Helm-port

The hands were now actively employed in endeavouring to keep the sea from breaking into the gun-room, which at every send-aft came in, in alarming quantities; which, however, was materially prevented by slinging gratings covered with tarpawlings, and applying them externally to the helm-port, with laniards somewhat similar to those of a ballast port, to confine them.

Source: wiktionary

... the helm-port, or opening in the centre, where the rudder-head enters, is so reduced, that the old dangerous rudder-coat of canvass or leather -- a very insufficient barrier to the breaking through of seas and swamping of the ship -- is now dispensed with, and woodwork, called canting, is substituted.

Source: wiktionary

There's something else – do you see the wide helm-port that the head of her helm goes through?

Source: wiktionary