C16 timber is the normal standard for structural timber in UK.  Better is C24.  Remember that this is graded for that piece of timber at that size. So if you cut it down you may end up with problem knots and a much weaker piece.  But you could end up limiting the knots and improving the strength.  

Many thanks for this. I'm using C16 as a kind of "worst case" example. I've been using this table to get figures. I can get quite reasonable C24 from Robbins in Bristol, I think.

Your point about knots is well taken. I've been looking at the pieces of graded C16 that I bought from B&Q this morning, as it happens, and noticed how consistent the knots are.  I can see what you mean about them not being "C16" if I rip them in half longways, for example.

I read some of the standard about how the machine grading works. It doesn't say so, but I bet the better bits are separated and graded higher, leading to a very consistent knot density in each grade.

[Edit: I've now looked at grading machines and this is exactly what they do.]

A wooden mast sees higher dynamic loads.

One thing we must remember is that a wooden mast rated at, say 1000kpm will break before an aluminium mast of the same strength.

The reason is that during violent pitching, or when hitting a rock, the wooden mast will see much higher peak load than the aluminium mast, and that for ywo reasons:

• ·         A wooden mast, in particular a solid one, will put much higher loads on itself due to the movements, simply because of its higher mass.
• ·         A wooden mast, being both thicker and with less elasticity than aluminium, will not be that able to relieve the loads by flexing, at least not as much as an aluminium mast.

Only accurate recording sensors in the mast step or partners could tell how hard these loads are, so without these I find it prudent to use at least 50 or better 100% stronger wooden masts.

Arne

Anonymous wrote:

Richard,

You are right that wooden masts tend to come out stronger than aluminium masts. However, I think this is necessary, since wood varies more in quality than aluminium.

Comparing the strength of two wooden masts are easy, if both are either solid or with the same wall thickness (relative to diameter).

One of the reasons I got interested in this process is that I noticed that the timber I bought to make a "crutch" for Tammy Norie was marked "CLS C16", which is the "Canadian Lumber Standard". I went off to find out what this meant, and found out that every piece of such wood is machine graded, and that the standard guarantees minimum properties for safe building. It's those properties that I've used for calculation.

I think that ought to mean that we could possibly shave a bit of diameter off.  In my article I calculate that 152mm is sufficient to match the strength of my aluminium mast, for example, using the standard engineering formulae (based on Euler-Bernouilli beam theory).

There ought to be quite a safety margin too, if that wood is guaranteed for civilian buildings.

I'm wondering if I'm right about that.

The bending  strength in Nm varies with the cube of the diameter:

Sooo... (164mm/134mm)³ = 1.83

I reckon the  masts coming out of the modified PJR formula to be ready for offshore work.

Arne

I reckon you're right! With maths!

Anonymous wrote:

Hi, Richard. 

One of us must be making a mistake here.
You write on your site that my method will result in a wooden mast with D=134mm

Now I used the 'modified PJR-method' by first calculating a Fake SA which produces a DA/Disp = 14.

Sailboat Data gives the displacement as 907kg = 0.907 metric tons

The fake SA thus ends up at 12.9sqm

Ah, my brackets are in the wrong place.  I wrote:

m = 980

SAAK = (14 * m / 1025.0) ** (2.0/3.0)

but it should be (as in your chapter)

m = 908

SAAK = 14 * (m / 1025.0) ** (2.0/3.0)

giving 12.9m², just as you say.

With that dimension, plus the LAP= 6.75m, I end up with D=164mm  when I put the numbers into the PJR formula ( See Chapter 6 of TCPJR).

Agreed.

This 164mm mast should be 83% stronger than that of 134mm.

What are you comparing to get that ratio?

Incidentally, it appears that my aluminium mast is considerably weaker than your formula's wooden mast, even with cheap wood. Your comment about "chickens" comes to mind.

Many thanks for checking this over. I have corrected the text!

Any further thoughts are very welcome.

Hello folks. I'm back after a long period of disability (and may disappear again) so I hope you won't mind me reviving this old thread of mine.

I've written up a summary of this thread on my blog as “Scratches and holes in the mast”.  There's nothing really new there for anyone who's read this far, but it's part of a series where I'm explaining my thoughts about a possible mast replacement, including experiments with birdsmouth construction, and a whole bunch of me trying to understand the engineering of masts. I have more articles to come and more things to try out, including glass and carbon sheathed wood.

I fully realize that this isn't the easy or possibly even the best (whatever that is) way to get a fixed mast, but it's the most interesting to me and this is my hobby! I'd be very happy to hear from any of you experienced folks about it.

Arne, you are mentioned and quoted!