Another mast problem

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  • 26 Sep 2019 21:42
    Reply # 7903326 on 7851186

    I guess there will be a bit of give in the timber packers. That said as long as the packing is less rigid than the aluminium and well fitted I guess there should be no problem.  With the cap, bond to the top tube but debone from the lower tube, then if needs to flex it will not crack away from the top edge

  • 26 Sep 2019 01:01
    Reply # 7901220 on 7851186

    Yes Mark, I agree it was a bit risky. What you suggest, and also the way Stephen is suggesting are both safer ways to go. (It worked though - thanks to baking paper and a bit of luck!)

    I am interested now in the question of glue/outer bandage.

    I am sure the flexible glue is any amount strong enough for the join, I have had exactly the same experience as Stephen with a sikaflex polyurethane rubber glue and had to shred plywood to break a join. And the glue area in the tube join is great. So, from an engineering point of view David's preference for the Simsons type makes the most sense. And not just because of different expansion rates but also other differences in mechanical behaviour.

    But it does not sit well with the final step of applying a fibreglass-reinforced epoxy bandage over the join.

    If the "Simsons" join will be impervious to any water getting in, without the need for an outer bandage, then I suppose that should be the best way to go. If an outer fibreglass bandage is thought to be necessary, then I am not so sure.

    (I went the all-epoxy way because I was unable to resolve that question, and to overcome paralysis, just decided "she'll be right!"  It looks OK so far, but only time will tell, I suppose.)

    I wonder if it is worth to consider a flexible glue like Simsons for the join, together with an outer bandage made of some material that would have just a little more flexibility than glass reinforced epoxy, assuming now that it is only for the purpose of covering and weather-proofing the join.  Dynel perhaps? Is an outer covering actually necessary?

    (In New Zealand tapered aluminium tubes and tubes of greater length than 6m are almost impossible to come by, so almost anyone here who wants to make an unstayed mast will have to be looking at fabricating in some way or other, unless they can afford a custom-made glass or carbon fibre mast.)



    Last modified: 26 Sep 2019 06:15 | Anonymous member
  • 25 Sep 2019 18:59
    Reply # 7900077 on 7851186

    Graeme,

    thanks for the very informative post.

    Does sound like a slightly risky method.  I think I would make the filler pieces slightly oversized ththen sand down to fit, with a gap just sufficient for the glue

  • 25 Sep 2019 13:56
    Reply # 7899592 on 7851186

    As usual I like to borrow others ideas and I agree with the suggestions.  I have dropped a line to the 3M product group asking what would their preferred product be for attaching an epoxy saturated wooden plug to the inside of a 150mm diameter aluminium tube, taking into account the different expansion rates. It will be interesting to see if they go down the flexible epoxy route or the one part adhesive route.

    I have had a bit of experience with 5200 adhesive and find it to be very strong. When I had my Folkboat I attached a varnished ply panel to a wooden frame with 5200 and once I realised that the grain was running in the wrong direction I tried to remove it. The ply laminates gave up before the glue parted.

    I will keep everyone informed as to the outcome.

    Steve

  • 25 Sep 2019 11:53
    Reply # 7899467 on 7851186

    Point taken.  Maybe it would be good to make the wooden packing sleeve in the manner suggested by Stephen, saturate it with epoxy resin, then when it is cured assemble the parts and glue them together with Simsons or similar. That still leaves the outer bandage which covers the join - at least it is external and can be inspected - or would it be better not to have it? I am assuming the components would all be fitted pretty accurately. It just seems intuitive to me that the whole should be covered with a fibreglass bandage, making a mixture of mechanically different materials inevitable somewhere in the scheme - again I stand to be corrected.

    Last modified: 25 Sep 2019 11:56 | Anonymous member
  • 25 Sep 2019 09:43
    Reply # 7899433 on 7899155
    Graeme wrote:

    It has been suggested that epoxy glue is not ideal for aluminium as they have different mechanical characteristics, and that a more flexible glue such as Simsons is preferable. I don't disagree with this, but my gut feeling is that, in this case, provided there is sufficient overlap (bury), the glue used need do little more than fill the annular gap between the two tubes.

