Blue Moon - A can of worms?

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  • 09 Feb 2020 01:48
    Reply # 8735225 on 8719784


    I am not convinced by the bandage, especially if it was not wet abraded with epoxy (and yes thats a very messy process).

    However, The problem is at the bottom of the mast where weight is not a major issue. Why not just fashion a piece of timber to fit inside the mast and insert from the bottom to well above the height of all the holes. Maybe 500 mm above. 

    It would probably mean removing the other fitting to avoid the fastenings when sliding the timber in and it would be nice if the inside of the tube could be wet abraded (maybe a flap disc on several extensions) and a nice mix of epoxy and microbaloons to bond it and fill any small voids.


  • 08 Feb 2020 23:24
    Reply # 8735109 on 8719784

    I’m not sure Arne. I’m not sure if the wood/aluminium glue joint plays a huge role here – assuming the wooden outer tube is up to mast specs in its own right, all we are looking for is a seal, and perhaps a certain amount of sheer strength between the two surfaces. Timber and aluminium extensions to aluminium masts have both proved to be strong enough in practice and in some cases epoxy glue has been used, evidently successfully, in their surface-to-surface joints. Also, assuming the tube encapsulates the previous repair, together with that fitting or the remains of it, perhaps there is enough mechanical interlocking between the two surfaces that not a huge demand is being placed on the glue, except to act mainly as a seal. 

    A combination with your aluminium strips would certainly lock things together better, but at the expense of more hole-drilling and maybe is just a complication. If the timber tube were as strong as a timber mast in this area, maybe that would be enough.

    Don’t know, I’m a bit out of my league now. My gut feeling is that the timber-alone outer tube would be not difficult to apply in practice, and would potentially add a lot more strength (perhaps even more than the original). Better than the current repair and little to lose, I say. Beyond that, who knows?

    Last modified: 08 Feb 2020 23:44 | Anonymous member
  • 08 Feb 2020 22:02
    Reply # 8735008 on 8719784
    Anonymous member (Administrator)

    Interesting, Graeme!
    What about mating my riveted-on aluminium strips with your glued-on wooden staves?
    The result could be glued-on aluminium strips/staves.
    If only the glued joint can be trusted...


    Last modified: 08 Feb 2020 22:03 | Anonymous member (Administrator)
  • 08 Feb 2020 20:18
    Reply # 8734893 on 8719784

    Hi Graham, I sympathise and agree with your solution for the time being, so this is not to hector further on the subject - but just to add one more thought, just for the record, to what has been an interesting collection of ideas, some of them perhaps not practical.

    If a built-up external tube in glass reinforced plastic has some merit in theory but would be messy and impractical in practice (I bow to David T's knowledge in this area) then how about a build-up in the form of wooden staves - similar to the mast-building system used by Bertrand Fercot in mag#59 and reprinted in the recent Anniversary Issue.

    I have built up a section of aluminium tube using this method. The staves were calculated for width, laid out on a length of duct tape then the entire set of staves at once wrapped tightly around the tube which had been prepared, primed and served with epoxy glue.

    The bundle was strapped on firmly by those plastic ties (I forget what they are called) which can be tensioned up. Later the gaps were filled with epoxy filler and the thing then shaped and smoothed before being given a glass and epoxy bandage. 

    The staves would be considerably longer and thicker than shown here (maybe 600 or 700 mm in length x 40mm thick, say). The principle would be the same, though. The staves could be tapered at the ends before applying, to reduce the time spent shaping later. 

    Admittedly I did not build up to such a thickness (perhaps you would add 3" or more to the mast diameter at the point of the repair) and I did mine on the horizontal - but it was an easy job from start to finish and I think it could be done in situ without too much mess. It could also, with a little bit of chiseling, fitting and epoxy, be fitted over the existing repair and over the existing fitting, I would think, and bury the lot. It could taper down somewhat towards the partners - and above the weak spot (at least the same distance from the repair as down to the partners, but more would be better) it can taper down to nothing. 

    I think this could be done in situ, in a few easy days, with a minimum of mess. 

    What say you David T. ? Would that add enough extra strength to the repair to be worth doing?

    PS   here is River Rat's topmast (an alloy tube almost completely encapsulated with wood) see mag#80   That was done by James G. using a different method and was quite a lot more work, but it does demonstrate another novel hybrid of aluminium and wood.

    Last modified: 10 Feb 2020 23:07 | Anonymous member
  • 08 Feb 2020 10:24
    Reply # 8734063 on 8719784

    It has been interesting reading the various responses to my original post.  I think an effective repair would require removing the mast and a significant amount of epoxy and glass, assuming I can get the glued-in mast out in one piece.  I think it might be easier to fit a new one.  So far I have not sourced a suitable tapered alloy pole.  I need 160-170mm dia at the partners, 10-11m LOA and 4-5mm wall thickness.  The other option is to build a hybrid.  A local company appears to offer a 6m by 162mm by 4mm wall tube.  I don't have a workshop to build the timber topmast but could outsource it.  I would assemble, glass-sheath and rig the mast myself in the local boatyard. 

    In the meantime I am going to rig the new sail (when the rain permits - we have gone from drought and bushfire to floods) on the existing mast and do some inshore sailing.

  • 07 Feb 2020 13:16
    Reply # 8732299 on 8719784

    Lying in my sick bed at the moment ( winter flu) adding to my thought of a long aluminium sleeve around the area.

    If an slightly oversize piece of tube can be found, cut it in half longitudinally, weld flanges onto it for bolt holes and short flanges to support the bolt hole flanges, taper the ends to reduce a stress-riser, pack the clearance between mast and 2-piece sleeve with some sort of structural adhesive and bolt it together.

    A good bit of work and more engineering than I know.  The oversize FRP patch seems like it would be easier.

