Here’s a photo of the heel plug being printed, about halfway through the process. The through hole is now starting to show, and the wall thickness can be seen.

The heel socket is now finished, and the square socket is a perfect fit with the square heel plug, which is a relief. You can see in the photos how I’ve introduced some recesses into the cylindrical shape, so that when it’s been bonded into the hull, it cannot possibly rotate or lift out, even if the bond fails. That’s all the heel components done - socket, plug, gasket, retainer and conduit bottom end, all seem good and fit for service to me. Now I wait with bated breath to check that they mate well with the 7in dia x 3/8in wall tube.

An update on the tube sizes:

The supplier has said that they can’t offer a full 5m length, only 4.1m  is available in the 7in x 3/8in tube we were looking for. That’s OK, 4.1m is as long as we need for strength - if there’s roughly 2m below partners and 2m above partners that’s fine, but it means that we need to find another metre of tube length at the mast head. So we're getting 1m of 4in x 1/8in to go at the top, which is actually an improvement, as it lowers the CG and reduces the mast weight. It does mean another joint, with internal sleeves and external tapered sleeves to be designed and printed; and a 4in head fitting to be designed and printed. The end result is that we’re doing what I'd hoped, and going further towards developing a library of parts designs that can be mixed and matched to make a range of sizes and strengths of mast, with diameters at the partners from 5in to 7in, wall thicknesses from 1/8in to 3/8in, mast head diameters from 3in to 5in, and 2,3 or 4 sleeved tubes of whatever lengths are needed (up to 5m each).

Graeme wrote:

Plenty of multi-part tube masts and tube/wood hybrids have been made already I imagine – I have made a couple and it is a good compromise I think. However I have only ever used adhesive, and fibreglass and epoxy filler for filling the annular gaps and fairing between diameters etc. That works OK. I don’t like to drill holes or introduce metal fastenings and I don’t see the need for that. Anyway, the 3d printed parts would make a very neat and tidy alternative to fibreglass and epoxy filler, so it is very interesting. 

I had also thought putting holes and bolts through the join would be a bad thing. However, the top of the bottom pole of any join is also the most over built part of that pole seeing as the strength is calculated at the partners (or the bottom join) while loading decreases going up. Likewise, the top pole while designed for strength at the join is over built at the bottom of it's bury. So I would suggest a bolt through the upper pole at it's bottom and a matching hole in the lower pole would not cause problems so far as strength is concerned.

I do understand the concern with metal fastenings, even though they are very common on most any sail boat. I think a tube (sleeve) made of nylon or some other machinable insulator could help. However, my 1969 mast has a stainless steel bolt through the mast to hold it down too.

The heads on both sides would need to be flush or nearly so anyway. It would be interesting to design a nut that would allow a wrench purchase while allowing battens and parrels to slide over freely. (Actually a design floated through my head while typing)  However, I could imagine temporarily replacing the nut with one that had a loop welded to it to give the crane some place to fasten to while lifting the mast in place to be later replaced with the one that doesn't stick out and impede the rig. This would be a good point of balance  if it was the top join in a 3 pole setup.

The boot, be it FG, wood or 3d print, probably also acts to extend the bury of the top pole much as a cord stiffener does for an electrical cord coming out of an appliance.

I am sure there are good reasons for shooting this idea down in any case. After all, I am older than my boat (41 hexadecimal) and have a cold to boot.

Len