kiltmadoc - You have asked two questions. Please see my earlier post and feel free to request more detail or clarification. I apologize for too much detail but want to give specific advice that's needed for correct masthead halyard upgrade. As always, wear a climbing harness in your bosun's chair and clip that separately to both your halyard and also to a bowline around the masthead that's also tied securely around the mast, so there's no way it can slip off. Also, be extremely careful not to cut things you don't want to while you're up there - A small slip of the wrist can easily kill you!
If your sheaves have literally a V shape, they will pinch and tear the inner (strength) fibers. The diameter of the groove in the sheave ought to be equal or greater than the diameter of the rope. The diameter of the overall sheave, seen as you look through the axis, needs to be at least 2.5 X the diameter of the rope. Many commonly available sheaves for rope are simply too fat to fit in the narrow slots on mastheads made for wire rope, so you will probably have to look at sheaves with smaller diameters. Take a set of calipers to accurately measure the available width of this slot and remember that there is probably 1/32nd inch you can get additionally by sanding oxidation or paint.
As I have probably mentioned elsewhere here, I strongly suggest you use (1) Use sheaves with bearings. You will create an additional 20% in drag, not even counting side friction, which is often considerable, with sheaves without bearings, for every 180° the rope bends. You'll love how easily your sails raise when you use them! (2) No plastic in masthead sheaves (3) Teflon washers the precise diameter of the sheaves, on either side. If you can double thin washers, you will have the best chafe protection. (4) Oversize guides (I usually use thin aluminum sheet, mounted to the sheave clevis pins and bolted on top, to prevent wiggling.) to make up the remainder of the width remaining space from the width of the sheave: You don't want to have any side slop and yet want the sheaves to effortlessly spin.
Because almost all sheaves are this large and because you don't need a diameter of groove larger than the rope, you can often (If you can actually remove them!) remove the clevis pins and replace your sheaves with smaller diameter sheaves that nevertheless are the correct size. There are some additional issues, as well: If your rope, with the smaller sheave, rubs, that has to be either cleared or perhaps the smaller sheave simply won't work. Garhauer has inexpensive, super-quality sheaves I use frequently.
Because constant-diameter, core-to-core splices are difficult, there are few who use them. This is by far the best way to join two double-braid (kern-mantle) ropes. The commonly used cover-tuck method is NOT strong and depends entirely on lock-stitching, which deforms the splice, often makes loops of cover that stick out and is highly vulnerable to tearing apart. It is NOT suitable for halyards. Remember that the cover is designed for wear and UV protection, while the core is designed for tensile strength, so splicing the covers together while ignoring the cores is essentially silly.
Why mention all this? Because an elegant solution, if you can not replace the wire V-shaped sheaves with rope sheaves is to replace them with wire U-shaped sheaves and use 1/4" Warpspeed spliced to 7/16" low-stretch line, for rope clutches, winch and hand use. Ooh - Cool! You have to have a strong constant diameter, core-to-core splice that will travel through blocks and masthead entry, though. If you're at all unsure about this splice, ask someone who does it regularly for a living. It takes me at least three hours and I'm working quickly all the time. It's the most complicated and one of the longest splices I make.
To keep the splice from jamming in the masthead as it passes around the masthead sheave, use a stop-ball. You can find them in different diameters. Thread it, wit a diameter that's tight, on the rope before you splice the shackle and then pull it onto the splice. It will stop as the rope bulges, preventing the splice from entering the masthead and thus avoiding jamming.
zz4gta - Excellent comment. The UV coating of cores does, however, wear off immediately. This isn't really an issue for a well-funded race boat but...
By the way, the Merit 25 is one of my favorite boats - They're just beautiful sailors on any point of sail and so much fun!
I'm a shipwright, rigger and professional sailor, in business as My Boat Works, in Los Angeles. I also make super quality, High-Definition instructional boating movies! I'm here to give my expertise to boaters, as I was helped. It's my pleasure to help you enjoy activities I love so. Cheers! - Jerr