Those little black rope locks pictured above cost me 2 DNF's at the Nataionals in Gulfport. The problem I had was the line was being pinched between the black ropelock and the tiny bullet block. Twice it cut through my (new) trap line and I went for a swim, then had to go to the beach to replace the trap line.

My fix was to;

1. get rid of the rope lock in favor of the little balls with the hole in the middle and
2. get rid of the little bullet block below the handle, just go with the thimble.

The drawback is, with the little stopper ball and figure 8 knot, it takes longer to adjust the line length if you ever need to move the ball. But at least the line doesn't chafe or get pinched against the little bullet block edges like it did with the little black ones.

I usually sail solo so I don't adjust mine very often, the crew trap gets addjusted only when taking on a new crew. My skipper line stays pretty much the same, the knot is so tight now I doubt if I could get it open anyway without using needle nose plyers.

I put the black ones on my kid's Hobie 14 which only has the thimbles, no turning blocks and I have not seen any chafe there yet, but it doesn't get used nearly as much.