What Are a Porthole and a Hatch?
The round side window and the opening deck hatch — gateways for light, air and, if neglected, drips.
A porthole is the small window set into the hull side or cabin wall, often opening; a hatch is the framed opening in the deck. In both cases, watertightness depends on the seal profile and how firmly the catch pulls it down.
Damp, mould and the dreaded ‘the berth’s wet’ complaints are most often down to these two; and the leak usually comes not through the glass, but from two places: the seal, which ages and hardens, and the bedding line where the frame sits on the deck.
Maintenance points
Hardened seal: Over time the rubber goes hard and loses its grip; on most makes the seal profile can be bought separately, so the seal is renewed without replacing the whole hatch or window.
Leak at the base of the frame: A drip seen inside is often blamed on the glass, but the real route is usually the frame bedding compound; a dye test (local wetting) helps pinpoint the source.
Ventilation: In winter, leaving a hatch slightly ajar is the cheapest cure for mould.
Do not rush in with sealant when tracing a leak; if you seal over it before finding the source, the water will simply find another and more awkward way in. Annual routine: silicone-based protector on seals, a light lubricant on hinges and catches, and a visual check of the frame bedding line.