Sheaves
Unless you have internal halyards it is unlikely that you can replace the sheaves while hanging on either halyard. In a standard rig with external halyards the main halyard goes up the front of the mast, over a sheave, across to the after side, over another sheave and down to the sail. The jib halyard is the opposite, up the back side, over two sheaves and down the front. The two sheaves in the back are on a common axle as are the two in the front. Any strain on either halyard will be resting on both axles. I doubt that you could pull the axle under load. I am sure you couldn't get it back in again. You might be able to change sheaves if you can find a way to take your weight off the halyards when you are at the top of the mast. There is a pin on the forward side of the mast head fitting which is for a spinnaker block (you should replace this with the spinnaker crane from Catalina Direct). If you can hook onto this pin and rest your weight on it you can probably get the axles out and replace the sheaves. I'd use a locking carabiner or a stout screw shackle and a stirrup to stand on then (taking a deep breath and invoking the name of my life insurance agent) unhook the bosun's chair from the halyard and hook onto the carabiner. During this maneuver use the other halyard as a belay, tying into it so that it will catch you if you fall. The axles are held in by large cotter pins. You should take new ones up the mast with you. They should be provided with the new sheaves. You will need a sturdy pair of pliers for the cotter pins and maybe a hammer and a punch to drive the axles out of the mast head fitting, especially if they are frozen in. There is a thin metal fin between the sheaves to keep the halyards separated. It is held in only by the axles and will move around when they are out. Don't loose it or the old or new sheaves down the mast. Do one axle at a time. I can't remember if they'll fit but in any case go carefully and don't drop anything. A phillips head screwdriver makes a handy alignment tool when you are trying to get everything lined up for reassembly. I did mine with the mast down. Safer and easier but not cheaper. For a look at a diagram of the masthead fitting for a Cat 27 with internal halyards go to the Catalina 27 unofficial web site (see the link below and select diagrams then masthead). It is not the same as the external halyard model but the main fittings are the same. Good luck.
