To set the scene-

I've just bought (again, but that's another story) Tropic Bird, a
particularly nice H28 which I have moored at a local marina. I've had
her there for 2 or 3 weeks now, using the mooring lines that came with
her- a couple of 4-1/2" and a couple of 3". I've been going to get a
new set of lines as the old ones are a bit tatty. I thought that 4-1/2"
may have been a bit oversize so I was going to use 3" all round- 2 bow
lines, a stern line and a spring.

Well they're predicting a bit of a blow overnight- 40 knots or so, so I
thought I'd go and check that everything was OK. Everything was, of
course, but when I looked at the 3" spring, which would take the main
force, in the light of a 50 knot blow, my lesson was immediately
learned- I doubled it and made a mental note that peace of mind in a
blow is worth the lines looking oversize when everythings balmy.
4-1/2" all round it is...
JM