In such a high lift twin-rudder system that: a pair of high lift rudders 1,
2 is arranged behind a single propeller 3; the respective high lift
rudder 1, 2 has a top end plate 6, 7 and a bottom end plate 8,
9 at the top end and the bottom of a rudder blade 4, 5; the respective
rudder blade 4, 5 is provided with a reaction fin 10, 11, protruding
from an inboard face of the rudder blade 4, 5 on a nearly same level with
the axis of the propeller 3, that is originated nearly from the leading
edge portion toward the rear and has a fixed chord length; the reaction fin 10
of the rudder blade 4 that faces on the board-side where the propeller
blades rotate in the ascending direction assumes a posture that makes such attack
angle that the ratio of a forward vectored thrust to a drag, both produced by a
propeller slip stream having a stream component in the ascending direction, becomes
maximum; and the reaction fin 11 of the rudder blade 5 that faces
on the board-side where the propeller blades rotate in the descending direction
assumes a posture that makes such attack angle that the ratio of a forward vectored
thrust to a drag, both produced by a propeller slip stream having a stream component
in the descending direction, becomes maximum, the respective rudder blade 4,
5 is so constituted that a chord length is of 6045% of a propeller diameter.
