That diagnostic temporary seal has taken many forms over the years, tho most frequently it has been blue tape. In approaching the possibility that my mast leak is actually between the deck ring and the deck, I had another option. Earlier attempts at sealing the top of the boot required me to peel off the 1" wide "self-bonding" silicone tape (in quotes because it really didn't do it all that well).
I had saved that tape, intending to reapply it later. But it was a perfect medium for creating a diagnostic seal at the base of the mast boot. I removed the base hose clamp, and formed the tape so that it went up over the boot and ring, and then splayed out on the deck, creating a skirt around the deck ring. (It's kind of hard to see in the picture above, but the two ends of the tape are directly beneath the hose clamp screw.) Re-attaching the hose clamp completed the diagnostic seal. Total time invested: 5 minutes.
This is not a permanent seal. Nor is it really waterproof - water can likely sneak under the skirt. But the skirt should serve to divert 90+% of the water coming down the outside of the mast away from the area between the deck ring and the deck. If the water flow down the mast inside shows a significant decrease at the next rainstorm, I've nailed the location, and I need to get out the polysulphide. If not, well then it is time for re-thinking. And the blue tape.
Such is life aboard.
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