Thread: Paulownia

Gday Catbuilder,

after a suggestion by Boatmik I went off and had a look at this stuff and was amazed with it.

I would say the next lightest thing would be balsa but it still has a lot of strength.

I've used it in my sons cadet to replace WRC parts.

The big thing is that if its gonna get knocks, walked on etc then find something else. If its for some of the hidden structural stuff, wall linings and I've heard that people have started using it as planking on the hull then give it some serious thought.

You can even laminate it for additional strength.

Oh yeah, I understand it has fire retardant properties (something to do with the lack of gum in it) and resistant to termites.

Check this thread out http://www.woodworkforums.ubeaut.com...ad.php?t=36091

What are you thinking of using it for ?

Regards

Wow Meerkat - what a perfect summary!

Put a bloke out of a job you will!!!

Timber strength goes with density. So Paulownia is weaker than western red.

But there are ways to compensate - and often no need to compensate at all.

For example Western Red has wandering grain so using it as a beam can result in a split travelling diagonally across the piece.

Paulownia has lovely straight grain - so is not prone to this mode of failure.

Tell us where you want to use it and we can respond with information!!

MIK

Originally Posted by Boatmik

Wow Meerkat - what a perfect summary!

Put a bloke out of a job you will!!!

Timber strength goes with density. So Paulownia is weaker than western red.

But there are ways to compensate - and often no need to compensate at all.

For example Western Red has wandering grain so using it as a beam can result in a split travelling diagonally across the piece.

Paulownia has lovely straight grain - so is not prone to this mode of failure.

Tell us where you want to use it and we can respond with information!!

MIK

Well you taught me everything I know.

....now wheres that brown nose icon

Boatmik/Meerkat

Thanks for the replies, the only thing is I have no idea what I would be using it for at this stage. I have been using WRC in my 16ft cat construction, gunwales, stringers and various other bits and pieces. I'm just furthering my knowledge, I will annoy all by doing this but if I don't ask I won't know. Would Paulownia suit the jobs above? In my line or interest, the usage of it would be interior similar to the above.

Thanks Again

Matt

Originally Posted by Boatmik

Wow Meerkat - what a perfect summary!

Put a bloke out of a job you will!!!

Timber strength goes with density. So Paulownia is weaker than western red.

But there are ways to compensate - and often no need to compensate at all.

For example Western Red has wandering grain so using it as a beam can result in a split travelling diagonally across the piece.

Paulownia has lovely straight grain - so is not prone to this mode of failure.

Tell us where you want to use it and we can respond with information!!

MIK

Originally Posted by catbuilder

Boatmik/Meerkat

Thanks for the replies, the only thing is I have no idea what I would be using it for at this stage. I have been using WRC in my 16ft cat construction, gunwales, stringers and various other bits and pieces. I'm just furthering my knowledge, I will annoy all by doing this but if I don't ask I won't know. Would Paulownia suit the jobs above? In my line or interest, the usage of it would be interior similar to the above.

Thanks Again

Matt

My limited rule of thumb is

Anything that will get some abuse Decks, Gunwhales etc - No
Anything that needs "real" strength Masts etc - No (of course this would really depend on what it is exactly. With poxy and proper support it may be ok)
Interior - Depends but most probably ok

It really depends on the application etc.

Mik ?

Catbuilder

There is always a risk when experimenting with lightweight structures.

But if it comes off - you end up being a guru to everyone else.

Worth the risk I say! This was my go at it.
http://members.ozemail.com.au/~store...alsacanoe.html

Anyway to get down to brass tacks.

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
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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.
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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.
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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.
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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!
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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.
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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 difference 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!)

MIK

Originally Posted by Boatmik

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

But breaking bits that weren't strong enough is pretty cool too.

They'll always remember the prangs long after the memories of the winner have faded!

If it breaks it wasnt' strong enough, if it doesn't, it was just tooo heavy.

P (who mostly didn't break stuff and is therefore long forgotten)

You only become a guru if you go light and get away with it.

So Midgy ...

How light was the Kevlar on your 32ft Cat Piglet - the one that won all those events up and down the Queensland coast?

How many people were needed to pick up one of the completed hulls?

How much would a glass hull have weighed.

Suck Rocks Midge - the secrets out.

None of this "I don't know anything about sailing but I know what I like" crap!

MIK

Originally Posted by Boatmik

So Midgy ...
How light was the Kevlar on your 32ft Cat Piglet - the one that won all those events up and down the Queensland coast?

Lighter than Paulownia, but no where near breaking point structurally.

How many people were needed to pick up one of the completed hulls?

Two, because it was a bit beamy to balance, but we did have a picture of someone doing a MIK/balsa canoe somwhere (but using two hands and the hull was upside down, and someone was steadying it out of the picture). To be fair that was the hull only, no lid, and it was only 30' with max beam at about 4' I guess.

How much would a glass hull have weighed.

About as much as our whole boat, rigged, and fitted with cruising gear. (But that's like comparing a spotted gum trawler to a paulownia canoe!)

Suck Rocks Midge - the secrets out.

Now there's a case in point, it was a long time ago, but we never capsized, rarely pranged, hardly broke anything, and basically had a nice little cruising boat which just happened to go pretty hard occasionally.

We won a bit, but we're the only ones that remember that. Everyone can tell you about the boats that broke though!

It was built so heavily that it's new owners bunged a whole heap of cruising gear in it, added five feet to the stick to compensate, and lived happily ever after. If that's not overbuilt, I don't know what is!

You may have a point though, Geoff Cruse built heaps of exotic structures at a time when they were pretty scarce commodities.... and became a guru!

Nice rocks BTW!

P

So wheres the plans for PPP (Petes Pawlonia Piglet)?

Gday Guys

Thanks for your input. I will see how i'm sitting when my current project hits the water, and see if it will handle a thrashin. It will probably be a consideration in the future. I'm almost finished my No2 hull, then a bit of filling and fairing, and paint. Drove to Adelaide the other weekend and picked up my mast. So its going to be all on for the next month or so, would like to make it for the start of the season, but the number one priority is the Australian Titles at Mcrae, Vic, just after christmas. I think from what I've worked out the finished product will be just under weight as per the class rules (102kg) completely rigged. I weighed one hull at one stage just before the decks went on, 14kg. Will weigh No1 hull tomorrow with decks on.

By the by, is Paulownia grown in Aus or elsewhere?

Thanks All

Matt

Originally Posted by catbuilder

Gday Guys

By the by, is Paulownia grown in Aus or elsewhere?

Thanks All

Matt

My knowledge is by hearsay and relies on my memory (for God's sake don't mention ethyl or methyl again for a month or three!)

Originally from China.

Some grown in Australia - but can be difficult to grow because it is adapted to getting most of its water in Summer and the rest of the year needs to be terribly dry.

MIK

I saw an ad in AWR by Empress Timber in NSW - have it for $1600 m3, air dried.

I'll dig out the phone number if you can't yellow pages it.
A lot of the aussie plantations are starting to harvest large amounts, there's a Qld plantation investment mob that irrigate it so that might deal with the summer rain issue.