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
Density	510kg/m3	425kg/m3	370kg/m3
Modulus of Rupture	86.2MPa	70MPa	51.7MPa
Modulus of Elasticity	12.2GPa	11.03GPa	7.66GPa
Crushing Strength	47.9MPa	38.2MPa	31.4MPa

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)
Douglas Fir	1196	2152	1.61
Sitka Spruce	953	1747	1.34
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
0	0.57	0.00	1196	953	786
0.2	0.37	877	1196	953	2626
0.4	0.17	1754	3036	2793	2626
0.6	0.03	2485	3036	2793	2626
0.8	0.23	2386	3036	2793	2626
1	0.43	2286	3036	2793	2626
1.2	0.63	2187	3036	2793	2626
1.4	0.83	2087	3036	2793	2626
1.6	1.03	1988	3036	2793	2626
1.8	1.23	1889	3036	2793	2626
2	1.43	1789	1840	1840	1840
2.2	1.63	1690	1840	1840	1840
2.4	1.83	1590	1840	1840	1840
2.6	2.03	1491	1840	1840	1840
2.8	2.23	1392	1840	1840	1840
3	2.43	1292	3036	1840	1840
3.2	2.63	1193	3036	2793	1840
3.4	2.83	1093	1196	2793	1840
3.6	3.03	994	1196	953	2626
3.8	3.23	895	1196	953	2626
4	3.43	795	1196	953	786
4.2	3.63	696	1196	953	786
4.4	3.83	596	1196	953	786
4.6	4.03	497	1196	953	786
4.8	4.23	398	1196	953	786
5	4.43	298	1196	953	786
5.2	4.63	199	1196	953	786
5.4	4.83	99	1196	953	786
5.6	5.03	0.00	1196	953	786

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.

In summary:

	Tube	Start (m)	End (m)	Length (m)	Weight (kg)
DF	Main	0.4	3.2	2.8	5.2
	Stren	0	1.8	1.8	2.9
	Top	3.0	5.6	2.6	3.2
	Total				11.3
SS	Main	0.4	3.4	3.0	5.6
	Stren	0	1.8	1.8	2.4
	Top	3.2	5.6	2.4	2.5
	Total				10.5
WRC	Main	0.2	3.8	3.6	6.7
	Stren	0	1.8	1.8	2.1
	Top	3.6	5.6	2.0	1.8
	Total				10.6

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).

Conclusions

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.