RUPAS (Rudder with Parallel Section)

RUPAS is shortened form of "Rudder with Parallel Section at the trailing part". Its rudder section makes rudder area smaller while keeping the same maneuvering performance. Therefore, it reduces rudder steering torque and accordingly smaller steering gear. Furthermore, the smaller rudder area provides better hull propulsion performance.

RUPAS

Cross section of RUPAS

Its radius of the rudder front section is smaller and its rear shape has hollow with parallel shape, comparing with the conventional rudder section. This provides higher lift and also smaller rudder resistance at midship rudder angle than ones of the conventional rudder.

Rupas Small rudius Rudder axis Hollow X/X Parallel Conventional Rudder axis X/C
RUPAS for the actual ship

Working force on the rudder

When steering in the propeller flow with rudder angle of δ, the normal force of Fry happens on the rudder as shown below.
Its quadrature component to the ship is the lift, and parallel component is the resistance.
This lift makes ship turning.

Rudder normal force and rudder resistance

Rudder normal force and rudder resistance

The left figure compares the normal forces of RUPAS with ones of conventional rudder at each rudder angle. It is found that RUPAS generates a normal force approximately 12% greater than one by conventional rudder at the same rudder angles.
The right figure compares the rudder resistance of RUPAS and one of conventional rudder at midship rudder angle. It is found that the rudder resistance of RUPAS is slightly less than one of conventional rudder.

Comparison of normal forces on RUPAS and conventional rudder Normal force on rudder
Comparison of rudder resistant on RUPAS and conventional rudder Coeff. of Ridder resistance

Actual RUPAS installed on a vessel

Below is a study of replacing RUPAS with a conventional rudder. Left figure shows that a rudder area becomes approximately 12% less, because RUPAS increases the normal forces. Right figure shows the results of Zig-zag model test of RUPAS and conventional rudder, where it is found that RUPAS with smaller rudder area has slightly better maneuvering performance than conventional rudder.

Shape of RUPAS and conventional rudder Ridder dimensions
Results of the Zig-zag model test equipped with RUPAS and conventional rudder Free sailing model tests
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