Good find!

More info - that's the UK version of Ebay.  The poles concerned are in Lancashire, and are quoted as:

Length 11m

Base diameter: 200mm

Top diameter: 75mm

Thickness: 6mm

Inspection cover - 3m up from the base

Price £295 + VAT

https://www.ebay.co.uk/itm/174517292303?hash=item28a20a8d0f:g:wBMAAOSw2KVfrt1M

Thanks for finding that!  I'm tempted, but probably won't bite...

I'm surprised nobody more knowledgable than I has come along...

I recall this has come up in discussion before.  The hole definitely weakens the mast.  I have seen people suggest welding the door shut; your idea of welding a plate looks better.  I think previously, I've seen a suggestion to not use that portion of the tube, but as it's reported being 3m up an 11m mast, that wouldn't leave so very much!

I suspect it partly comes down to how good a weld you can achieve, and whether the welding heat would affect the temper of the alloy; I'm afraid I'm no metallurgist, and all my own welding has been to steel, and of "agricultural" quality!

As my need would be for a longer mast than 11m, if I were to use this, I would be using it as a top on to a probably wood lower section.  I would consider lengthening the wooden plug to extend beyond the opening.  Heavy, yes, but the weight is relatively low.  I won't though.  I'm not 100% convinced about all the details in the listing (eg wall thickness - at top or at the base?).  It's a few hundred miles from me, so a long way to go to inspect it, and a long way to arrange transport for it.

But it's a good price...

It's impossible to say what can be done, without a physical inspection. Commonly, tubes like this, with an access hole, have some form of reinforcement such as a sleeve at that point. What can be said for sure is that any form of further work will weaken the tube. Welding certainly will. Rivets certainly will.

Supposing that the lengths quoted are accurate, this might serve as the foremast for a schooner, or as the mast for a single sail of modest AR when cut down to ~ 8 metres. I remember that the 24ft Mingming II has a mast of this diameter, so maybe it would be suitable for a similar boat meant for rugged cruising. Whether it would be serviceable at a greater length would depend on an inspection.

As it happens, I am due to be in Preston one morning this week, and it's just possible that I could get to Nook Lane to have a look. If so, I'll report back, as this tube ought to go to a good home on a JR boat of some kind. 

How about using a lamppost salvaged from a car accident with the no parking sign still attached - see file from 1964

Chris wrote:

Seem to recall critical loading for a regular mast is the compression induced via the stays. However for unstayed JR mast should be the righting moment with maximum bending stresses near the partners. For argument's sake if you positioned the access hole between the keel and partners then avoids region of highest shear force and bending moment. Having said all that, I'd fair off the hatch and over-wrap with carbon/epoxy prepreg. Shrink tape and a hot air gun. Cheap, quick and reliable solution?

As I've had occasion to say before, aluminium and carbon should never be placed in close proximity in a marine environment - they are at opposite ends of the galvanic table, and the aluminium is quickly eaten away. And materials of differing elasticity should not be asked to carry a load together - the stiffer one carries all the load until it breaks, then the less stiff one carries all the load.

But still, if the full length of the tube is needed, and if the access hole is well above the partners and has already been sufficiently reinforced with an internal sleeve, and all that is needed is a fairing layer, then glass/epoxy will do well enough.

Another way to lose the potential weakness of the access hole is to cut the tube at that point and mount the mast in a tabernacle, so that it is ~ 8m LAP, or apparently ~ 9.5m LOA, rather than only ~ 8m LOA.