Propeller placement affects the flow of water available to a ship's rudders. The propellers move water past the rudders, giving them more flow to act upon. A rudder's turning effect depends on that moving water. Like an aircraft wing in a faster flow of air, a rudder can exert greater control force when more water flows past it.
Notable Point · 28:23-29:18
Opal's propeller arrangement and water flow over rudders
Opal's construction photographs prompt an explanation of how propeller flow helps a rudder turn a ship.