I am replacing my H16 mast base and see a flexible (and leaking) rubber membrane inside the mast. The boat is 1981 and has rivets
and screws securing the existing base to the mast, so I presume this has been replaced already.
Hoping this link works here...

Herein I found some advice to silicone and A/B foam. I wish to repair it as robustly as possible and propose the following, advice appreciated.
- drill/tap four holes in the the new mast base (holes exist in mast) and ensure a nice fit. Do not re-install rivets since the holes are blown out (presumably why the existing base was drilled and tapped)
- wire brush and soap/water scrub inside of mast
- alcohol wipe inside of mast
- push cardboard plug/disk about 6" into the mast
- mix and dump A/B foam into mast (tape off outside of mast to not make a mess)
- cut off excess foam
- install mast base with copious amounts silicone in the first inch of the inside of the mast
- Stainless SHCS screw base to mast
- Silicone outside of base/mast interface
Seems OK? Perhaps do the same at the top of the mast?
Any sound advice will be appreciated!