One widely used way to extend an aluminium tube is to use a wooden topmast. This way we get the strength of the aluminium where it counts and we can use wood where the ability to taper is more critical.
Let’s start with a 2 3/4 x 10swg aluminium tube:
|Tube||OD (mm)||t (mm)||Wt (kg/m)||Yield moment (Nm)|
|2 ¾ x 10swg||69.85||3.251||1.85||1,840|
We’ll do the same as the other aluminium masts and add a strengthener at the base & a topmast at the top. We’ll work out the figures for three types of timber (figures explained on a previous post):
|Douglas Fir||Sitka Spruce||Western Red Cedar|
|Modulus of Rupture||86.2MPa||70MPa||51.7MPa|
|Modulus of Elasticity||12.2GPa||11.03GPa||7.66GPa|
The aluminium tube has a nominal internal diameter of 63.35mm (±1mm) so we’ll use this as the outside diameter of our wooden section. We’ll assume that the wood is solid.
Using the calculations on a previous post we get this:
|Timber||Max moment using crushing strength (Nm)||Max moment using modulus of rupture (Nm)||Weight (kg/m)|
|Western Red Cedar||786||1290||1.17|
So now we need to work out how long the strengthener, main tube and top-mast need to be. Referring back to the table on the aluminium tube mast post we can work out how long the strengthener and topmast need to be to get the necessary strength up the length of the mast:
|Index (m)||From partners (m)||Moment (Nm)||Douglas Fir||Sitka Spruce||Cedar|
I need to generate these figures in a better way – probably graphical – but this will do for now as it is quick and easy. A graphical method would yield a more accurate fit to the required strength and thus probably a lighter mast. I’ll investigate that later, if necessary.
|Tube||Start (m)||End (m)||Length (m)||Weight (kg)|
It is worth noting that this table assumes that the topmast tapers from 70mm down to around 40mm diameter.
All three woods produce a mast of similar weight. The timber of choice depends on availability and price. Cedar might be a bit soft.
It would be possible to make a lighter mast by making it hollow. Typically the centre 25% of the mast is removed by using two halves and hollowing them out. However the hollowing is supposed to be tricky. A router would make it easy but I don’t have one (yet).
This is a promising way to produce a mast:
- Lots of masts in the Junk Rig Association have been made this way, so it works and is robust.
- It allows the topmast to taper reducing weight aloft and windage.
- The lengths of timber are all less than 2.4m (approx 8′) so are easy to obtain and handle. I won’t have any issues transporting the timber in my car or fitting the lengths into the workshop.
- The extra strength of the aluminium is used where it is needed – at the partners. I don’t need to worry about finding extra strength from somewhere.
- It is straightforward to make the wood to fit the tube – the tolerances of the aluminium tube are much less important.
- It should be possible to make the topmast removable, which would make transport and storage of the mast much easier.
Things I still need to worry about:
- Need to be sure that the tolerance of the aluminium tube will allow the tube to fit into the partners.
- Getting the strengthener to fit inside the main tube will be fiddly but not impossible.
- I’ll still need to paint the Aluminium tube and varnish the wooden topmast.