Thanks for the great info, Joe. The diagram is especially helpful.
Both my main and jib halyards are 5/16 rope which I think is the right diameter. The jib halyard moves freely over the sheave but the main creaks and groans as it makes its way up the mast and the weight of the sail increases.
Dumb question: I've never been up a mast. How many halyards do you need to hoist an average size dude up there? I ask since obviously I can't change the sheave using the same halyard I'm up there on. I would have to go up the mast twice to change each sheave.
Measure the new sheaves that you already have. Just the width across the channel. If you have some 5/16 handy, see how it runs in the groove. My halyards are 5/16 with core AND cover, but the part that goes over the top sheaves is core only, and that is 3/16. Thats how I accommodated the 1/4" limitation of the original sheaves. Most people will say it's okay if the rope rides up a little.. the clearance is so tight it is unlikely the rope would pop off and get jammed. Especially since so many people have roller furling and don't really use their jib halyards that much.
To answer your other question, one halyard is strong enough to hold, the others are back upo. If you can, tie the halyards on rather than using the shackle. When I've used a bosun's chair to climb the mast I'll use the spinnaker halyard as back up.. but you could use the mainsheet. Switching between all of them as you progress. For added safety and extra convenience, riggers take a length of strapping with stirrups on each end ( or loops sewn in) to straddle the top fitting. These act as foot rests, allowing you to stand to get up even with your work so your arms don't get fatigued as fast as they would sitting in the chair. My plan would be to connect two halyards, one for lifting the other for backup, leaving the third free to work on. When you're finished with one, you can let that one take the load while you direct your attention to another, and so on. Obviously, you'll need some help. Just make sure you explain precisely what it is you want them to do. The other alternative is to call a local independent rigger to do the job for you. It'll cost $200 or so. It shouldn't take them more than an hour.
Finally, this may sound complex, it isn't really. Like all things boat related, you just need to think it out ahead of time. Step by step. For instance, how are you planning to get your tools an parts up the mast with you. And how to you minimize losing a tool, seeing it hit the deck and cracking the gel coat before doing a swan dive into the water. Remember, think ahead, anticipate the job, so you take only what you need. Get lanyards for your tools, a small bag on a long cord will help ferry parts up from you deck assistant. Get a head light, a belt with a pouch, wear long pants and sneakers. ask questions... okay. I'm sure you have more... good luck. it's a nice project. btw, if you get curious about building "tapered" halyards do a search, I've written about it a few times over the years.
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