This formula 4.1 applies only to rudder arrangements in which the rudder is located directly behind the propeller. For any other rudder arrangement Det Norske Veritas requires an increase in the rudder area by - at least - 30 percent. A twin screw (or more) arrangement should be combined with rudders located directly behind the propellers for maximum low speed maneuverability. A single rudder placed between two propellers may be inadequate, because the rudder blade does not swing sufficiently into the low of a propeller to generate the needed turning moment.