A couple of extra points:

Suppliers

Finding suppliers for tubes that will definitely fit is proving harder than I expected. In particular I’d like a 70mm diameter by 3mm wall thickness tube for the main tube, as this should fit a 2 1/2″ inner tube with everything at its nominal size. However 70mm is very close to 2 3/4″ so 70mm is hard to find.

I’m also struggling to find suppliers who will measure what they have in stock.

Adjusting the size

I found a discussion on using Car Exhaust Tailpipe Expanders to adjust the size of an aluminium tube. Neat idea but rather scary – I’m not sure what that would do to an aluminium extrusion with a T6 temper.

However:

• It might be possible to make the internal diameter 0.1mm larger, should that be necessary;
• I could always wrap abrasive paper around the tool and use it to make the internal diameter larger using an electric drill on low speed.

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.