The primary consideration with your
headsail sheets, IMHO, should be how it feels in your hand and how it fits in your winch. You can look at strength charts to determine if stepping down in size is advisable. With your
halyards, the most important thing is how the line runs through the blocks. Unlike sheets, which normally run through just one lead block at an obtuse angle, your halyards will be making a 180 deg turns up top, and probably 90deg down below if they runs aft, not to mention any deck organizers and clutches that can add friction to the operation. The winch fitting is important also, but less critical, while the hand I think is not that important, With that said I am a strong proponent of dyneema cored halyards. They allow you drop the line diameter, allowing smoother operation by vastly reducing friction. On my boat I built my own halyards by purchasing a single braid dyneema for the core and sheathed it in a dacron/polyester cover. The conver can be purchase separately, but it's more inexpensive to buy a lower quality double braid and use its core to fish the new dyneema core into position. I also only cover the back half of the core, since it's the only part that is handled, wrapped on the drum and cleated. I prefer Samson rope, but most any of the popular brands do the job. I use Samson Trophybraid for my sheets. I has a fuzzy spun polyester cover that feels good in the hand and hold well on the winch and cleat.
running rigging size chart
As far as size goes, I can't imagine putting anything larger than 7/16 on a 35 foot boat. Google "running rigging size chart" for more detail. On my 27 the owner manual pretty much recommended 3/8" for everything. After converting to the dyneema cored halyards, and researching the actual loads on other applications, the only 3/8 I have now is the jib sheets, because it feels better. Everything else was downsized to 5/16... even less for cunningham, outhaul, etc.
Finally, I am not a proponent of the "cow hitch" attachment for your headsail sheets. If you think it will make your tacks smoother, well, not really, only proper tacking procedure will solve that problem. Two separate sheets with bowline knots.... call me old fashioned. If you cow hitch your sheets, it is highly likely that you NEVER remove them. Which means they are in the elements 24/7 which means it won't be long before they start looking faded and become puffy and swollen like your existing sheets (which is why they are measuring 5/8" Okay, good luck. Be smart.... remember .... two sheets, bowline knot
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