ago.

Regards

Paulownia has a nice straight grain, straighter than WRC

I have not used it but thought if I could source it when I build my next boat I would.

It's something like 20% lighter which is a plus and around 20% weaker. But given where it would be used, provided the gunwales are taped which they should anyway, I don't think this would be a problem.
If using it in C/Bs and Rudders I'd be laminating uni carbon over it anyway. I'd just consider the difference in compressive strength when considering the layup.

Given its lighter weight, and considering the supplies of good quality Gaboon ply getting harder to come by, it would be interesting to do the numbers on building a stripped beach cat from this material. You never know it may be another viable building material.

Regards,
Phill

Some more information, and perhaps a source of experience can be found here: http://www.storerboatplans.com/Faq/paulownia.html

Looks like he is on the same continent as you guys, so should be accessible.

Rolf,
Interesting link.
Claims Cedar varies from 300 to 320kg per m3.
My own measurements put it at around 310kg per m3.
I'm surprised- got it right in the middle.
The weight can vary a lot more than 20kg per m3.
The lighter cedar by colour is also the lighter timber on the scales. So if building from cedar look for the pale timber. From my own measurements the darker cedar can be up to 50% heavier.

The same link indicates the Kiri (Paulownia) weighs 250kg per m3.
Now the important issue is how this compares to 4mm gaboon ply.
Well you need to allow 2 kg per m2 to be sure for the 4mm gaboon or okume ply.

Cedar at 300kg per m3 works out to be 1.2kg per m2 @4mm thick.
This would allow 200gm plain weave on the inside and 200gm on the outside which at optimum would add 800gms per m2 bringing the weight up to 2kg per m2. while this looks good we are not really out of the woods (no punt intended here) as the strips need to be glued together or kept apart with a low desity filler. This would increase the weight per square metre. Sorry I don't have figures for that.

Now if we look at Kiri (Paulownia) it comes out at 1kg per m2 plus 800gms that allows for 200gm inside and out and we have a bit of the allotted wieght left over to join the strips together.
Looks like it would be worth a try. Bloody shame cedar is a much better looking timber.
Interesting....
Regards,
Phill

The trick is clearly to make the strips as tight fitting as possible, and pick the planks you want before ripping.

I think 800grams m2 for 2x200g glass + epoxy is very good. Multiplying 400g glass with 1.6 to get 640g epoxy for a total of 1440g would in my experience be better as some of the epoxy will soak into the wood.
On the other hand, you will apply two or three layers of epoxy to the ply as well, which also adds considerable weight.

Rolf,
Yes.
Now if it was a labour of love you may consider employing the idea of running a saw blade between the planks and follow this up with 60, 80 or 100 grit sand paper.
That would give a nice neat fit.
I could do that if I liked the look of the timber I was using. Then just paint some glue or filler on the edges while a bunch are clamped together and then just assemble.

Sounds like fun.

Regards,
Phill

phll wrote
But given where it would be used, provided the gunwales are taped which they should anyway, I don't think this would be a problem.

i havnt taped in the inwales on my current boat they are just glued on with epoxy resin, or are you talking about if they are on the out side of the hull

I'm talking about glass tape on the outside.
A light layer of glass on the decks and glass tape on the gunwales reduce spitting of the grain which allows water in past your epoxy coating. I bought a timber Cobra nearly 30 yrs ago and its gunwales had not been glassed. It opened up between the deck and hull on the port side of the starboard hull just in front of the front beam on a distance race. Very lucky to get back to shore.
Once the gunwales are glassed the strength of the gunwale material is much less significant.

I know another chap who built a ply cobra back then and did not glass the decks. Within 12 months he had cracks in the deck as the grain was opened up.