Okay. One point though, in my first post I wrote :"To be precise I used a swiftcord hollow line as mantle and ran a 3 mm dyneema (spectra ?) line as core through it. Only partly. Than I stitched it and tapered it"

So I didn't remove any cover. I bought two different lines. One section of swiftcord line (this had no inner core); and some 3 mm high tensile line (Dyneema/spectra). I then ran the 3 mm line through the "tube-like" line that swiftcord (D12) is. I then stitched it in place on both ends but after letting the swiftcord grip itself along the inner 3 mm line. I than cut away individual strands of teh swift cord weave and stitched the remaining strands in place. This will give you a nice taper over about an inch. I think I left some 2.5 mtr of 3 mm line un-swiftcorded; thinking that I could always trim it. It turns out that this was about the right length for a 1:7 system on my boat. However, guys, each boat/sail combo can be different so you just have to measure what you need and give yourself some margin that you may trim of later.

Pretty much I rigged my mainsheet system at home and from memory moved the blocks apart to the minimum distance that they would have when fully sheeting my mainsail. I then measured the distance used up in the system when the taper was just past the ratchet wheel. This ratchet doesn't grip 3 mm line very well so wanted to have the 9 mm line there, especially when fully sheeted = max load. Best advice I can give you is to rig your boat sheet it to the max, mark you sheet line, take the line out and use this measured distance as the require length of line after the taper.

I also got another idea. Next time I will buy a full length Swiftcord line of 6 mm (mainsheet and traveller system combined) and run the 3 mm line ( as a core) only through the first 2.5 mtr of the swiftcord. Than sticht the end into place. Now I will have a continiously line that goes from a 3 mm section that runs through most of the blocks to 9 mm sections that you hold in your hand when sheeting the main to 6 mm section that runs through the traveller system which in turn will have a 3 mm line insert in its end to make up the spit tail that will help centre the traveller better. I found these diameters give the best handling of the named systems, with a comfortable grip and for the lowest weight. My mainsheet line is noticeably lighter then the standard 9 to 10 mm lines manufacturers use. I'm think about doing a similar thing for my spi halyard and spi sheet. Here the intend is to thicken the line locally where it must hold in a cleat or ratchet block while keeping the remainder of the line flexible and small/light.

I'm think about using 4 mm swiftcord for the spi halyard and only thicken the halyard to 5 or 6 mm at the points were you need to pull hard on it (the last bit) and where you cleat it.

Tricks like these you can perform with lines where you can only remove the outer mantle. Mostly because the cores are to slippery to hold.

I will try to make a few digital pictures and post them.

Wouter