Hi Andrew, and all, It is great to be back on here, with a new project ahead of me, and to hear of yours and this extraordinary discussion between everyone. 3D printing parts for a junk rig!? Fantastic!

I think you have pretty much answered your first question yourself. I have similar experience of selling and buying boats to yours. And when it came to selling our junk rigged Cornish crabber pilot cutter, I had absolutely no interest apart from some delightfully complementary and supportive comments. But no buyers with her junk rig sadly. That was asking £34.5k, which is a lot of money, but these boats do keep their value very well. Or so I thought. Sadly, I put her gaff rig back in her and we sold her last July in a part exchange. David Tyler has it dead right: you just have to have the boat you want, involving a wonderful project and a transformation of a boat which may be quite ordinary, into something that you are going to enjoy more than any other sailing boat before!

We exchanged our Annie, the pilot cutter, for a smaller sister which is 24 feet long and a mk1 Crabber from 1978. She will be getting a junk rig of course. There is quite a lot of work to do, to replace parts of her plywood deck etc, so the new rig may not be this season although I'm going to have a good go at it.

This new boat is 24 feet on deck, two tons, and has a centre board. I would like to make a bird's mouth mast out of Douglas fir or spruce, (I'm in the trade), but I think I need to be a bit more careful about the weight than I was with Annie's solid Douglas fir mast. She was six tons,  and as Arne said at the time, the heavy mast was a small % of her displacement, and dhe turned out a dream, with the motion at sea to match.

I need to do a little bit more research on the difference in weight between an aluminium mast and a timber mast for the same boat. I'm guessing my mast will need to be between 28 and 32 feet, depending on whether I step it on the keel or in a tabernacle.

Anyway, I will watch your project with interest.

Pol

I made a 3-part aluminium tube mast for a 5.2 m trailer boat, filled the annular gaps and faired between outside diamaters with glass cloth and epoxy. It seems to be fine after a few years. I see no need (and would prefer not) to drill holes and add metal fastenings. A polyurethane rubber glue might be a better idea than epoxy for bonding the tubes and taking care of rotational load.  The fairing should be made to take care of telescoping load. Having said that, a kitset of 3-d printed parts would be very neat and tidy but I still see the addition of metal fastenings to be undesirable and unnecessary - at least for the mast on a small boat.

Cruising Club wrote:

Wow, fascinating.  3D printing the collars and taper(s) sounds great!  I made mine from fibreglass, but it was a messy job and the taper took quite a lot of work to get fair.  I epoxied the top section into the bottom tube and then set a couple of pieces of 6082T6 10mm bar through both tubes at each of the collar positions to help stop the top tube 'twisting' inside the lower tube.

Peter, that looks good. Did you just have 2 lengths of tubing? And how much overlap did you have, one inside the other? I'm thinking approx 0.5m overlap, but the less I have, the lighter the mast will be.
Mine is only a fairly light 20' boat so I'm going to have 5", 4" and 3" tubes.

Andrew wrote:

The thinner tube would need some form of physical stop, I was thinking of perhaps riveting a ring inside the larger tube at 460mm depth?

The professional mast-maker's way of joining is with ¼” monel rivets, in a pattern spaced at 3 diameters apart. I propose to put, say, 30 rivets around the circumference of a 7” tube. At this spacing, it’s possible to be way over-strength with monel rivets. so the more commonly available aluminium rivets should suffice. We’ll need to do some research on this.

OK, a quick research and number-crunching session tells me that 4.8mm stainless steel pop rivets have a minimum strength of 3kN in shear.  Downrating that because our tubes are spaced apart by the sleeves and are not in true shear, I think we can rely on at least 1kN per rivet, or about 100kg.

A ring of 20 x 4.8mm stainless steel rivets, or two rings of 10 rivets, around a 5” tube would be spaced at around 20/40mm, and the mast would fail at around 2 tonnes in compression.

A ring of 28 x 4.8mm stainless steel rivets, or two rings of 14 rivets, around a 7” tube would be spaced at around 20/40mm, and the mast would fail at around 2.8 tonnes in compression.

That sounds OK to me. 

I suggest that we standardise on 1 inch steps in tube sizes, to keep proliferation of component designs to a minimum. The smaller sizes of tapered tubes have a top diameter of 76mm, so it seems to me that 5” > 4” > 3” is good, as is 7” > 6” > 5”. Only the larger sizes of tapered tubes have a 100m diameter top - I remember that this was the case on my 220mm dia mast.

I’ve got the arrangements of the deck partners and heel worked out for Jeff’s 7” mast, and so I’ll put in a couple of teasers here, before starting up a new topic dedicated to that size.