Jib-sheet lengths
To calculate jib-sheet length, add it up:
Clew to the winch;
Enough for the tail (past the winch) and cleat;
Foot of sail;
Twice that-- because the other sheet has to make it to the winch before you come about.
Therefore on my 1974 H25, for the 110 jib:
10
+4
+11
----
25
x2
----
50 ft.
For a 150:
6
+4
+15
----
25 ft
x2
----
50 ft
See how that works?
Actually each of mine are like 55 ft.
Note that spinnaker sheets are actually shorter than jib sheets, because the two clews never actually change places; they just stop at the headstay (for an asymmetrical/'cruising spinnaker' even less). Mine are 35 ft each.
All my sheets are 3/8"-dia Sampson XLS or Yale ULS. Perhaps some could be 5/16"; but no one wants to hold a line that's too small for comfort.