Kurt Jon Ulmer wrote:

An inner heavy-walled aluminium sleeve, fitting loosely, (because fitting perfectly is not a sure thing, and getting it in there isn't either) with the space around it carefully filled with a uniform rigid-setting pourable substance (epoxy glue mix an obvious one) would, I think, prevent the holes near deck level causing any mischief.

Why be limited to aluminium? Presumably we could reinforce the inside of the mast with other materials and even other shapes.

Here's the crudest idea: what if you jammed a wooden plank in there? That would reinforce the mast quite well, but in one direction only.  What about a cross-shape? 

How about an octagonal birdsmouth mast type insert?  If you made it the right size you wouldn't even need to glue it -- just mallet it in.

These are not ideas I've thoroughly thought through though. Just kicking off some lateral thinking.

Kurt Jon Ulmer wrote:

Although reinforcing the outside of the Coromandel mast-stub might be awkward, the inside could probably be sleeved to bombproofness.

I like this thinking.  I'll investigate.

And the outer sliding sleeve, outer-sleeved?

I think the outer sliding sleeve is OK, but if it were too thin it would probably be simpler just to replace it, provided a tube could be made with just the right inner diameter.

Or, perhaps best, a new mast with no interruptions, that simply speaks confidence...

The best option, I think, for long-distance.

David Tyler wrote:

…there's a professional method in which a brightly coloured compound is rubbed well into the surface, then wiped off completely clean. Cracks may show up at this stage as a coloured line. If not, as a further stage, a thin coating of a chalky white substance is applied. The colourant then leaches out of the crack and stains the white coating. You might try improvising this method using some artist's oil paint and some Tippex correction fluid.

It turns out my dad is familiar with this method from aircraft engineering. Unfortunately the cracks are likely to appear underneath the riveted collar, making the likelihood of sudden failure more likely. Not a good cruising design.

What about simply beefing up the section around those holes with a number of (20cm long?) strips of aluminium, like the staves on a wooden barrel? With the metal surfaces perfectly clean and lightly sanded, they should hold epoxy glue very well.

Or have I missed something? Am I about to produce a ‘stiff knee’?

Arne

In my former life as a designer at Kemp Masts Ltd, I did a lot of fatigue testing on components riveted to aluminium masts. Where there was a load in the vertical direction,  cracks would eventually start to propagate horizontally outwards from holes in the mast wall. 

It would be worth doing some DIY crack detection around holes near deck level. First, examine closely with a magnifying glass and a bright light. If this reveals no cracks, there's a professional method in which a brightly coloured compound is rubbed well into the surface, then wiped off completely clean. Cracks may show up at this stage as a coloured line. If not, as a further stage, a thin coating of a chalky white substance is applied. The colourant then leaches out of the crack and stains the white coating. You might try improvising this method using some artist's oil paint and some Tippex correction fluid.