The sewing will start soon, but one question seems to keep me on my toes: how wide should one make the vertical seams of the panels in order to ensure the strength?

20mm overlap? 40? 50? More?

I'm using 190g/sqm polyester (Swela Outguard).

The cloth width is 170cm and the panel width is about 400cm, so I'll have two seams - or possibly three, if I end up using broadseaming (which seems probable). The third seam would be placed at about the 35% spot, i.e. max round.

Jami,

The  lower battens of the 35sqm sail on my IF are 35 x 2mm, and they cope well. Your sail is so much smaller and the boat so much lighter, so I think 35 x 1.5mm should be OK.  For batten 2 from top, I guess I would prefer the 35 x 2mm version, and maybe even for the boom.

Arne

Alll right, folks - I have started crafting the sail by cutting panel patterns from thick paper.

I'll go for Arne's sail type with AR=2.15 and sail area of about 27 sqm. The cloth will be Swela Outguard (190 g/sqm) in bright yellow :)

The mast issue is still pending, but I would need advice on the batten sizes. They will be aluminium tubes, but would 35/2 or even 35/1.5mm be enough or not? 

Correct, the stepped poles from Nedal and others are flow-formed, like the conical poles. I guess it must be easier for them to run the machine along at the same diameter than to be making the diameter decrease as it runs along. When I mentioned separate tubes and a conical adaptor, I was thinking of the masts I've seen made this way, when full length tubes haven't been available.

If you want to see how it's done:

https://www.youtube.com/watch?v=ZAnzRRjKPpc

Jami Jokinen wrote:Is it possible to use a lighting pole that is not tapered, but "stepped" (I have no idea of the right term in englis) like this?

They seem to be available in 4 and 5 mm wall thicknesses in various lengths, although sold only in 6060.

I see no reason why you shouldn't simply fair from one step to the other, with epoxy filler, or similar.  If the poles fit all your other specifications, the steps wouldn't really be an issue.