Jamie we can not always achieve the ideal and we have to make do with the materials which are readily to hand, and with what skills we have. Sounds to me you have done that, and done a very good job, including your mast collar and masthead fitting which are a credit to you. And I will bet if you ever have a mast failure, it won’t be any of these items, or the join that fails.
Jami, it has indeed become an interesting thread, and since you have now glued your mast, I hope you will not mind if I weigh in here with a few more ideas, because I am sure there will be many more composite masts made in the future. I don’t want to pretend to be an expert, I just want to bounce a few more ideas with anyone who has made or will be making a composite mast by joining two unequal aluminium tubes.
This afternoon, after applying three layers of glass tape to the two places where the bandage packers will go – I began to worry a little. First, I could see it was going to use up a lot of tape and epoxy to fill the gap – and second because I was afraid that by the time I had built up enough thickness it might not be perfectly circular any more – I even dreaded that it might sag or become out of alignment. In order to be able to do lots of “trial and error” while building up that annular gap, to make sure it would fit nicely and also maintain alignment, I wondered how I could manage, as my topmast is 5.5m in length and weighs about 20kg – quite unwieldy, especially inside the shed. I would be interested to know how you did yours.
Anyway, so I hit on the idea of reversing the topmast and sliding it almost full length inside the main tube, supported at each end by a sliding collar which allows the top mast to slide in and out at will, in situ, maintaining concentricity, while I build up and trial-fit that annular gap.
The sliding collar would best be a grommet made from that cheap slippery kind of rope, if a rope with just the right diameter could be found. But I was too impatient so I just took the nearest bit of old synthetic rope which was lying around, about the right diameter, and just wound it around the topmast one turn and a bit of overlap, then taped over it with duct tape and kept applying duct tape until it was a nice tight sliding fit inside the main tube. I did that at each end and the result looks rough but works perfectly. It slides in and out and maintains concentric alignment to within a mm or two which is as close as I can measure. I can now focus on building up the gap, and some more possibilities have now opened up.
If anyone is interested here are a couple of photographs of my “trombone.” I can now work on the base of the topmast, protruding from its cradle (the main tube), and slide it in and out at will, filling, sanding etc until a sliding fit is achieved. I’ll do the inner one first, then remove the rope grommet there, slide the topmast further in, supported at this end now by the newly made epoxy collar - and then do the outer one.
This seems to be a workable procedure – and I am now wondering what would be the best way to fill that gap. (The clearance is just about 1cm all round.) It doesn’t seem necessary now, to fill all that gap with epoxy, and if it gets out of shape, I hate sanding that stuff. One way would be to cut up about 40 little strips of wood (say 8mm x 8mm) and lay them all parallel across a strip of duct tape, then wrap the bundle around the tube (layered up with glue) and lash the bundle in place there until the glue cures. The result at this stage will look like a “spline”. It can slide in for a test fit. The interstices can then be filled with epoxy and microballoons, sanded – and the final clearance and fit achieved with an outer coat of just a layer or two of glass tape and epoxy. It won’t matter if its not perfectly circular, it could be multi-hedral, as long as it can be bounded by a perfect tube, ie as long as “it fits where it touches” - and the two tubes remain concentric.
I’m doing an experiment tonight – gluing two aluminium surfaces together with a thick layer of thickened glue, but separated by four layers of “grease-proof” or “baking paper” which has been sprayed with that spray-on parting agent. If it falls apart tomorrow morning, I might consider doing the final fit “wet”, wrapped with four layers of sprayed baking paper, and use the outer tube as a mould. 4 layers of paper will give just a nice sliding clearance, and if all went according to plan, would allow a perfect “casting”, the topmast to be pressed out from the other end after it has cured. I am just not sure if I have the guts to try it. So much for Jami worrying his tubes might not stick. At this stage my top mast is in backwards, and I’m more worried they just might! As stated, I probably won’t have the guts to try it.
A real “quick and dirty” way to do the whole job in one go, and have it stick, might be to wind on a lining of the right diameter rope like this. The rope would be saturated with epoxy resin, and might as well be wound on for the full length of the join, lather it all up with lashings of epoxy and that white power which makes it thixotropic, slip the two parts together and job done. Alignment, fit and adhesion all taken care of in one go. But I am not sure if I want a hard rigid bond between the tubes.
(Winding with rubber round bar instead of rope, and gooping it with simsons or polyurethane from a tube would be an interesting variation!)
I think I prefer to go more slowly and just aim for a nice dry sliding fit. If I can do that (haven’t done it yet) then I am only going to use minimal glue or maybe even just a dab of cheap silicone rubber mainly just to help it slip together. I am banking the cone fairing will take care of telescoping, and relative rotation of the two tubes, and if I am wrong then it will come apart won’t it? and it can easily be remedied. I can appreciate that ocean-going sailors might perhaps rightly worry about possible slow deterioration or fatigue of the bond between the epoxy fairing cone and the two tubes.
Any actual mast failure is going to occur outside of the overlapping part, I am confident about that. And top mast failure is a possibility in my case, maybe just above the fairing cone which I think will be the weakest part, as I think my top tube may be just a little on the light side. If it ever happens I want something I can hopefully salvage and repair.
I may be way out of line here, but apart from Arne I think there has been too much worry given to bonding the two tubes together on the inside, and rivets etc. I am sure the two Davids are each correct in terms of what would be the ideal glue to use, but I can not see why the inner tube needs to be glued to the outer tube at all - any more than a mast has to be glued into a tabernacle – it’s the same thing isn’t it? Provided its tight, can’t slide down and can’t rotate. I suppose rivets could be put in later if it were ever proved necessary, but I really can’t imagine the need, and it would seem a shame to drill holes there unless really necessary.
Didn’t Annie just stick her topmast in with what glue she had at hand, probably correctly (see pps below) choosing the simsons in preference to the epoxy - and hoist Fantail’'s sail a couple of days later? We’re not building a Lotus here. I do admit that too much of a nonchalant attitude would not do if preparing for an ocean crossing.
I hope at least this will bounce a few more ideas, and would be more than happy to receive negative criticism – sooner rather than later please!
PS sorry to wade in over the top of Arne, who is an expert - I posted this before I saw Arne's previous post.
PPS after going back and reading the posts of Arne and Annie, I think we can now fairly conclude that both epoxy and simsons have proved to be effective enough in this application, and Jami's original question (the topic of this thread) has been answered.