Freeing ports or scuppers are openings in ship sides or bulwarks, allowing for the draining of accumulated water from the decks. Green water can present a risk to stability if not drained in a timely matter, as it can be of considerable weight and induce a free surface effect. Freeing port sizes are regulated by the ILLC, flag states, and classification. Fishing vessels sometimes carry methods of plugging freeing ports.
Freeing ports over 300mm long generally require some form of protective arrangement, i.e., spaced bars. In ships with limited sheer, freeing port sizes will generally be suitably increased. Freeing ports should be enlarged in way of the ends of deck-mounted breakwaters. For aesthetic reasons, or to limit ingress of green water through the freeing ports, they will sometimes be fitted with outward opening flaps.
Freeing ports should preferentially be placed at the bottom of a vessels sheer profile, at the low point between discontinuities, and at the end of breakwaters. Otherwise, it is generally preferable to evenly space freeing ports along the length of each section of deck (known as a well).
Alternatively, bulwark plates can be stopped above the deck, providing a continuous freeing area along the length of a vessels decks. Vessels with a rolled deck edge, or open railings all around do not need any freeing ports (as the entire deck is exposed). Similarly, pilot boats may have no bulwarks or outboard railings, relying on exterior storm rails and safety lines for crew safety (and maintaining a clear area for boarding operations).
Plugging the freeing ports may be done to keep waves from coming up on the deck, or to gain a small amount of extra displacement. This is generally ill-advised, as any water on deck will then accumulate, and could quickly lead to a loss of the vessels stability, or swamping.