Hi Peter,

You're talking about a deck plate, like the Beckson that CLC sells? Then yes, you want silicone- a great seal, and it remains flexible. The 3M marine grade stuff is pretty great. No idea what went wrong with yours- clean it off, get a fresh tube, and give it another try.

Good luck,

Patrick

   I'm assuming you used a standard silicone caulk. All the silicone caulks that are single part and smell of vinegar/acetic acid actually cure by exposure to atmospheric water vapor.  So if you caulked it up in a dry environment, and then screwed everything down tight and closed up, there may not have been enough water vapor to cure all the caulk.  I used 3M 4200 polyurethane sealant for my Beckson ports on my Skerry, since I use it or the more permanent 5200 on almost all my big boat goop jobs.

I used silicone to rebed plexiglass portlights, but used a special GE silicone specifically for that task, not bathroom caulk, because the portlight material isn't really compatible with polyurethane.  The GE caulk is not an acetic acid cure.  No smell and no acetic acid to attack anything.

In either case, I always put a big bead of caulk on the port ring and screw it down loosely but evenly, squeezing out some and leaving it about 1/16" from tight, wipe the excess and let the caulk setup.  Then go back later and finish tightening it down.  This also works for things like deck fittings on my big boat.