The armed launch used by the US Navy had pivot sockets at the bow and stern from which the boat-howitzer could be pivoted.
Two tracks, the width of the field carriage ran fore and aft. These tracks could be slid down to the shore to get the field carriage ashore.

The boat-howitzer sat on a sliding carriage when positioned on the launch.

An armed ships launch crewed by elements of the Naval Battalion, 13th New York Heavy Artillery, 1864.
The coxswain and quartermaster are positioned at the stern. The gun captain and second gun captain stand either side of the boat-howitzer in the bow.

FROM: Osprey Publishing's: 'American Civil War Amphibious Tactics'

PRICE: £200.00 (Painting unframed)
SIZE: Image approx. H 34 cm x W 24 cm (H 13.5 x W 9.5 inches)
MEDIUM: Watercolour and gouache