Mast repair advice

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  • 02 Oct 2020 10:05
    Reply # 9278451 on 5262807

    A link to Bertrand Fercot method of building a wooden mast.

    http://storage.ning.com/topology/rest/1.0/file/get/1799493931?profile=original

  • 28 Sep 2020 11:48
    Reply # 9269297 on 9269205
    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.]

    Last modified: 30 Sep 2020 08:00 | Anonymous member
  • 28 Sep 2020 10:30
    Reply # 9269205 on 5262807

    Richard,
    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.  

  • 27 Sep 2020 20:57
    Reply # 9268347 on 5262807
    Anonymous member (Administrator)

    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


    Last modified: 28 Sep 2020 09:45 | Anonymous member (Administrator)
  • 27 Sep 2020 20:20
    Reply # 9268316 on 5262807
    Anonymous member (Administrator)

    Richard.

    As for calculating the aluminium mast, I start with guesstimating the righting moment of the boat, as shown in Chapter 6b about hybrid masts.

    I want an aluminium mast to have about twice the yield strength, Mb yield, of the boat’s maximum righting moment, Mr.

    The challenge is to guess on the maximum righting arm, that is, the maximum horizontal distance between the centre of buoyancy (CB) and the centre of gravity (CG). As can be seen in that chapter 6B, this arm is a function of the beam and the sort of ballast the boat has. In Tammie Norie’s case I guess the ‘fudge factor’ to be 0.19, so the righting arm will thus be:

    Lr = 0.19 x beam = 0.19 x 2.18m =0.41m

    That results in an Mr = Disp x Lr = 907kg x 0.41m = 372kpm

    I cannot find the strength of the 6062-T6 alloy. However, I use 6082-T6 alloy with a yield strength of Sigma = 250MPa= 250 N/mm2 in my masts. I use that strength at your mast as well. In case the yield strength is only 230MPa, or whatever, you just reduce the resulting bending strength accordingly.

    The strength of a tube with outer diameter D = 100mm and inner diameter 94mm should then be

    Mb yield= Pi/32 x 250 x ( 1004 – 944) / 100  = 5381Nm = 549kpm

    With the mast strength below 1.5 times that of Tammie Norie’s estimated Mr, I would say that your mast might be on the slim side. The mast I had for my 750kg Frøken Sørensen was Ø100 x 4mm, and didn’t appear to bend.

    If, when you sail around with Tammie, can see that the mast bends from the cockpit, then it might be an idea to replace it, at least if you have some deep-water ambitions.

    Note: My very home-made “rules” are not very accurate. Still they appear to produce strong enough masts without making them over-heavy. That’s what I aim for...

    Arne


  • 27 Sep 2020 18:40
    Reply # 9268219 on 9268145
    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!

  • 27 Sep 2020 18:20
    Reply # 9268145 on 5262807
    Anonymous member (Administrator)

    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).
    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


  • 27 Sep 2020 17:57
    Reply # 9268113 on 9268052
    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.

    Last modified: 27 Sep 2020 18:01 | Anonymous member
  • 27 Sep 2020 17:15
    Reply # 9268052 on 5262807
    Anonymous member (Administrator)

    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

    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).

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


    Cheers,
    Arne

    PS: I see that you reckon Tammie Norie's displacement to be 980kg instead of 907kg (SailboatData.com).

    In that case the fake SA raises to 13.6sqm,
    and the mast's diameter will rise to D=166mm.



    Last modified: 27 Sep 2020 17:30 | Anonymous member (Administrator)
  • 27 Sep 2020 10:46
    Reply # 9267591 on 5262807

    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!

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