The United States Navy built and commissioned nine Canonicus-class improved monitors, losing three of them in combat. Intended to be more viable at sea, they carried two 15-inch Dahlgren smoothbores but achieved about eight knots against a planned thirteen. Their low freeboard left little margin in rough water even when hatches were designed to exclude the sea. Armour and heavy guns could not replace sea room and survivability. Tecumseh's loss and Manhattan's aggressive participation put that design trade-off into practice at Mobile Bay.
Notable Point · 42:09-45:06
Monitor design: armour and guns versus seakeeping
The monitor discussion explains why low freeboard and limited speed constrained early armoured warships at sea.