Species of timber

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Species of timber

Post by topherdawson on Sun Dec 04, 2016 8:03 pm

1   St Ayles Skiff oars are being made all over the world and locally available timbers vary widely in their properties. It makes sense for oar builders to use the best local timbers which are likely to be cheaper than imported timber and it is also easier to keep an eye on quality.

2   Many websites have tables of timber properties but they are usually specific to only one continent. The [You must be registered and logged in to see this link.] is the best I've seen which covers the whole world.

3   Traditional UK timber racing oars use a very select grade of sitka spruce imported from North America plus a small strip of ash on the compression side. Sitka spruce is pretty expensive but it is strong and stiff for its weight.

4   At a micro level all wood is a collection of hollow tubes of cellulose, more or less parallel, bound together with a natural glue called lignose and sometimes other fibres at right angles. While the tree is alive these tubes carry sap, in seasoned timber they are full of air.

5   In light timber the tubes are thin walled and large diameter. In dense (heavy) timber they are thicker and narrower. But the cellulose material is more or less the same stuff in all timber.

6   As a result the properties of timber like strength and stiffness are more or less proportion to its density. A timber like oak which is about twice as dense as cedar is also about twice as strong and twice as stiff.

7  Timber is a natural product so its properties will vary from tree to tree and even between the parts of the tree.  Strength, lightness and stiffness are best at low moisture content so oars need to be kept dry. Knots and curly grain need to be avoided so we are really talking about straight clear timber here.

8   I have posted a table of timber properties at

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9   Cells with particularly promising values are coloured green. The dense species Douglas Fir (oregon) and bamboo stand out, but the figures for bamboo vary so much it's hard to know what to believe. The light timbers Western Red Cedar and Sitka Spruce also stand out. The parameter strength/density is important, but the parameter root stiffness/density is regarded as very important for lightweight beams, and lightweight beams is where we are. For its cost, Western Red Cedar comes out looking good.

10   Canada and the USA are spoilt for choice and could consider tulipwood and aspen for the lightweight timber. Not sure what light woods can be obtained in Australia and New Zealand?

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Re: Species of timber

Post by Don Currie on Mon Dec 05, 2016 7:44 am

Re point 10, we're spoiled rotten in NZ for lightweight timbers - which is one of the reasons why I think that the timber species should remain outside of any rules. I've not tried even half the different species I'd like to experiment with! The "best" species of timber will vary from place to place, time to time. Strong recommendation - leave the timber species out of any rule. Also, looking beyond NZ it's worth noting that identifying the species of timber from a sawn stick of timber is a pretty inexact science - it will be a very sad day when we get distracted arguing whether a particular set of oars is made out of any particular species of timber. It would be most useful for each country to list what timbers might be suitable, so that new builders have an idea of where to start, but for a rule to lay down what species ought be used - no we ought not even consider going down that road.

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Re: Species of timber

Post by topherdawson on Mon Dec 05, 2016 9:43 am

I should perhaps have said that there is no intention of specifying timber species, as local builders will always have a better knowledge of what's locally available. Glad to hear you have lots of light timber! I used rimu and pohukatawa and a wee bit of kauri in a little pram dinghy I made in Aukland.

But if we make a design spreadsheet available, or perhaps some simple design calculations, builders will be able to check that using a particular wood will work and be strong and stiff enough.

I'm thinking I need to find a way of testing small samples for strength and stiffness.

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Re: Species of timber

Post by topherdawson on Mon Dec 05, 2016 9:50 am

What timbers do you have? We're looking for around 0.3 to 0.4 density, round about 30N/mm^2 (same as MPa) compressive strength and E round about 7Gpa. And obviously straight grain and clear.

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Re: Species of timber

Post by Don Currie on Wed Dec 07, 2016 6:04 am

Hi Topher,
most of our early oars were made of either Kahikatea (NZ white pine), density 450kg/cu M, MOE 10.7, or the ubiquitous Pinus Radiata, which is the most extensively grown timber in NZ. Density 500kg/cu M, MOE 9.0. These produced reasonable oars, and underline why I'm so keen on encouraging people to build hollow oars - it just opens up so many more choices for making cost effective oars so long as you take a deep breath and keep the pieces thin. We use 6mm sticks top and bottom, and have had no problems. When you're half a world away from the action, and stumbling around in the dark, you don't want to be chopping into great sticks of expensive timber. Radiata is cheap as chips here, so a great material to try for those wild ideas. My favourite timber for oar shafts is Lawson's Cypress/Lawsoniana/Port Orford Cedar. It is around 480kg/cu M, but MOE is 12.1. It can be a bit tricky to get clear lengths, but for hollow oars it is grand stuff. For inboards I've used Eucalyptus Delegatensis, which is one of the Australian Gums, often sold as Tasmanian Oak (!) Density is up there at 585kg/cu M, but MOE is 12.4. What you would call a composite oar with Tassy Oak inboard and through the lock area, then Lawson's outboard is a strong and most excellent oar. Tanekaha is magnificent timber - heavy at 610kg/cu M, but MOE is 11.4, and it's a timber that bends and bends, with very benign failure characteristics. If I had to make a solid oar I'd think about a Paulownia centre, 330kg/cu M, MOE only 4, but who cares if it's only a sandwich material. We can get Douglas Fir too, but it tends to be heavier than Lawson's whilst being weaker and more expensive - not a good option for my penny pinching nature. Western Red Cedar in NZ is around 370kg/cu M, MOE 4.7. I'd only use it if I had to make a solid oar. Pacific Coast Hemlock would be worth a look for you northern hemisphere folks - it's used by some aircraft home builders - density 465kg/cu M, MOE 12.3. I've made a few masts with it, but it's hard to get in NZ now. No Rimu, Kauri, or Pohutukawa - even if they were any good I'd have to sell the house to pay for them!

