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Had a look at a 26 swing keel, and in the area close to the pivot point, there are two plugs, one was actually missing. The owner, seemed to indicate they’re access to the bolt through the keel, but I couldn’t be sure.. Unfortunately one of the plugs were missing, and so it seemed you had a direct entry into the cavity there for water (however I can’t tell if it was a large area).. This approximately 1.5” hole didn’t seem to provide water into the boat, but still wondering what it was, anything that should be plugged, or if water entering this area was a concern..

??

Maybe a photo?

The plugs are to stop marine growth when the boat is in water.
Mine are both missing.
I am not worried as I am in the water for no more than 5 days at a time.
These inspection openings are to remove and drop the swing keel or daggerboard for service. They are not part of the water ballast system.
Have a look on this forum. . There is a huge volume of knowledge pertaining to this