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Choosing rope: material and size
Rope is not simply rope. The important question is not how much it holds but how much it stretches. That is what decides whether a line belongs where you put it or does damage there.
Polyester, the default
For sheets, halyards and most lines on deck. Low stretch, good resistance to sunlight, inexpensive. Where nothing argues against it, polyester is the right answer.
Nylon for mooring lines
Polyamide stretches under load and so absorbs the snatch that swell and gusts bring into a berth. The very stretch that would be a nuisance in a sheet protects cleat and deck here.
Dyneema where it has to be stiff
Very strong and practically stretch free, but expensive, slippery and sensitive to heat. Good for halyards and strops, poor for anything that gets knotted: many knots slip in it.
The size
Thicker is not automatically better. Beyond a certain diameter it is no longer breaking load that decides but what fits the clutch, the winch and the hand. A line you cannot grip becomes a problem in a manoeuvre.
What this has to do with insurance
A mooring line in a low stretch material passes every snatch straight into the deck. The damage then shows not in the line but in the cleat, its bolts and the laminate, and that repair costs a great deal more than the right mooring line. If you lie in swell, this is not the place to save.
