You should be able to add the tensioner without unstepping the mast. I can't think of a safe way to put on a back stay adjuster without unstepping the mast. If you have your boats manual, it should have a drawing of your tensioner. This is the drawing for a 1987 Standard rig. If you don't have all the parts, I would think it would be about the same cost or cheaper and much better to change to the back stay adjuster system. To see the adjuster system, go to catalina direct, select catalina 25, then select rigging, standing rigging, shrouds. It will be in the top row. There are two there for standard rig, not sure what the difference is in them. If you select Catalina 22 and follow the same selections they have a picture of the backstay adjuster installed. I believe it is very close to the same design except the 22 is 12:1 as opposed to the 25's 8:1.
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