Stuart wrote:

Could this system be used with pre made standard size carbon fibre tubes? A cheapish carbon fibre mast :)

 Yes, if such tubes can be sourced at sizes, prices and specifications that make them suitable for assembling into unstayed masts, then 3D printed components can be used instead of having to hand-make joining sleeves, which tends to be a tedious and messy business. However, I’m not aware of any such tubes being available. There are CFRP and GRP tubes in sizes suitable for making battens and yards, though, I tend to prefer them to aluminium tubes, and 3D printed additional components are a convenient alternative to other means of manufacture.

In essence, a 3D printer can replace the lathe, mill, drill etc needed for machining components from solid stock, or the welding and bending kit etc needed for metal work, with only a very reasonable up-front financial investment and a less steep learning curve than for machining and welding.  

Anonymous wrote:

Could this system be used with pre made standard size carbon fibre tubes? A cheapish carbon fibre mast :)

There might exist some large diameter carbon fiber tubes, like those used for antenna masts, but most standard off-the-shelf carbon tubes do not have the majority of their fibers running unidirectionally parallel with the length of the tube, which is absolutely critical for an unstayed mast.

Standard tubes are typically engineered for stayed rigs, where the mast is primarily under heavy axial compression (pushing straight down) rather than pure bending (flexing). For an unstayed mast, the structural load is almost entirely bending moment, which requires massive tensile and compressive strength along the longitudinal axis.

Because standard commercial tubes are wound with a high degree of diagonal or circumferential fibers (to prevent crushing and buckling under compression), they are highly likely to fail in bending if used as an unstayed mast.

Anonymous wrote:

Thomas wrote:

I wrote:

The heel plug is now complete, quite chunky at 1070 gms, but should be plenty strong enough. The heel socket that it is to fit into is now printing.

Is anyone considering making a similar size of mast, with a lower tube size of the same 7inch or 180mm diameter but with a thinner wall? I have a heel socket already printed as a test piece, with the same square shape but with a lighter internal structure, that would fit this heel plug. I also have a heel socket with a hexagonal interior shape, from when I was thinking of going that way. I could print a heel plug to suit either one, if I know the internal diameter of the mast tube. 

I really like your approach with 3D-printing between the tubes and as cones to even out the difference in diameter, but I am very skeptical about using a PETG square heel plug if it is meant to take up the torsional forces from the mast.

Since your layer lines are perpendicular to the mast, the twisting forces will put the layer bonds under direct shear stress. 3D-printed parts are notoriously weak along the Z-axis, meaning the square plug could easily shear off right between the layers. A high number of walls won't save it from splitting along those lines under load, and over time, PETG will also suffer from creep under constant pressure.

The circular plug inside the lower part of the mast, on the other hand, seems like a very good idea to strengthen the point of load and prevent the aluminum tube from crushing.

If you want to use your 3D printer for this, a much stronger alternative would be to 3D-print a mold (matrix) instead of the final structural part. You could print the plug and the square heel as a hollow shell, and lay up solid carbon fiber and epoxy inside it. This would give you a plug with incredible strength.

To prevent galvanic corrosion between the carbon and the aluminum mast, the outer few layers could be of fiberglass and epoxy to act as an electrical insulator. This way, you use the 3D printer for its true strength—precision geometry—while leaving the heavy mechanical loads to real composites.

Thomas,

I note from your profile that you have some experience of sailing under bermudan rig, and here, you seem to be claiming some knowledge of 3D printing and mechanical design.

However, it would be wise also to gain some experience of sailing under JR with an unstayed mast, before expressing such definite opinions. 

There is very little torsional load on the heel. It tends to come from the halyard, when the sail is squared away on a run for a long time, and so it can be in either direction, with any ill effects often cancelling out over the long term. The wedges at deck level can soak up a lot of it, due to friction, so that the heel plug sees very little. Any shaping of the mast heel that is other than round, is useful in a mast that is frequently stepped and unstepped, to get it into the correct rotational orientation automatically, but many unstayed masts have had no such shaping. In fact, there is an effect that is of more importance than torsion; and that is a tendency for the mast to kind of ‘lever' itself upwards out of the step, over the long term.

