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?

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.

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

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!)

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...

Arne

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