Maine Sail's butyl tape is great for fittings. For bedding portlights I much prefer caulk, which fills gaps and is easy to clean up afterwards (outline the exterior part with painters tape).
Butyl needs to be compressed to be effective, whereas Ericson portlights sit in their openings, held by the exterior flange against the pressure of breaking waves. The interior trim is mostly cosmetic and not intended to be tightened hard like the bolts of a deck fitting.
Choose a low adhesion caulk, there are many good ones. No 5200, which is a strong adhesive. Boats flex, which is why Ericson portlight openings are slightly oversized (and crude)--a permanent bond is to be avoided.
Butyl needs to be compressed to be effective, whereas Ericson portlights sit in their openings, held by the exterior flange against the pressure of breaking waves. The interior trim is mostly cosmetic and not intended to be tightened hard like the bolts of a deck fitting.
Choose a low adhesion caulk, there are many good ones. No 5200, which is a strong adhesive. Boats flex, which is why Ericson portlight openings are slightly oversized (and crude)--a permanent bond is to be avoided.