Yes, I know how to splice multiple types of line. Yes, I know washing can help--published an article on that. Yes, I know a knot is an alternative. Yes, I could get new line. Easy answers.
I have a unique rigging problem (the sheets go between shrouds when setting a specific sail close to the wind) that I believe a splice and soft shackles would solve. Knots--I've tried all of the standard suggestions for sheets--work but are not as smooth as I would like. I have an older halyard in perfect condition that will be good for the application, but it's way too stiff for me to want to try splicing. I have sewn splices before in similar cases, and invariably the line eventually failed... away from the splice.
Typically I sewed enough passes (part way into the rope, round stitching on both sides) that the thread count would carry the load, and then added some whipping at the opening and the taper. Sometimes I covered it with heat shrink, sometimes rigging tape, sometimes nothing. Takes about the same amount of time as a double braid splice in new line if sewn by hand, but generally less. Machines do it MUCH faster. Common practice for construction trades.
Rope Splicing | SherrillTree.com
On a boat UV is more important than for these other applications, but any sort of covering handles that, and chafe is only a factor in certain boat applications. Someone is going to say it can fail if sewn wrong; these are used for climbing and I've seen conventional double braid splices work apart too. I've sewn and failure tested climbing equipment; I understand the methods.
A seizing is also a possibility, but sewing is really just a variation on that that can be made smother (the link above did not show a taper, but it is easy to do.
Anyone have actual experience with actual applications on boats? A better method?