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Re: Species of timber

Post by topherdawson on Wed Dec 07, 2016 10:00 am

Fascinating! I have also used Kahikatea and Radiata and Tassie oak but not the others.

I agree we need an oar with a solid dense inboard and a light outboard. I've just discovered a way to make 4mm ply spoons which are stiff and light, but that's for another post.

6mm strips for a hollow oar, that's daring.

We could make a YouTube video on how to make our oar, when we eventually design it, plus plans.

I will put a post up on hollow oars.

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Re: Species of timber

Post by John Mitchell on Wed Dec 07, 2016 2:00 pm

The only clear, straight grained timber I can source on the Isle of Lewis is Douglas Fir unless I am prepared to pay a fortune shipping it over from the mainland.

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Re: Species of timber

Post by topherdawson on Wed Dec 07, 2016 2:52 pm

it is also possible to pick through CLS timber at a builders merchants for clear stuff, as we have done. But for wood like Western Red Cedar you would probably need to go to Inverness as we have to, and put it on a roofrack. The amount needed is not large.

We are aiming to make a design which can be adapted to the available woods anywhere in the world, and I think we will need to make the spreadsheet available so people can check what dimensions will work for the timber they have. By varying one or two of the dimensions it should be possible to get the lightest possible oar despite the properties of the timber varying.

What I think is very clear is that the timber of the outboard shaft can't afford to have any knots at all, so whatever is used needs to be totally clear.

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Re: Species of timber

Post by Don Currie on Mon Dec 19, 2016 8:13 am

Topher made the following comment in an earlier post: "I'm thinking I need to find a way of testing small samples for strength and stiffness."  Somewhere in the dark recesses of my peanut brain I seem to recall that the homebuilt aircraft movement  (mostly wooden aircraft back in the day) used to rely on a simple bend test of a standard sized strip of timber to assess its suitability.  It might be worth taking a (say) 15 by 25mm  strip of timber off each stick that is used for building prototype oars.  If the strip is about 750mm long it should be simple to do a basic bend test (from memory the home builders used to cantilever the sample off the end of the bench by a set distance, then bend it a set amount and measure the force required for the standard deflection)  So long as the dimensions of the samples are tightly complied with, (and the sample is not tested on edge!) you might be able to come up with a useful and cheap comparative test that could be done before clubs spend lots of money on timber for oars.

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Re: Species of timber

Post by topherdawson on Mon Dec 19, 2016 11:07 am

For strength tests:

What I'm concerned about is a way to apply force to a lightweight and easliy crushed sample so that it is not locally damaged at the grip point. Such damage weakens the timber in compression, and compression failure is going to be the way the sample fails. One way round this is to make a necked sample and grip it at the larger dimensioned ends, but making an accurate sample this way is fiddly. It is more accurate to bend a sample at four points so as to have a section in the middle with all the same bending moment, and then the failure might be away from the grips.

On the other hand the stiffness would be better measured as a simple cantilever test and I think I've already convinced myself that stiffness is critical not strength. So OK Don, I'll test a sample of the WRCedar and see what I get. I'll see what weight is needed to get a reasonable bend without breaking it.

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Re: Species of timber

Post by Don Currie on Mon Dec 19, 2016 7:18 pm

Yes, I should have said, as I recall it the idea was to stay well away from the limits of the test sample - they were testing at deflections that purely reflected MoE, and where the loads applied ought not result in failures due to localized crushing. Simple stuff, but as I recall, it gave a surprisingly good indication of how the timber would behave in use.

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Re: Species of timber

Post by Finlay Robertson on Wed Dec 21, 2016 1:27 pm

Just for the record, Topher, I've not forgotten the Excel document that I promised I'd have a look at for you. I'll also upload my own Excel tool, though I may need to update it first. I'm in Germany at the moment so don't have access to all my own files, but hoping to have time when I'm home between Christmas and New Year. In the meantime, I've uploaded a document in the Dropbox folder called 'Report on Options for 3rd NBRC 3rd Generation Oar Design' (I know you've already seen this, Don); it contains some graphs that my own tool produced which I hope illustrate its indended uses. It should be useful for identifying the suitability of a given wood for a design or, indeed, the suitability of a given design for a wood!

(Note that the oar was never actually built.)

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Re: Species of timber

Post by topherdawson on Mon Dec 26, 2016 9:27 pm

Don Currie wrote:Topher made the following comment in an earlier post: "I'm thinking I need to find a way of testing small samples for strength and stiffness."  Somewhere in the dark recesses of my peanut brain I seem to recall that the homebuilt aircraft movement  (mostly wooden aircraft back in the day) used to rely on a simple bend test of a standard sized strip of timber to assess its suitability.  It might be worth taking a (say) 15 by 25mm  strip of timber off each stick that is used for building prototype oars.  If the strip is about 750mm long it should be simple to do a basic bend test (from memory the home builders used to cantilever the sample off the end of the bench by a set distance, then bend it a set amount and measure the force required for the standard deflection)  So long as the dimensions of the samples are tightly complied with, (and the sample is not tested on edge!) you might be able to come up with a useful and cheap comparative test that could be done before clubs spend lots of money on timber for oars.

Although we could agree a standard size for samples it's easier just to go with whatever rectangular section stick you have, and do the sums. I've just made a calculator for this

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You clamp the strip in the vice with one end overhanging, hang enough weight on it to bend it appreciably, write down the length vice to weight, weight in kilos, deflection at the weight, width and depth of strip.
Feed the numbers into the calculator and hey presto E in units of GPa.

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