RUDDER BASICS
Rudders, in any types and forms are used to maneuver the ship. There are many types of rudders, balanced (compensated), un-balanced (not compensated) and semi-balanced (semi-compensated) when talking about, let’s call them “classic”, rudders. They can be horn type, flap type and spade type. According to shapes and technical solution they can be shilling type, rotating cylinder type, twisted leading edge type and integrated propulsion-propeller systems as azimuth thrusters, Kappel systems, Kort nozzles, Pleuger rudder, Voigt-Schneider propellers. We will try to see few basic types.
A rudder has to be designed to be effective for the type, size and speed of ship. A major data for rudder design is the turning circle of the ship, which influences the rudder area on the basis of the ship’s hull characteristics. After determination of rudder area to be used the shape, dimensions and suitable location must be determined to obtain the most effective hydrodynamic compromise for the real ship. Rudders are shaped as a wing with symmetrical profiles on active and passive faces.
Basic forces on rudder are as shown in the figure.
On a symmetric wing profile immersed in a fluid moving with a speed v and angle of attack α (angle between direction of ship movement and direction of fluid movement – usually 0 to 35 degrees) fluid will have different speeds on active and passive faces. According to Bernoulli’s law, this generates different pressure on the sides of the rudder. The resulting pressure difference is actually the rudder force. This force is made of two components: FL the force perpendicular to the direction of speed v, and D, the force having the same direction as v. The creation of the perpendicular component FL is the only purpose of the rudder’s use. The force P having a distance to the rudder shaft (rudder center) generates a twisting moment, while on vertical force P for will produce a bending moment depending on the distance to point of application. These are the main forces involved in calculation of rudder shaft, rudder stock, mechanical, hydraulic or electric steering system. We will not get into calculation details here, we recommend you to visit
Rudders, Sole Pieces and Rudder Horns - IACS
Strength analysis of rudder arrangement – DNV exchange
Calculation of main structure or rudder – Veristar
The maximum lift generated by a rudder is limited by hydrodynamic rules. There is a point at which the flow changes from laminar to turbulent determining rudder to stall and is mainly depending on Reynolds number, defined as
Re = ρvL/μ = vL/ν = QL/ νA
Where:
v = speed (m/s)
L = travelled length of fluid (the distance from the leading edge -where the fluid first makes contact- for flow over a plate; DH when about pipes)
μ = dinamic viscosity of fluid (kg/ms)
ν = kinematic viscosity (m2/s)
ρ = density of fluid (kg/cm)
Q = flow rate (m3/s)
A = pipe cross-section area (m2)
Going further the turbulent flow will create the effect of cavitation which will further reduce the efficiency of rudder, higher the Reynolds number meaning more turbulent flow.
Depending on number of propellers also rudders can be one, for single-screw ships, one or two for double-screw ships, one for three-screw ships and two for for-screw ships. Number, sizes and types depends on solution adopted by designer.
Other types of rudders
| Schilling rudder | Twisted edge rudder | Kappel system | Energopac system | ||
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| Becker Marine Systems | Becker Marine Systems | MAN Diesel & Turbo | Wartsila | ||
| Kort nozzle | Flap rudder | Voigt-Schneider propeller | |||
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| Van der Velden Marine Systems | |||||
