Grant Ziebell wrote:
Jib sheets which came with the boat are in need of replacement. The
existing sheets sem o be excessively long leading to much clutter in
the cockpit.

Is there a rule of thumb for determining the correct length of the
sheets?

The old "rule" was that the jib sheet should be approximately equal to boat
length. Depending on the LP of the sail, one could adjust this up or down
according to preference. However you may want to consider keeping the
sheets a bit longer. This will allow you to adjust the length for wear or
chafe, reverse the sheets more easily, whip or re-whip the ends, etc.

Another idea is to use a single length of rope about twice the boat length,
and then set it up as both sheets with a simple loop through the cringle.
Many people prefer this since there is less likelihood of the sheet hanging
when tacking. Of course this means dedicated sheets for each sail and
consequently more expense.

What line diameter would be best for this application? Jib
is a 135%.

You don't indicate boat size, but I don't think we're talking about an 80
footer (correct me if I'm wrong). For the typical cruising sailboat, the
sheet diameter is usually chosen more for "hand" and ease of use that for
strength. Even thin cordage (say, 1/4 inch) is very strong these days, but
except for special applications (e.g., light air spinnaker sheets), you will
usually be better off choosing the sheet that feels best in your hand. This
turns out to be somewhere between 7/16 and 5/8 for most boats.

What about cruising spinnaker sheet lengths and sizes? Boat came
with the sail but no running rigging.

The old rule of thumb was that the spinnaker sheet was twice the boat
length, plus or minus. I am specifically talking about conventional
spinnakers. The "proper" length will depend to some extent on how the
spinnaker is gibed.

Take a look at Wally Ross's "Sail Power" for an extended discussion.

--
Good luck and good sailing.
s/v Kerry Deare of Barnegat
http://kerrydeare.tripod.com