Portlights and hatches are holes in the deck and cabin side, closed with a frame and a lens of acrylic, polycarbonate or glass. When the bedding dries out, the frame leaks. When the lens cracks, it can let go in a sea. And sometimes the leak is telling you the cabin side is moving.
Why they leak
Darrell Nicholson in Practical Sailor separates two cases. The easy one: the bedding has dried out and a removal, clean-up and rebed is all it takes. The harder one: in all too many cases the leak is a symptom rather than the problem. Holes in the cabin side reduce its stiffness, and loads from the chainplates, mid-boom sheeting and genoa tracks flex it until the ports work loose.
Lenses suffer too. Good Old Boat's Kim Efishoff says plastic lenses become weathered, scratched and crazed, and that holes must be oversized for thermal expansion, at least 1/16 inch per foot of span. Nicholson notes a fastener can twist and crack the lens.
How a surveyor checks it
Inside first. Look below every port and hatch for water stains, soft or black veneer and swollen joinery.
Hose test. With someone inside, run water over each port. Note which drip.
The lens. Look for crazing, which weakens it and spoils the view, and for cracks, especially radiating from fastener holes.
The structure. On repeat leakers, look for cracks in the cabin side round the opening and movement under load. Practical Sailor says port leaks can be the sign of much more serious structural problems.
What decides the grade
Whether it would stay in place in a sea. A cracked lens or frame where green water can reach it is an A. Active leaks or lenses cracked at the fasteners are a B. Crazing or old bedding with no leak is a C.