To counter both of these effects, it has long proven to be effective to put a length of bar through the mast near the step and through a pair of brackets on either side of the mast that are securely bolted down to the structure of the boat. It’s no more than a minor irritation if a mast tends to creep around its axis, or upwards, but a through pin effectively prevents both.

Also, extruded aluminium tubes withstand compressive crushing loads very well - but there are no such loads in play here. There is a bursting load, but this is only an issue if there is a longitudinal flaw in the extrusion, and fortunately these are rare; and in any case an internal plug wouldn’t help resist this. If there is any doubt about the extrusion’s quality, an aluminium mast can be stepped directly into a socket -  and then you give yourself the additional problem of drainage, as sitting permanently in a puddle of brackish water is not something that aluminium enjoys. Yes, you could over-engineer the mast step using other materials; but the aim here is to get away from the process of laminating glass and carbon where it’s not strictly necessary.

So no, I’m not greatly concerned about the soundness of the design in the area of the heel.

You say that you are in the process of converting to JR. Perhaps you might share with us the materials and methods that you are employing, if you are indeed experienced in marine engineering?

Thank you for the practical insights regarding the JR behavior. You make an excellent point about the mast's tendency to lever itself upwards over time due to cyclic wave action, and that the rotational forces on the heel might be lower in practice due to friction at deck level and alternating sail angles. 

I am an amateur when it comes to junk rig, but my plan aligns very closely with your telescope aluminium mast. I have been looking into building a carbon fibre mast, but it will probably be too expensive for my budget (and for the value of the boat). I had planned on using fiberglass for the overlapping transitions between aluminium tubes. However, I realized after reading your posts that 3D-printing the internal bushings and external cones provides a level of geometric precision and elegance that is hard to match with manual lamination. 

I have owned three 3d-printers the last 10 years, and have some experience with that. I do not trust PETG when it comes to loads in z-axis. Whether it is torsion or sideways force, it is prone to "delamination". 

Regarding the mast step, my intention is exactly what you suggest: to physically bolt the mast down using a through-bolt. I would not rely on a 3D-printed block of PETG to absorb primary structural loads except direct pressure supporting the layers, not even if the forces are much lower than what I have thought. My mast heel will be deeply buried in layers of glassed in plywood, with drainage. Perhaps I will make a square plug as well, from your advice. If I can take up the forces elsewhere than the square foot, and It is only a guide, then I would still probably print it from something like PET-GF/CF (Not PETG-GF/CF), and print it laying down.

Anonymous wrote:

Thomas,

Do you have any experience of 3D printing ASA? I thought that this would be good for the the cones, but couldn’t get good layer/layer adhesion near to the top where there is very little thickness, so eventually gave up on it and used PETG GF - at least this should be fillable with polyester car-body paste if/when it gets beaten up by chafe from the battens. I used ASA for the masthead, where the best possible UV resistance is called for, but the design doesn’t require it to carry any load; this is taken through to the aluminium with the ASA only acting as fairing.

I am actually planning to use ASA-GF for the external cones myself, precisely for its superior UV resistance and weatherability compared to PETG.

I have never printed anything that tall and thin, but I completely recognize the layer adhesion issue you ran into near the thin top. With ASA, if the walls get very thin and the print speed is too high, the nozzle revisits the same spot before the previous layer has cooled enough to stabilize, causing the structure to overheat and the layer bond to collapse.

To overcome this, I will try to tweak the minimum layer time settings in the slicer to force the printer to slow down to a crawl near the top, while keeping the chamber well-heated. Printing two cones at once or adding a sacrificial 'cooling tower' on the build plate could also help, as it gives each layer on the cone time to cool down before the next one is laid.

Your practical approach of using PETG-GF and body filler is a solid alternative. It will last for many years, and you could also spray them with some UV-protective topcoat to make them last even longer. I'm going to give the ASA-GF a shot with carefully tuned cooling profiles to see if I can get that crisp geometric precision all the way to the edge—just printing the thin parts first until I can get it right, and then making the full part.