Mast materials and Specifications

  • 10 Sep 2017 21:48
    Reply # 5073457 on 5073379
    Anonymous member (Administrator)
    ueli lüthi wrote:

    edit: i just took a look at your changed formula:

    when you fix a ‚fake‘ SA/disp of 14 for the calculation, you could take the SA out of the formula and work directly with the displacement – and the length above partners shouldn’t be an essential part of the formula, as its effect is just the impact of the inertia of the rig in a swell.

    ueli

    Ueli,

    my not so scientific hunch is that the load on a freestanding mast comes from two sources; windloads and loads because of the boat jumping in waves. The last factor makes me think that a long mast sees some harder load than than a shorter mast fitted to a sister boat with the same displacement and SA.

    For this reason I find the modified PJR formula useful, including the LAP part, and et appears to produce useful mast scantlings.

    Arne


    Last modified: 10 Sep 2017 21:49 | Anonymous member (Administrator)
  • 10 Sep 2017 20:23
    Reply # 5073379 on 5073326
    Arne Kverneland wrote:

    Ueli, I have twisted the PJR-way of calculating wooden masts. In chapter 6 of my 'The cambered panel Junk Rig'. That formula seems to produce quite good mast sections, still stout, but not too stout.

    thank you – i will check it out.

    PS: Who has written the mast calculator? I am always sceptical to spreadsheets unless I have access to the formulas behind them.

    the calculator is from oscar fröberg.
    i compared the wooden mast part with other sources and found it good enough for my needs. but i'm not an ingeneer…


    edit: i just took a look at your changed formula:

    when you fix a ‚fake‘ SA/disp of 14 for the calculation, you could take the SA out of the formula and work directly with the displacement – and the length above partners shouldn’t be an essential part of the formula, as its effect is just the impact of the inertia of the rig in a swell.

    ueli

    Last modified: 10 Sep 2017 20:43 | Anonymous member
  • 10 Sep 2017 19:23
    Reply # 5073326 on 1306051
    Anonymous member (Administrator)

    Ueli, I have twisted the PJR-way of calculating wooden masts. In chapter 6 of my 'The cambered panel Junk Rig'. That formula seems to produce quite good mast sections, still stout, but not too stout.

    Arne

    PS: Who has written the mast calculator? I am always sceptical to spreadsheets unless I have access to the formulas behind them.

  • 10 Sep 2017 12:29
    Reply # 5072951 on 5072773
    David Tyler wrote:

    …Work out the sizes of a solid wooden mast to PJR scantlings for the same height and sail area…

    i don't think we should use PJR to calculate mast dimensions any more. (imho it's the only part of PJR where hasler and mcleod are really wrong.

    as they used smaller sails for a given boat weight than we do now, you will always be on the safe side, but the calculation doesn't represent the needed strength of the spar…

    once again i recommend this calculator for an estimation of the breaking strength of a mast profile and the righting moment of the hull. (the latter will be a rough guess, as a traditional full keeled boat will have a lower righting moment than a modern 'dinghi like' hull.)

    ueli


    Last modified: 10 Sep 2017 12:30 | Anonymous member
  • 10 Sep 2017 09:06
    Reply # 5072773 on 1306051

    Graeme, how about this as a check on your proposed wooden components. Work out the sizes of a solid wooden mast to PJR scantlings for the same height and sail area, then compare the diameter at points of interest with your proposals. If the diameters are not too dissimilar, all should be well.

    My mast is of the same thickness as your flagpole, and tapers from 165mm down to 76mm. I should think that the top 5m is very near to the size of your flagpole, so I wouldn't be too concerned about using a little more of it, and a little less wood at the heel.

  • 10 Sep 2017 04:08
    Reply # 5072634 on 5072631
    Graeme Kenyon wrote:

    The spec sheet states, by the way, that is is moisture cured - but you have proved that it will cure between epoxied wood and aluminium so that should not be an issue. I notice they recommend a join gap of 2mm. Thanks for that link.

    Do you know, I checked the spec sheet before posting, to ensure that it wasn't polyurethane, but I literally didn't see that it was moisture cured.  Apologies all round.  It just proves that we tend to see what we want to see.  Simsons was recommended to me by a marine engineer as being superior to Sikaflex and 5200 and I have been very satisfied with it wherever I've used it.
  • 10 Sep 2017 03:49
    Reply # 5072631 on 1306051

    Well, if the spec sheet is anything to go by, it  looks like great stuff. Just what the doctor ordered. The spec sheet states, by the way, that is is moisture cured - but you have proved that it will cure between epoxied wood and aluminium so that should not be an issue. I notice they recommend a join gap of 2mm. Thanks for that link.

    Last modified: 10 Sep 2017 03:50 | Anonymous member
  • 10 Sep 2017 03:15
    Reply # 5072598 on 5072591
    Graeme Kenyon wrote:

    Annie I am getting a bit confused now too - evidently there isn't just one Simpsons. I am just wondering if you could give the full product name of the Simpsons adhesive you are using.

    I'm not surprised you're getting confused, Graeme and it's made worse in that it's sold under the name Simson marine adhesive, but is occasionally advertised as Simpson.  However, if you follow the link, you'll see exactly what it is that I'm wittering on about. 
  • 10 Sep 2017 02:51
    Reply # 5072591 on 1306051

    Annie I am getting a bit confused now too - evidently there isn't just one Simpsons. I am just wondering if you could give the full product name of the Simpsons adhesive you are using.

  • 10 Sep 2017 02:12
    Reply # 5072550 on 5071543
    David Tyler wrote:If you make a long joint with impervious surfaces and Simsons, it will eventually cure right through, but the process might take weeks or months rather than days. The cure must start at the ends exposed to atmospheric moisture and work its way along. As Paul says, it's just what happens when you've  broken the seal on a tube of Simsons - the cure starts at the nozzle and works its way inwards. However, where the joint is short, as with a heel plug, it's possible to coat with epoxy, so long as load is not applied too soon.
    I think you, too, David are confusing Simsons with polyurethane.  It isn't and the instructions, moreover, state the wood ought to be sealed before the glue is applied.  The reason I'm going on about it is because Simsons is such a good product for a lot of jobs.  And it stays usable in the tube for several months.
    Last modified: 10 Sep 2017 02:14 | Anonymous member
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