    My concern is that there really needs to be adhesion, as well as gap filling. If a rigid adhesive cracks away from the aluminium it will let water percolate down through the join. There were signs that this was the case when I used fairly rigid casting polyurethane to extend Tystie's mast. This may or may not matter. If the water is in small quantities and doesn't cause problems, then it doesn't matter. In fact, in the case of a wooden annular spacer, it might actually help, by causing the wood to swell. But all things considered, good adhesion is to be preferred, and Simsons certainly gives that. If epoxy is preferred, I would go for one of the flexible epoxies that are now available. they do tend to be more expensive, though, compared to a tube of Simsons or Sikaflex.
  • 25 Sep 2019 08:02
    Reply # 7899395 on 7851186

    Brilliant. Its worth swapping ideas isn't it? That's a better one. Make a wooden cylinder to the correct ID by forming it over the small tube, then put it in a lathe to make the OD - and a taper as well. I've got a big old wood lathe with a 3' bed and I could have done that too, but I never thought of it. I guess the wooden tube would just need a temporary plug in each end, then it could be turned between centres. I'll remember that for next time. Thanks, lets know how it works out - an article for the JRA mag.?

    Last modified: 25 Sep 2019 08:06 | Anonymous member
  • 25 Sep 2019 07:13
    Reply # 7899354 on 7851186

    Wow, thanks for all the information and suggestions.

    Arne, the 4 1/2" tube has a wall thickness of 1/4". There is not a lot of choice when it comes to the wall thickness of the tubes. The only other option was 10g (3.25mm) which I calculated was too thin.

    Graeme, thanks for the detailed suggestion/article. It pretty much detailed what I was thinking about although I had not thought about casting a sleeve; very ingenious. I am more used to working with metal and was thinking along the lines of a machinist. My thoughts were to use the upper tube as a former to build up an oversize cylinder using the same methods as you did. I was then going to cheat as I have a large metal turning lathe in the garage. It would be easy to make up a couple of wooden formers to go inside the wooden "tube" that could beheld apart with screwed rod. I would turn down the outside diameter to a snug fit in the lower tube and if I made the "tube" long enough I could turn a taper to smooth out the transition. It would be covered with a bandage on completion. The inside diameter would be good enough and allow a bit of a gap for the epoxy. I do also have the luxury of plenty of outside space in the boatyard where I would be able to build supports for laying out the mast. This is not a commercial boatyard but a club yard that more resembles a boat graveyard!

    Again many thanks to everyone who has taken an interest in problems.

    Steve

  • 25 Sep 2019 01:43
    Reply # 7899155 on 7851186

    I have done it the way Arne suggests and where the difference between the two diameters is small, that, as Arne says, is easier than making a wood sleeve. But if the difference is great (say between 6" and 4.5" - I think wood sleeve is better and accuracy can be achieved without a lot of effort. I offer you a sort of "article" about it:

    Mast made from lapped aluminium tubes

    I have made a couple of “composite” aluminium tube masts, one was done in three pieces and one was done with two. The aluminium/aluminium overlap epoxy-glued join seems to be successful, though in neither case have they been rigorously tested. I can say only that I am satisfied.

    It has been suggested that epoxy glue is not ideal for aluminium as they have different mechanical characteristics, and that a more flexible glue such as Simsons is preferable. I don't disagree with this, but my gut feeling is that, in this case, provided there is sufficient overlap (bury), the glue used need do little more than fill the annular gap between the two tubes.  A necessary step is to add an external fairing of thickened epoxy to smooth the transition between the two diameters, then an external fibreglass bandage applied over the join - and this alone ought to be strong enough to resist the twisting forces that the rig imposes on the top of the mast, and the downward force (which wants to make the mast “telescope” shorter). Actually, I think it would be preferable if the join could be made so that the two tubes could be disassembled with minimum damage, in the event of a breakage, but I don't know how to do it without the two parts being irrevocably stuck together. This is why I don't think it matters much what type of glue is used, though I may be corrected on this.

    In the three-part tubular mast the differences between the diameters was small enough that an epoxy/glassfibre bandage was sufficient to fill the gap between the tubes at one joint, and on the other join, the gap was a little larger so I made a bandage of what seemed to be a sort of thin polyester carpet which I thoroughly soaked in epoxy before sliding the tubes together, and the composite was resilient enough to ensure that there was no floppiness while the epoxy was curing – ie that the tubes would remain co-axial. The above seems like a lot of words (it’s a bit hard to explain) but the process was simple, straight-forward and intuitive. I think there will be a photo of that mast in the next JRA magazine. I agree with Arne that is a much easier solution than making a tapered wooden topmast or trying to make an accurate, thin, wooden sleeve. It really was easy.

