We know the sound of a traveler screaming across, and we all cringe, waiting for the "bang" that follows an accidental jibe. During a proper jibe we brake the traveler car's motion by controlling slack and easing it out, but mistakes happen. Some times we're short handed and a flying jibe in light air is not a terrible thing, not if the car was at least brought to center first. Why not use nylon--better yet, highly dynamic climbing rope--to absorb the energy?
The same 8mm line I use for tethers. Notice the sewn eyes covered in rigging tape for UV protection; a knot would do, but testing for an upcoming Practical Sailor article about stitched eyes and another about chafe protection started some time ago.
It has been suggested--by folks that haven't tried it--that nylon traveler line will stretch too much. Nonsense, it's just a matter of selecting the correct size for the boat. Yesterday I took my PDQ for a blast in 15 knots sustained, right at the edge of reefing and hence at maximum main sheet loading. Slamming waves and powering through gusts, the traveler car quietly working through a 1/2-inch range of motion. For test purposes I have crash jibed in 15 knots intentionally, just to see what would happen; 2-4 inches of give and harmless thud rather than sharp impact. Obviously the jibes that can cause damage and normal working pressures are of a different magnitude. Unlike stretch which allows a genoa to power up, stretch in the the traveler releases pressure in the correct way, without affecting sail shape.
Yes, I can see and feel the line stretch in a breeze. The traveler may be pushed an inch further with the same settings as compared to light air, but a traveler is meant to be adjusted frequently and I would never notice were the line not marked. Why is it marked? In order to assure jibe shock absorption on gusty days it is important to maintain a 3-4 inch cushion from the traveler end stop, and a whipped marking shows that position at a glance.
What line size? For hand-tensioned travelers, 8mm should be about right for any size boat. For larger boats 10-11mm climbing rope is available. Simply use the same size as appropriate for polyester.
Climbing rope is available in sizes ranging from 7-11mm. Use dynamic rope (identified ans UIAA single ropes, 1/2-ropes, or twin ropes), not static rope.