Mast materials and Specifications

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  • 28 Sep 2018 09:25
    Reply # 6696472 on 1306051

    Google search results has defeated me once again, but I think this spot is OK. I had a visit from a local marine surveyor today, and the topic turned to polystyrene and masts.. I recall polystyrene cropping up here somewhere too, but i bit my tongue at the time. I suggested he avoid polystyrene, because:-

    Expanded Polystyrene is hygroscopic.

    It is toxic (deadly) in a fire. This was learned in aircraft fires.

    It leaches plasticisers from other polymers when in contact, eg PVC (think electrical insulation, conduit etc, making it brittle, and potentially causing short-circuits and perhaps fire). This is known in the electrical industry.

    Also, expanded polystyrene is transparent to sound. Ie it offers no sound insulation/attenuation.This is known in the building industry.

    I am not a fan of hidden expanded polystyrene anywhere. 

    Walter



  • 11 Sep 2017 05:18
    Reply # 5073742 on 5073506
    Arne Kverneland wrote:

    I also made a study, in 2006, based entirely on the righting moment of the boat.

    in tihs study i recognise the base to your 'letter from mr spruce h mast', which i find really useful.

    i agree, that your tweaked PJR formula will give a good estimation for mast dimensions. …and – yes, there is a lot of guesswork in finding the 'real' righting moment of the boat and the loads generated by a personal style of sailing in specific weather conditions – and there are different needs of redundancy in strength of the whole system to feel comfortable.

    i'm not searching for the exact formula. i just like 'matematical aided guesswork' with the right inputs, and i don't like to see people thinking, they could reduce the mast diameter safely when reducing sail area or mast length…

    all in all, i like your technical stuff (and i work with it…)

    ueli

    Last modified: 11 Sep 2017 11:12 | Anonymous member
  • 11 Sep 2017 00:18
    Reply # 5073542 on 1306051

    David, thank you for the suggestion to compare my mast base with the standard H/McL specification for solid tapered mast – and thanks to Arne and Ueli for their follow-up comments.

    I have now done this, but first, a little background:

    My plan is to stretch the mast length out to the maximum I can achieve with the materials I have at hand. If this proves to be over the limit of what the boat can carry, I will reduce it accordingly. Any reduction will come first from the wooden extended heel plug.

    The current untapered Bermudan mast stands at 9.05m above where the partners will be, its weight is estimated to be over 56kg (not counting spreaders and rigging) with CoG estimated to be 4.5m above partner. (To lift this mast from horizontal would require a “couple” with moment of greater than 56 x 4.5 = 252 kg-m. I might be mistaken, but this seems to me a good basis for comparison.) 

    The proposed composite alloy/alloy tapered mast will be initially 9.7m above partner, weight of that part above partner calculated to be 52kg with CoG carefully calculated to be at 4.31 above partner. This gives a comparative “moment” of 224 kg-m. These numbers actually represent an improvement. However this does not take into account windage – and nor is it proven that the boat could carry the sail area which this new mast height could accommodate (an increase of about 30%). But part of the purpose is to explore these limits in preparation for a future sail-making project. I plan for the moment to use a sail which I currently have, which is 30.1 sq m (14% greater than current sail area), and can be fitted to this new mast with half a metre to spare.

    Now for the “heel plug extension.”

    (Thankyou David for that nice piece of nomenclature.)

    It may prove to be temporary, but the issue now is, how would this "heel plug extension" compare in strength with a standard H/McL solid tapered mast base?

    Using H/McL formula, the diameter of a solid mast should be

    90% of (LAP+sqrtSA)/.85 = 90% of (9.7+sqrt35)/.85 = 165mm

    (Here I can pay homage to Arne also by noting that my proposed maximum sail area of 35 sq m on an estimated 3.25 ton represents a SA/Displ ratio of about 16. The Arne “fake” sail area, using 14,  would be 30.1 sq m – exactly equal to the sail I will be using for the immediate future – and using that in the above formula the diameter would change to 161mm.)

    My new alloy composite mast, at partner, will be actually 152mm diameter, 5mm wall thickness. At the point where the mast suddenly transitions to solid timber heel plug extension (0.8m below partner) the diameter will be 140mm. This is the diameter of the heel plug which is inserted into the alloy tube. Here is, I suppose, the weakest point in the base assembly. (diagram)

    A Hasler/McLeod solid timber mast, if tapered to half its diameter at the heel, will, by simple geometry, be 128mm at this point 0.8m below partner. So, luckily for me, the geometry is favourable, and I think my 1.0 m heel plug extension, at 140mm diameter,  is well within the Hasler/McLeod recommendation.

    That is, if my arithmetic and reasoning is correct.

    Actually I don’t really trust either, which is why I have shown the “working” – maybe one of you good people would be kind enough to check.

    However I must say it all looks “intuitively” OK, and I feel better now having done the calculation.

    David’s comment about Weaverbird’s mast top also makes me feel better about using my flagpole for the top 4.5m of my mast. Thank you for that, David.


    Last modified: 12 Sep 2017 08:32 | Anonymous member
  • 10 Sep 2017 22:46
    Reply # 5073506 on 1306051
    Anonymous member (Administrator)

    Ueli,

    my modified PJR method may be un-scientific, but it still has produced some useful masts.

    Before using the modified PJR method, I also made a study, in 2006, based entirely on the righting moment of the boat. When recently trying to find the best aluminium tube for a hybrid aluminium-wood mast, I basically use that method.

    In any case, there will be some guesswork needed and fudge-factors  added, depending on the kind of use the boat will see; how hard it will be driven, etc. This isn't rocket science.

    Arne

    Last modified: 10 Sep 2017 22:52 | Anonymous member (Administrator)
  • 10 Sep 2017 22:01
    Reply # 5073483 on 1306051

    hi arne

    the windloads won't bring any bending moment to the mast, if there is no hull with its inertia and righting moment to work against heeling. that's why the needed breaking strength of the mast is only slightly affected by the length of the mast (as i understand it, just by its inertia…)

    ueli

  • 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.

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