Re: What Size Dock Lines do I Need ?
Personally I don't use double braid - not enough stretch. I'd use 3-strand.
Given the fact your ropes parted, I think that is a clue that 1/2" was not sufficient, so 5/8 or 3/4. If in doubt, why not overkill? That way you can feel confident.
Also old and frayed ropes are more likely to break. Consider a chafe guard - mine has heavy-duty canvas at the chocks. Avoid tight corners - my port bowline if tied off at the starboard cleat. And if a rope begins to fray - especially a double braid - it is suspect, and can be used as a backup, but not a primary.
My 2 cents... ;-)