If you are going for dyneema, 10mm is a good compromise between strength and line handling requirements. We use 10mm on our 17.8m mast for the main halyard. It is over specified for your loads I suspect but any smaller is hard on the hands.
I think you will find that 10 mm is more than adequate. When I was destructively testing Dyneema soft shackles the break load of 6 mm line was higher than the 10 mm stainless steel shackles tested earlier. It is incredible stuff.
Not cheap, but this Liros Dynamic Plus is good stuff - and certainly don’t need anything bigger than 10mm Liros 10mm Dynamic Plus Dyneema - Sheets, Halyards, Control Lines
Great, that is worth knowing, I have been looking at some of his offers. I only have to figure out if I can get away with 25 metres now.
We use Marlow basic 12mm polyester line on the genoa and jib halyards. Little point on dyneema for this application in my experience if not racing. We use 10mm marlow racing dyneema on the main and mizzen halyards. Far more benefit here in my experience ?
When I changed the original Doyle mainsail supplied with my boat as standard to a better quality one I had to change the halyard. This was because I could see the creases coming & going in the sail as the braid on braid halyard was stretching so much. The original sail was poor quality & was also very elastic so it was not obvious.
I was specific about the genoa halyard on a smallish pure cruiser. Your comment refers to your mainsail.
As you probably know I do a fair amount of sailing miles. Always very keen to get the best of of my boat. Mainsail tension can be improved dramatically by a dyneema halyard but in my experience genoa/jib tension is far less variable. I could pretty well tension my genoa and not need to adjust that tension with a polyester halyard for the whole trip. It may need tweeking if the the wind goes from light to strong or vice versa. With the same polyester halyard on the main it would never be at the correct tension. I used to have a dyneema halyard on the genoa but put the polyester one back on and now use that dyneema halyard for the spinnaker.
