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September 2004

Article selected from our quarterly magazine dedicated to the largest and most luxurious boats with information, interviews, technical articles, images and yachting news



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Article by
Angelo Sinisi


In the previous article on maneuverability, I pointed out to the fact that the maneuvering qualities of a ship, as well as her various characteristics, depend mostly on the rudder's type and size.

For that reason, starting from the requirements supplied by the customer, the designer must obtain the rudder's characteristics that satisfy such requirements. Subsequently, from such characteristics he must define the dimensions that affect, on one hand, the structural size of the rudder and, on the other hand, the power of the system that will drive it. The two problems related to rudder design involve a quite ample field of ship hydrodynamics, which could be briefly synthesized with the expression "the rudder hydrodynamic design". The precise definition and the subsequent analysis of the factors that have to be considered require quite an exhaustive study thus, for a magazine article, a synthesis is necessary.

The problems concerning rudder design have been treated simultaneously by experts in aerodynamics and experts in hydrodynamics. Therefore, the naval architect may use such research studies to develop a good design. One of the major and most useful data for rudder design is the radius of evolution of the ship, which influences the rudder area on the basis of the ship's hull characteristics. After defining the rudder area to be used, shape, size, dimensions and suitable location must be fixed in order to obtain the most acceptable hydrodynamic compromise. Rudder area means the area of the symmetry plane of just one of its sides. Even though it is very difficult to relate the forces and the moments generated by the rudder to the ship's maneuverability characteristics, the knowledge of such data is essential for design, because the rudder thickness, the rudder shaft diameter and the steering gear size - the system needed to move it - depend on them.

Figure 1

Figure 1 shows the involved forces that must be taken into consideration for rudder design. Among the most widely known profiles, the most suitable for rudder construction are the so-called wing profiles with the rudder shaft in the symmetry plaque. A profile of this type, immersed in a fluid having speed U and angle of attack ???as shown in Figure 1, creates dissymmetry as for speed: speed on the rudder's left side (passive surface) increases and on the rudder's right side (active surface) decreases. According to Bernoulli's Law, this generates a pressure decrease on the left side and an increase on the right side. The resulting pressure difference is the rudder force that, in this case, is directed from right to left. Usually such force is resolved into two components: force FL perpendicular to the direction of speed U, and force D, having the same direction as U (see Figure 1).

The creation of the perpendicular component FL is, in fact, the only purpose of the rudder's existence. The product of force P by distance cCP (see Figure 1) between its point of application and the rudder shaft generates a twisting moment, while the product of such force P for the vertical distance between its point of application and the load-carrying bearing, generates a bending moment. As is well known, the two above-mentioned moments are the main elements for calculating the dimensions of the rudder shaft and of the hydraulic steering system.

Figure 2

The maximum lift that may be generated by a rudder, as a function of its angle of attack ?? is limited by a series of events that cause the rudder to stall. When a rudder stalls, lift suddenly falls to very low or null values, therefore, in the design phase this possibility must be carefully studied and avoided. Stall occurs when the flow separates from the rudder low-pressure area and envelops an area of vortical flow. As previously mentioned, this separation generates an abrupt decrease in lift. The point at which the flow changes from laminar to turbulent is mainly a function of the Reynolds number, that is, a function of chord, relative speed and angle of attack. Three are the events related to stall: separation of the laminar flow, cavitation and ventilation. The most detrimental of them, that is, the one that by itself may generate stall, is the separation of the laminar flow, followed by ventilation and, lastly, cavitation. Cavitation, even if in a minor way, may reduce rudder thrust. The effect of cavitation is not as harmful, for it results in the reduction in inclination of the lift curve, compared to speed, rather than in a real decrease in thrust (see Figure 2).

Figure 3

Ventilation, like cavitation, is a consequence of the low-pressure values occurring in the flow adjacent to the rudder's passive surface. Ventilation, in this case, is the air suction appearing between the atmosphere and the low-pressure area occurring on the rudder's passive surface. Generally, this phenomenon happens only when the rudder is too near the water surface and when the pressure difference, between the atmosphere and the rudder's passive surface (see Figure 1), overcomes the resistance to the air passing through the interposed water. In practice, as everybody knows, this problem is solved by fitting a plate (see Figure 7) between the rudder's top and the water level. In many cases this plate is represented by the hull itself.

Figure 4

Another important factor to be considered before starting the design of a rudder is to establish the number of rudders to be used and their location. The hull wake diminishes the speed of the laminar flow that hits the rudder, while the propeller produces the opposite effect. In single-screw ships and narrow stern the two effects mostly cancel each other, while in twin-screw ships with rudders behind the propellers the effect of the slipping wake is considerable. The propeller not only modifies the speed of the water outflow going to the rudder but it also substantially slows the stall, thus remarkably improving the rudder performance for angles of attack greater than the angle of stall.