    Back to my sleepy sickbed for now.

    Dave D

  • 07 Feb 2020 08:28
    Reply # 8732092 on 8719784

    I, too, shudder at the thought of adding more holes and a riveted patch, without the use of the design tools such as Finite Element Analysis that an aircraft designer will have at his disposal. It may make the problem worse, it may make it better, there's no telling.

    Graham has rightly discounted an aluminium tube. I have never been able to find pairs of tubes that will sleeve together neatly, in mast sizes, and getting a tube over an existing mast is easier said than done.

    A short GRP patch is doing more harm than good, in that it is only putting the problem "out of sight, out of mind", it's still there.

    Laminating a long, strong FRP tube in situ has merit. Only vertical fibres will do any good, as cracks, when they begin, will propagate horizontally. Extruded aluminium is stronger in compression than in tension, and compression will not be tending to form cracks. This makes my thoughts turn towards unidirectional aramid (Kevlar, Twaron) cloth or tape, which would add a lot of strength in tension, not so much in compression. Carbon is out of the question, next to aluminium, and glass would need a lot of thickness to get up to the strength of the aluminium tube (though possibly S glass would be OK).The laminated tube would need to be twice the height of the gooseneck above the partners, with the top tapered in thickness. A long, difficult, messy job, I don't think I'd want to try it without taking the mast out and setting it horizontal and able to be rotated, but it would add strength.

    The holes are at gooseneck level, which is not as bad as if they were at partner level. As Graham says, it's lasted this long, so it can't be that bad. Some under-spec unstayed masts have broken at the gooseneck (eg Hunter Liberty), but if the mast has lasted this long, it's valid to think "well, it's not ideal, but it will last until it breaks, and then I'll replace it".

  • 07 Feb 2020 01:38
    Reply # 8731700 on 8719784

    Hi Arne, what you describe as 6 to 10 rows of holes along the length of the tube, I see as a matrix of holes, around the tube as well as along its length – whatever, I just shudder at the thought of drilling yet more holes. However its food for thought, and David D's idea of an outer sleeve to add as much strength as possible, and as far as possible to match the specs of another mast, seems a good one – though for at least two reasons it would be better if the outer tube did not simply truncate at its ends, but tapered back to the diameter of the mast itself.

    This leads to the thought of fabricating a tube, in situ, of some kind of reinforced plastic. I believe masts have been made out of fibreglass but I don’t know what the specs would be – anyway a heavy build up in diameter at this point wouldn’t hurt – the remaining fitting and the repair already commenced could be simply encapsulated in it. I don’t know what length of mast is accessible between the part where the existing holes are, and the deck partner, but there may be enough room for a reinforced plastic sleeve, tapered top and bottom, as strong as a fibreglass mast would be. It would also mean simply building on the repair which Graham has already done, as I presume the obvious reinforced plastic to use would be GRP. (I would also presume that carbon fibre is out of the question here, not only because of cost.)

    David T – is there a technique and a material which would be practical here? I know its not ideal, in that it means two materials of different elasticity – but if the outer sleeve were strong enough in its own right (as a metal sleeve would be) might it not still be the best compromise, if it could be done? What Graham has already done (and "she'll be right") is a reasonably sensible (practical) option in the circumstances, I think - but I wonder if going further with it and building up an even stronger tube might even be a bit better?

    (By the way, a 6" diameter T6 tube is available in NZ for less than the cost of the tube David W refers to - same as Annie has successfully used and, I believe will use again - though its only 6m in length. Adding a tapered extension is not such a big deal though.)

    (I’m trying to stay off the forum at present – but its hard to quit!)

    Last modified: 07 Feb 2020 02:55 | Anonymous member
  • 07 Feb 2020 00:51
    Reply # 8731634 on 8719784

    It would not be possible to slide a tube over the mast due to the other fittings on it, and the inability to lift the mast out of the boat.  I could put a patch on it the way Arne suggests, though, using vertical rivets. I may even have such a patch in the form of the discarded gaff jaws from the junkette mainsail.  Though there are two other riveted fittings in the top section of the mast, and the other gooseneck fitting just below the removed one.  I like the sound of David Webb's Chinese mast section.  Thanks, everybody. For now I will continue with my plan to rig the sail while thinking about the situation.

  • 06 Feb 2020 19:36
    Reply # 8730998 on 8719784
    Anonymous member (Administrator)

    Patch it.

    The real masters of riveting metal structures are the aeroplane manufacturers. They have developed riveting into high science.

    I surely am not there. My quasi-engineering instinct tells me that if riveting is to be used on a freestanding mast, it is better to align the rivets along the mast than across it. The row of holes across the mast of Blue Moon must in my view weaken the mast considerably. On the other hand, I would not have worried about a row of holes from, say, a sail track up the whole length of the mast.

    If I were given such a mast, I would patch it rather than discarding it. To save fancy brainwork about the various materials’ strength and elasticity, I would make the patches of aluminium (same thickness or thicker than in the mast). The ideal would be finding a tube which fits exactly over the mast’s tube. In the real world, that will not happen. I would then pick a tube which fits as well as possible, and then rip it into 6 to 10 strips (straps?). These I would rivet over the weakened area of the mast, using 8-10 rivets (lined up along the tube) on each strip (the strips could be 60cm, for instance).

    I would of course combine the riveting with epoxy-gluing, and take care to make use of Graeme’s method of sanding an priming the area right afterwards with epoxy resin.
    These strips should add both stretch and compression strength. I am less concerned with the third stress  -  the shear loads. I have never seen a bent or broken metal tube splitting lengthwise.
    I could be wrong there...


    PS: The photo below shows a quick-and-dirty patch on a (faulty cut) batten. Works fine.

    Last modified: 06 Feb 2020 23:55 | Anonymous member (Administrator)
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