Free-standing mast: Aluminium Tubes

Aluminium tubes are easily available in the UK and fairly cheap. Aluminium is strong, light and has very predictable properties.One issue is that aluminium is only available in lengths of 5m, so we can’t make the mast out of a single tube – we’ll need to extend it somehow.

Another issue is that cutting and welding aluminium destroys the properties – you are likely to end up with a very soft material. So we can’t cut and weld the tube to create a tapered tube – we’ll have to stick with the tubes as delivered.

First we’ll create a table of the strength required at different heights in the mast. We’ll take zero as the mast foot; the partners are at 0.57m and the top is at 5.6m. We’ll assume that the loading is all at the top of the mast. This isn’t actually the case with a junk rig – the load is distributed down the mast to a large extent – but all the load at the top represents the worst case. Thus the bending moment at any point is a proportion of the maximum bending moment at the partners.

Index (m)From partners (m)Moment (Nm)
00.570.00
0.20.37877
0.40.171754
0.60.032485
0.80.232386
10.432286
1.20.632187
1.40.832087
1.61.031988
1.81.231889
21.431789
2.21.631690
2.41.831590
2.62.031491
2.82.231392
32.431292
3.22.631193
3.42.831093
3.63.03994
3.83.23895
43.43795
4.23.63696
4.43.83596
4.64.03497
4.84.23398
54.43298
5.24.63199
5.44.8399
5.65.030.00

The bending moment may go to zero at the mast head and foot but other loads don’t so we do need some strength at these points. We’ll just ignore these loads for now.

I’ve created a page on this blog showing the properties of some aluminium tubes. We’ll pick suitable tubes from that list.

Let’s start with a 2 ¾ x 10swg tube. This is around the maximum size that will fit through the slot in the deck ok. 10swg is 3.252mm wall thickness, which is about as thin as we want to go to retain stability under general loads. 16swg (1.626mm) would be a bit fragile – certainly at the bottom of the mast where it might get bashed.

Our main tube has the following properties:

TubeOD (mm)t (mm)Wt (kg/m)Yield moment (Nm)
2 ¾ x 10swg69.853.2511.851,840

The yield moment of 1840Nm is a bit small, plus the mast will need something at the top to make it long enough. How about 2 ½ x 16swg? This has these properties:

TubeOD (mm)t (mm)Wt (kg/m)Yield moment (Nm)
2 ½ x 16swg63.51.626
0.86
810

Nice, light, and reasonably stiff. If we could put some of this tube inside the bottom of the 2 3/4 x 10swg tube then we’d have a yield moment of 2650Nm. We could use the same tube at the top to make a top-mast with a yield moment of 810Nm. People have done this and reported good results. The wall thickness is a bit thin but gut feel says we can get away with this at the top of the mast.

Snags

There is a snag though. The internal diameter of the 2 3/4 x 10swg tube is 63.348mm – about 0.15mm less than the outside diameter of the 2 1/2 x 16swg tube. How have people joined these tubes if they won’t fit?

The answer is tolerances – the sizes given are not exact. They vary depending on how worn the dies were that were used to make the tubes. One supplier quotes a tolerance of ±1.0mm, which means that the inside diameter of the 2 3/4 x 10swg tube could be anywhere between 62.35mm and 64.35mm. Similarly the outside diameter of the 2 1/2 x 16swg tube could be anywhere between 62.5mm and 64.5mm. We might be putting a 62.5mm tube into a 64.35mm hole, which would rattle somewhat. Or we might be trying to put a 64.35mm tube into a 62.5mm hole which won’t fit – even with lots of force.

So if we want to do this we need to get the supplier to measure the tubes and provide actual sizes before we order. We can then make a judgement on whether it will fit together.

To work out the tube lengths we can add our hypothetical mast to the height-vs-moment table above:

Index (m)From partners (m)Moment (Nm)Mast strength (Nm)
00.570.002650
0.20.378772650
0.40.1717542650
0.60.0324852650
0.80.2323862650
10.4322862650
1.20.6321872650
1.40.8320872650
1.61.0319882650
1.81.2318892650
21.4317891840
2.21.6316901840
2.41.8315901840
2.62.0314911840
2.82.2313921840
32.4312921840
3.22.6311931840
3.42.8310931840
3.63.039942650
3.83.238952650
43.43795810
4.23.63696810
4.43.83596810
4.64.03497810
4.84.23398810
54.43298810
5.24.63199810
5.44.8399810
5.65.030.00810

So we need the stiffener plus main tube from 0m to 1.8m. The main tube on its own is enough from 1.8m to 4m. After 4m the topmast is strong enough on its own. Note that the topmast does need to overlap with the main tube for at least 1/10th of its length; we’re using 0.4m here.

Given all this we can calculate the weight and cost. We know the lengths of tubes required and their weight per meter, so the total weight will be 10.8kg, while the critical top 1/3 of the mast will weigh around 1.6kg. Not too bad.

We’ll need two tubes – 1 off 5m 2 3/4 x 10swg and 1 off 5m 2 1/2 x 16swg. Prices vary between suppliers but we’re looking at around £110 + delivery. Pretty good, especially as we’ll be paying for delivery of aluminium tubes for the yard and battens anyway. We also need to factor in the cost of a decent riveter (£40) and some monel rivets to join the tubes together

If the supplier measures the tubes and they won’t fit then there are other possibilities. 70 x 3mm would work as an outer tube, although metric sizes are not as widely available as imperial. Similarly 60×2 would work as a strengthener and topmast. The lengths of the tubes would need to change to suit the strengths of the tubes chosen but that is easy to work out.

Conclusion

An aluminium mast looks good:

  • It would be strong enough to capsize the boat.
  • The weight would be ok – not a lot more than the existing stayed rig (9.6kg). The weight of the top section of the mast would be light enough.
  • The mast wouldn’t be particularly thin as there isn’t a way to taper the mast down to less than 2 1/2″.
  • The mast would fit through the existing slot in the deck without any issues.
  • The mast would be cheap – I can’t complain at around £110. At this price I might be able to afford to replace it with something else in a few years if necessary. Delivery costs will be shared with the yard and battens.
  • Assuming that the tubes are correctly sized the mast should be easy to make. Since it wouldn’t need weather protection I could make and store it outside if necessary. Painting the mast would be a good idea but not really necessary, so that step can be avoided at least until the weather would permit painting outside.

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