Michael Storer Boat Design 

FAQ - Paulownia/Kirri - the newest lightweight timber

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Paulownia is a very lightweight timber from China that has started to become available in quantity in Australia and many other western countries.

It is an excellent substitute for Western Red Cedar (thuja occidentalis) for the internal framing of lightweight boats or for the strip plank building method.

It can generally be used in either situation without having to increase the size of the timber or the thickness of the strip planking.  
After all I got away with a balsa strip canoe here - it came in at around 12lbs.

In fact some of the material handling is quite similar expecially if you dent the paulownia and the prevention of the dents in the first place.

I have been in contact with a number of people who have built heavy duty sea kayaks out of paulownia.  They have found that the boats are just as durable as the cedarstrip ones once the fiberglass goes on the inside and outside.

As far as framing for lightweight racing craft  - or just for boats that need to be portable it seems rather good at that job too.

And it seems to be largely plantation grown.

So I was asked whether paulownia would be suitable for a highly stressed Taipan racing catamaran.  A similar approach would work for any highly (or less highly) stressed dinghy or cat - Fireball, Sabre, Contender, Mirror, Cherub, Moth, International Canoe, Lightening, Windmill, Optimist, El Toro, PD Racer or any other class where some boats are still being built in wood.   Rowing shells like the Bangor Packet, the Kingfisher and other sculls being built in timber would also benefit.


It would also be a good timber for foils for any senior class - 505s, skiffs, NS14s, International 14s etc - as a wooden core to be glassed or carbon fibre.  A lost of professionals use Western Red Cedar as the core because it makes a reasonable light foil that is very reliable.

If fact so few break these days it indicates that perhaps the cedar is now TOO strong - ie too heavy for the application.

But back to using Paulownia in building the hull - it would be suitable for such boats - but I suggested two approaches.

A cautious one

And a "go for it" approach.

The advantage of the cautious approach is that the boat can be built that little bit lighter than everyone elses' for a small performance advantage.

The advantage of the "all out" approach is that despite the risk of breaking something there is the chance of real GURU status if you pull it off - and a step forward for building in the class.


Physical Properties
Paulownia's specific gravity (density compared to water) is quoted as between 0.23 and 0.30. - lets call it 0.25

Western Red Cedar is quoted at 0.30-0.32.

Strength and stiffness go in line with density.

So Cedar is around 0.30/0.25=1.20 times the strength and stiffness of Paulownia

Compensating for lower stiffness and strength
First a little naval architecture nugget. Generally it is more important to design for stiffness rather than strength. In most contexts if you get the stiffness right then the item will be strong enough.

So how to compensate?

There are two types of structure in a boat.

Case 1/
the timber has to deal with the loads by itself - eg stringers where the ply is only attached to one side of the timber. Or in the case of strip planking - (though my balsa canoe does show that strip boats are much stronger than they need to be)

Case 2/
where the timber is used to tranfer the loads from one piece of ply to another - so ply is attached to two or more faces.

Case 1
The Paulownia is less stiff, but you can make the beam/stringer deeper to make it stronger. The WIDTH of the item need not be changed.

The amount that the depth of the beam needs to be increased is
the cube root of the difference in density. This comes from classical beam theory where stiffness is given by

Stiffness of a rectangular beam = b x (d cubed) / 12

To cut to the chase the paulownia beam needs to have 1.2 times the stiffness of the Cedar one.

You could increase the beam width by a factor of 1.2 but this would end up with a beam the same weight as the cedar one.

The best way is to increase the depth by a very small amount by finding the cube root of the extra stiffness: ie cube root of 1.2 = 1.06

So just make the beam 6% deeper.

Case 2
is more complcated - there is no real way of knowing just how much load is being taken by any of the pieces of timber in this situation. Even the best computational methods can be quite wrong (note the America's cup boats that have sunk.)

So some general advice. If there is timber holding two pieces of ply together and you are already putting glass tape on the outside of the join - then just use paulownia on the inside to the same dimensions as cedar - almost no worries at all.

If not using glass then we have ourselves a problem. I can't give you an answer - and you certainly don't want to add the weight of glass to every join!

Strategy 1 - All or Nothing
It would be possible to just use the Pulaownia - which means either the boat will break or you become a guru.

If the boat breaks in one place only then you know where to strengthen it next time.

My feeling is that you might just get away with changing everything over to paulownia.

Strategy 2 - Risk assessment

But if you don't want to take that much risk then a part use of paulownia might make more sense.

So I would suggest a path of reasoning instead.

1/ Cat structures with Cedar are highly reliable even with differences in material quality and workmanship.

2/ this would indicate that the structures are stronger than needed

3/ So it might be possible to cut down on the strength without affecting the reliability of the structure.

4/ There are some areas that are higher stressed than others. Chainplates, crossbeam mounts and centrecase. Maybe leave Cedar in these areas

5/ All others are fair game to be changed over to Paulownia.

Up to you!

(If it was me I would take the risk - guru status is rather cool!)

Bigger boats - Does the Boat use fastenings or does it rely on glue bonding
There are two ways to go with modern boatbuilding - you can use fastenings or leave them out and use epoxy for bonding the timber structure together.

Both are nice approaches but for modern lightweight structures eliminating fastenings has considerable advantages.  
I've written about the general approach here.

If you are planning to glue everything up with epoxy and coat all parts of the boat then both strength and durability of particular timbers is not so critical. The gluing of all parts to each other makes the boat so much stronger than the original the timber doesn't have to be quite as strong. So if you pick a timber of similar density with reasonably straight grain all will be well - because the boat will come out at around the right weight.

You could even use lighter timbers like Paulownia for a fair bit of the structure and easily get away with it (not engine bearers or keel structure if there is a big donk going in her - or the floors the engine bearers intersect with). Also if the piece of timber is only glued to the structure along one face (stringers, frames) I would suggest increasing the depth dimension (from glued surface to opposite face) by 6% and so on as stated above for the smaller boats.

The durability of the timber is also not so important using epoxy to seal. The timber will never get enough water and air going in and out to start rotting if the job is three quarters decent.

If building traditionally then the important thing is to pick timbers that have a similar density to those specified and will hold fastenings to the same degree - as well as have similar durability if they get wet and stay wet.


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