A tapered tube mast is well and fine if available. If not, the 2-section mast should work well enough, and make use of (some of) Jami's existing mast for top-mast. 

Below is a sketch which explains how I would have done it: The top section would first be given two 100mm tall and 10mm thick waistbelt from grp tape, set in epoxy. When this has hardened and the belts have been adjusted to fit closely into the lower section, a generous amount of glue (epoxy or whatever) is added to fit the topmast permanently. This method should ensure good enough alignment. When the glue has set, it is time to add the fairing, of thickened  (medium density) epoxy with one layer of glass on top of it. I don't think there is any need to use any bolt or screws in the joint.

Arne

David,

this is exactly what I was starting to fear. Ahh, the sail is coming along nicely - why can't the mast be a smooth project, too!?

I still have to visit a supplier of aluminium lamp posts (and the Nedal 145/2.8 flagpole) and see the quality and sizes of their products.

In this picture their non-tapered posts seem to have very nice and smooth transition zones. However, the alloy is T5. Could one live with this if the wall thickness and diameter would be enough - or does it mean a very heavy mast?

Yes, pourable polyurethane is a possibility, but the practicalities would take some thinking about. You would need to stand the tubes vertically, having made a setup that held the tubes concentrically and was guaranteed leak-proof at the lower end of the upper tube, and then be in a position, several metres off the ground, to mix and pour the compound. Not a trivial problem to solve, and so it would be a good idea to investigate having two rings turned from aluminium or hard plastic, to be screwed into place. Also non-trivial is making a smooth conical transition between the two diameters. I confess that I can't think of a satisfactory way of doing that, with only home workshop facilities.

So I would continue to look at the one piece tapered tubes, both aluminium and GRP.

Jami

A mast from two tubes

If you are now going for the AR=2.15 sail, then the total mast length should be 9.4m. That could be made from a 6m long 130/5mm lower section plus a 3.9m top section (3.4m + 0.5m bury). As basis for my calculations I have estimated a max righting moment of 681kpm. All bending strengths below are yield strengths, not ultimate breaking strength, which are much higher.

The lower section:
That 130 x 5mm tube should have a yield strength, Mb= 1505kpm, if the alloy is of 6082-T6 standard. Weight= 32kg.

The 150 x 3mm tube should have an Mb=1313kpm if 6082-T6 alloy is used. Weight about 23kg.

Although the thin-walled 1500mm tube looks tempting because of its light weight, the wall thickness makes me  a little worried that it can be dented and thus buckle in use. Handled with care, it should still be OK. I would then add a thick half-metre long grp wrapping at the partners.

If only 6060-T6 alloy is available, then I would go for the 130 x 5mm tube at Mb=903kpm. This is only 130% of the estimated ultimate righting moment, but I would still try it, if the boat were mine. I would go sailing with it, at if it looked ok, I would carry on. If it looked to bend alarmingly near deck level, I would beef it up with a 2-3m inner tube. Remember, the Brits have for decades been using very thin masts from Needlespar, and these masts seem to have held for at least  20 years before coming down. A mast section of 13 x 5mm is generous compared to these masts.

The upper section:
At the joint, the bending moment will have been reduced to 40% or less of the moment at the partners. If you use 3.9m (3.4 + 0.5) of your old 100 x 4mm mast for top mast (Mb=710kpm  with 6082-T6 alloy), the joint should not be a weak point. The top mast should be 12.7kg.

All in all the mast should end up at 45kg or 36kg. That is 2.8 or 2.3% of the boat’s displacement  -  very good numbers.

Cheers, Arne

Jami Jokinen wrote:

The mast issue seems to stay problematic, because the availability of aluminium tubes in different sizes is quite limited, and lighting poles even more so.

The options so far for a mast with total length of about 10 meters:

1) 140/5mm al-tube, 6m plus a grp flagpole on top of this. Pros: easy to get, cheap. Cons: Heavy

2) 145/2.8mm al-flagpole (2.8 meters parallel at the bottom part, total legth 12m -> 2m will be cut off at the top). Pros: Easy (not a hybrid) and light, Cons: Not strong enough. Can something be done?

3) Grp flagpole (actually a pole for airport wind indicators, 10m or 12m for added strength (2m cut off)) with 7mm wall thickness. Pros: Easy, Cons: Heavy and possible installation problems (tapered all the way to the lower end)

4) Something else?

Is option 3) really out of the question? The density of GRP is about 2/3 the density of aluminium, so a 7mm GRP tube and a 5mm aluminium tube  should weigh about the same, at the same diameter. What is the diameter of the GRP pole? More importantly, what is the construction? We would want to see most of the fibres longitudinal or at a small angle to the axis. The full length taper wouldn't matter: with a cylindrical hole at the partners, wooden wedges would work well.