Originally Posted by Stumble
The issue here is the "well weathered". Dock lines don't last for ever and nylon looses a lot of strength over time. For you boat I would go with 1/2. If it's too big the boat won't generate enough force to cause the line to stretch.
For comparison I use
3/8 - 26' powerboat (hauled out for storms)
5/8 - 38' Beneteau
7/8 - 58' powerboat
I agree - 1/2" is all that is needed or they line will not stretch, putting more load on cleats. I have a CS27 of about the same displacement and have always used 1/2" double braid nylon. I also have a small snubber in the bow and stern line. I have had no issues in the 5 years I have had the boat. It does blow here - the wind hit 40 knots in the harbour last night and was measured at 52 knots a few miles away. It is blowing as I type - speed 28 knots with gusts of 38 knots.