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.

Anonymous wrote:

So I’m sure we can do something similar for your conversion, first choosing the tube size to be stepped into the boat, then the diameter of the next one, and that should be enough to get the required height. Then between us we can design and print all the components to make a mast and step it.

A modern version of the hybrid mast that has a wooden topmast going into a tube. Might this be something that other JRA members would be interested in?  

David, thank you for your help with this. And thank you for the original concept, which i think is superb.

I have drawn the junction between 4¹/₂" and 4" tubes, using a printed collar/fairing piece similar to your design.

I have assumed that the top of the collar needs a physical thickness, it can't taper to nothing. I think 2mm would be the thinnest I would want to go with PETG. The bottom of the collar is approx. 3mm. I've allowed for the tube to insert into the other by 460mm. I hope a 2mm step wouldn't snag anything.

I would plan to print on it's side, to avoid layer line weakness.

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?

This is all on the assumption of using 4m of 5" tube, 4m of 4¹/₂" tube and 3m of 4" tube. With 1.2m buried and 2x460mm joints, this would give me just under 9m above deck, which may even be too much.

As you have suggested, a combo of 5", 4", 3" may be strong enough. This would give more substantial joining collars and would be slightly cheaper. Either way, the whole mast would come in at less than £500 delivered, much less than the £800 for a Norsk Hydro tapered pole.

What do you think?