Jami,

Do you know what alloy (temper) is used in those aluminium tubes?

One factor I use to check when fitting a new mast on my boats, is its weight related to the weight of the boat. I try to keep the mast down on 3%, or better 2.5% of the boat.

Now I have guesstimated the maximum stability of your boat to be about 715kpm (+/- 10%?). I would aim for a mast with a yield strength (bending moment) of about twice that. I could be willing to go a little below this and see what happens. The lovely thing with a freestanding tube mast is that one can easily see if it is struggling during sailing. This is visible long before any damage is done. If I felt uncomfortable with it, I would insert a 3-metre length of tube up the lower end of the mast. The highest stress in such an un-tapered mast is no doubt at deck level.

Arne

I never like to mix materials that have different mechanical properties in this way, to carry the same kind of load at the same time. They might, or might not, reinforce each other. It's different in the case of, for instance, steel reinforced concrete, or glass reinforced plastic, where one material carries tension, the other carries compression. Here, you're asking both materials to carry tension on one side, compression on the other, as the mast flexes. But I don't have a definitive answer on this, either on theoretical grounds or from practical experience. It might work, I don't know, and the only way to find out is to try it. It would be necessary to use a timber with compressive strength equal to tensile strength, which means not the lighter softwoods. 

Because you're aiming to go with a high AR rig, I believe you'll need the strongest, and more importantly, the stiffest of those options. That's the 140 x 5. My 165 x 3 mast is a little stronger and stiffer and a lot lighter, because of the thinner wall and greater diameter, and that's what I'd prefer you to have, but if it's not available in Finland, then 140 x 5 is a reasonable choice.

David Tyler wrote:

Yes, depending on the spec of the GRP. But if this is possible, then  it is also possible to use that portion of the aluminium flagpole that has a diameter of 140 at its bottom end, and make a joining sleeve or spigot.That would be better, but rather wasteful.

¨...and too expensive, considering the waste. I can get my hands on cheap grp flagpoles. If I remember right, one of the members has done this to a 20" Havsfidra.

David Tyler wrote:

I've just done some figuring, and 140/4 is coming out OK, as the lower tube on its own, so long as it is T6 temper. Could you accept this, with a wooden topmast?

I would, gladly - but estimating from the first sketches I need a 9,5-10 m mast to keep the AR high enough, which in turn would mean 4 meters of wood (the al-tubes here are sold as 6000 mm). I'm not sure if this is achievable by myself.

How about a 4m grp flagpole plus this 140/4 tube?