Ernie (and Phil),
I'm not a Nonsuch 30 expert. (There is one in my marina, but the closest I've gotten to it was watching it sail away from me once and internally debating how much I love my 26 vs. would I like to have a bigger boat. For the record, I'm sticking with my 26.)
I did a little research before writing this response, and was surprised to notice that the N30C manual specs a 7/16" halyard, while the N30U manual specs 1/2". I'd love for a N30 guru to chip in and explain why. Joe Valinoti, are you reading this?
IMHO, rope technology has improved enough that it's feasible to downsize a bit with the usual suspect products I mentioned. I'd be conservative about going aggressively smaller with them. If you want to go significantly smaller and lighter, you're probably getting interested in newer, higher tech.
At 8,135 lbs breaking strength, the Marlow Doublebraid 3/8" that Paul M reports using on Sandpiper is comparable to the strength of the ropes I suggested, certainly lighter -- and pretty affordable. too. I don't know how his sailing conditions compare to Phil's, but there's lots to be said for doing what works for others with the same boat.
Cedric's right that smaller lines may be hard to grip by hand. If you use rope clutches, whether going smaller is a problem or not likely depends on the age of the clutches. Newer ones grip a wider range of line sizes. I think it's pretty safe to bet that lines down to 5/16" will work with most clutches and self-tailers. Below that, especially if using some of the high-tech slippery stuff, it might be worthwhile to check your hardware's specs and do some testing with a line sample.
-- Bob
Solar Wind
Nonsuch 26C #143