    With the other mast, the one made from just two tubes, the difference in diameters was great enough that I was worried about how to keep the two tubes accurately aligned, and there was no way the annular gap between the two tubes could be filled with just epoxy-and-fibre bandages. At least, that didn’t seem to me to be an easy or economical way to do it. Instead, I filled the gap with wooden staves.

    The staves were laid out on a length of duct tape, like this.


    then wrapped around the mast, glued and held with plastic cable ties until cured.

    Two of these wood/epoxy packings were made, the gaps filled with epoxy and roughly faired.

    They were deliberately undersize and the intention was to make an accurate fit with the addition of thickened epoxy.

    The mast was to be about 10 metres and I did not have the space to lay it out and ensure that the finished mast would be straight. To save space in the garage the two tubes had been stored, one inside the other - and from this happenstance I hit upon the idea of making the wood/epoxy packing for the join while one tube was still lying inside the other, ie of reversing the topmast section and sliding it all the way into the bottom tube – with just enough sticking out to allow me to apply the wooden packing. I then applied a layer of thickened epoxy resin, wrapped it all in two layers of baking paper

    and slid the top mast further into the bottom section so that the wood/epoxy/thickened-epoxy packing could cure in place. The nice part was, perfect alignment was easily obtained by putting a rope grommet around each end of the inner tube and covering the grommet with enough layers of thin packing tape to make a close fit. This ensured perfect alignment and the result was a casting which was a near perfect fit, and with accurate alignment (ie accurately coaxial.) The temporary rope grommets were discarded later, of course.

    There was only one worry – after the epoxy was cured, would the two parts come apart, to allow the topmast section to be reversed and re-assembled as the desired join?. I had previously ascertained that baking paper will stick to epoxy, but does not allow epoxy to soak through it, so I thought to use it as a parting membrane. I also used a liberal spray of an aerosol parting agent on the inside of the “mould” (the outer tube.) I then said many prayers, because if it would not come apart the result would be a very expensive failure. With a fair bit of beating it did come apart, and the result is a near perfect coaxial casting which, after the baking paper is sanded off, is a sliding fit.


    A few drops of epoxy (or pretty much any sort of glue) and I think it would be impossible to separate the two parts.

    After the topmast section is reversed and the join  assembled and glued, an external conical fairing of thickened epoxy can smooth the transition between the two diameters and then, as described by Arne, a fibreglass bandage applied over the joint. This last should on its own provide enough strength to prevent rotation or telescoping. It is my surmise (correct me if I am wrong) that this last necessary step ought to make the glue which is inside the joint redundant except as a gap filler.

    I am sure there are many better ways, but this is just what I did, with what I had to hand, and maybe will spark some more thinking and some improved ideas. I can say that it was quicker and easier than the description would imply, and accuracy was obtained without a lot of sanding and fairing. Personally, I would do it again that way if it were again necessary to fill a largish gap between the diameters of the two tubes. (But I would be very careful to ensure a minimum or two layers of baking paper and to ensure there are no deformities in the outer tube (effectively a non-tapered mould.) There's quite a lot of friction to overcome when separating the casting.

    [I just re-read Stephen's proposal (much more economical with words) and infer that he is pretty much suggesting to achieve the same thing. If you can lay the mast out fully, keep it straight and do it all in one go, so much the better I suppose. Laying out the wooden sleeve in the form of staves, thickness and spacing carefully calculated, and initially stuck to duct tape, was, I thought, a good and easy way of making (assembling and gluing) the sleeve.]

    Last modified: 25 Sep 2019 02:50 | Anonymous member
  • 24 Sep 2019 20:32
    Reply # 7898162 on 7851186
    Anonymous member (Administrator)

    Stephen,
    you may of course make that sleeve between the two mast tubes from wood, but that would call for quite some precision work. Another method is shown at the end of this chapter. It has been used and should be a good deal easier to make.

    Btw, what is the wall thickness of the 4 ½” top section?

    Arne

     

    PS: See also: https://junkrigassociation.org/arne/

    Last modified: 24 Sep 2019 20:34 | Anonymous member (Administrator)
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