Figure 5

Consequently, the rudder maximum efficacy occurs when it is hit by the propeller race. Single-screw ships always have one rudder; twin- screw ships may have one central rudder or two rudders behind the propellers; generally, three-screw ships have just one rudder behind the central propeller and often, four-screw ships are steered by two rudders that are hit by the stern-propellers race. In twin- or four-screw ships, fitting two side rudders behind the propellers is much more efficient than fitting just one central rudder, even when the total surface is the same. This is true because the feeding speed increases as a consequence of the low coefficient of hull wake and of the effect of the propeller race. Moreover, total area and rudder height being equal, generally affected by the ship immersion, the two side rudders have greater height and thus greater lift coefficient, tiller angle being equal (see Figure 6).

Figure 6

As everybody knows, lift increases with the increase of the angle of attack and with the square of the impact speed. When fitting the rudder astern of the propeller, one must verify that, even though it benefits from greater speed, it is not hit by the propeller hub vortex (see Figure 8), which not only has the effect of reducing the lift, but also of causing erosions and eventually annoying vibrations. For this reason, the rudder is often installed slightly towards the inside compared to the propeller center. If the above-mentioned conditions are satisfied, the rudder shall be fitted the most astern as possible, with the clear purpose of obtaining the maximum arm K compared with the center of gravity of the ship and, as a consequence, the best turning characteristics (see Figure 1).

  Figure 7

Figure 8

Figure 9

There are three main types of rudder: the Flap, the Horn and the Spade types (see Figure 3). The Flap and Horn rudders may be useful whenever, for reasons of directional stability, an additional surface is needed besides the existing one and when all of the above- mentioned surfaces should not be mobile. Moreover, they should be fitted when a fixed supporting structure is preferable in order not to load excessively the rudder shaft and its supports. If the turning qualities are to be improved, the rudder surface compared to the lateral ship surface is to be increased; yet, if both are increased, the hull dynamic stability improves and, as a consequence, also its straight line path stability. Nevertheless, one must be careful in fitting the rudder and the lateral ship surface because they may affect in a negative way the ship's dynamic stability. In fact, as explained in the previous article on maneuverability, when the rudder is taken to tiller angle ? and the ship starts turning, various forces are generated the moments of which ( F * h ) make the ship list. If the rudder is pushed to the right (see Figure 4) and the moment given by FL * hL (the force generated by the rudder multiplied by the vertical distance between the center of application P of the total transversal resistance that the ship encounters when turning) is greater than the moment FC * hC (the centrifugal force of the ship applied to the center of gravity G multiplied by the vertical distance between G and P), the ship shall list to the right. Vice versa, if the rudder moment is smaller than the moment given by the centrifugal force, the ship shall list to the left. Therefore, the rudder surface, location and shape affect the listing moment during the turn and, as a consequence, also the listing angle. In following seas, even a prominent lateral ship surface generates a transversal listing force that, if not adequately countered by an efficient rudder, may cause dangerous transversal listings, especially if the ship has a low stability index. One of the rudder characteristic data is the figure ratio a defined as the ratio between rudder height b and its medium chord c (see Figure 5).

In a Spade rudder (see Figures 5 and 9) with a high figure ratio a = b/c there is:

  • a greater vertical distance from the center of pressure and, as a consequence, a greater bending moment for calculating the rudder shaft,

  • an increase in the lift coefficient (Figure 6),

  • a low angle of attack ? at which stall occurs (Figure 6),

  • a greater listing during the turn.

The area forward of the rudder shaft is called compensation area (Figure 5). This area and thus the compensation degree to be assigned to the rudder, i.e. the ratio between the rudder surface forward of the shaft and its total surface, must be such as to be able to obtain a low torque both in forward as well as in reverse gears.

NACA sections are to be considered among the various profile sections for rudder construction and NACA 0015 offers the best combination of hydrodynamic qualities and construction possibilities for Spade rudders. Usually, this type of rudder has a trapezoidal shape, which not only increases the center of pressure by reducing arm hT, but it also has an elliptical distribution of load along its height. This distribution opposes minimum resistance and improves the lift curve.

The bottom end of the rudder may be squared, that is with a sharp edge, or rounded. Testing showed that rudders with squared bottom have greater lift than rudders with rounded bottom. Nevertheless, design practical conditions such as resistance to progress, rudder weight, rudder support weight and steering gear weight impose the construction of a rudder with the smallest possible surface. Therefore, the designer is faced with a problem the solutions of which can only be a compromise and the technical approach of which is complicated by the influence of several factors which come into play in addition to the rudder's main ones.