606

SEAMANSHIP AND NAUTICAL KNOWLEDGE

Fig 13.—A Freeing Port.
Fig. 13 illustrates one type of closing port. It is hinged about
one-third from the top edge. The end elevation shows that when
the vessel rolls the water runs to leeward, pushes outwards against
the port, which it opens, and flows overboard.
The area of freeing ports depends upon the area of the bulwarks
according to the following scale :—
Well deck in feet 25 ; area of openings in sq. ft. 9
>» » 4x* > 99 » ,,11
»» » 65; „ „ „ 13
In well decks above 65 feet in length the area of freeing port
is 1 square foot for every 5 feet of bulwark, thus, 80 feet bulwark
would require lt> square feet of openings.
Ventilator.—Fig. 14 illustrates a common type of ventilator,
It is trimmed an required by turning the cowl A on its coaming B,
which is secured to the deck by a watertight collar G. An inner
tube of smaller diameter than the coaming leads down to the lower
hold. The current of air passes down the inner tube of the ventilator
to the lower hold, but some of the air is diverted into the 'tween
deck through the annular space between the tube and the coaming,
Two or more ventilators are fitted to each hold in tramp steamers,
one, the lee one usually, being trimmed mouth to wind and the other
one back to wind, thus forming a downtake and an uptake to create
a continuous current of air throughout the hold. Some cargoes give
off gases lighter than air, coal for example, and m such cases surface