9. 
   CONTROL DEVICES

1. Active and passive control devices

All control devices could be divided in two main categories: active and passive.

Active control devices: are devices actively fed by external energy.

Passive control devices: are devices absorbing energy from the ship speed (wake) and propeller race.

Active	Passive
Thrusters; active rudders; azimuting propellers; Voith-Schneider propellers	rudders; stopping shields; fins;

Fig. 9-1

Application: the effectiveness of active control devices is generally reduced with increasing the ship speed; while the effectiveness of passive control devices is increasing with the ship speed.

2. Characteristics of rudder profiles

Rudders are the most common control devices. Rudder is a profiled foil or flat plane which is developing lift when inclined to the flow. Profile (or flat plane) when at an angle S to the incoming flow develops lift and drag forces. The lift causes the turning moment of the ship, while the drag causes the reduction of ship's speed. Both forces depend on the rudder deflection angle

Fig. 9-2

.

Fig. 9-3 Explanation of the stalling phenomenon at critical angle

A rudder is more effective as a control device if the developed lift is larger. The ratio of drag/lift forces is also important and it has to be made as small as possible.

Rudder effectiveness depends upon:

• profile (form, thickness)

• aspect ratio

• rudder outline

• rudder location

• special features (flaps, foils etc.)

1. Rudder effectiveness

Profile

All modern rudders have cross-section in the form of a symmetric airfoil profile. Some older ships are fitted with the flat plate rudder. Profile is better than the flat plate as it is seen from the table below, where values of the lift and drag coefficients and their ratios are compared:

rudder angle [deg]	plate rudder			NACA 0015 Profile
		CL	CD	CD/CL	CL	CD	CD/CL
10	0.323	0.132	0.409	0.289	0.042	0.145
20	0.654	0.311	0.437	0.622	0.135	0.217
30	0.915	0.593	0.648	0.926	0.320	0.346
40	1.000	0.873	0.673	0.685	0.605	0.883

Providing almost the same values of lift coefficients the profile offers much lower values of the drag/lift ratios. The difference between various profiles used for rudders is not very large, but the thickness has some effect.

Aspect ratio

The rudder aspect ratio affects the rate of change of the lift coefficient with the rudder angle and it also affects the rudder critical angle (angle, when the flow separation occurs at).




Fig. 9-4

Rudder outline

The rudder outline has small effect on the rudder characteristics. The outline (planform) could be rectangular, trapezoidal, rounded edges, etc.

1. Rudder force

The rudder force or the lift of the rudder is calculated with the formula:


, where: C_L is a lift coefficient


Rudder force depends on the attack angle of the rudder relatively to the water velocity. When the ship is moving on a straight course, this angle equals to the rudder angle approximately (Fig.9-5).

Fig. 9-5

When the ship makes a turn, the direction of the water velocity at the rudder position does not coincide with the plane of symmetry of the ship.

The angle of attack of the rudder is reduced and - if the hull and propeller have no influence on the water flow - is equal to the rudder angle minus the drift angle at the position of the rudder (fig. 9-6). Since the hull and the propeller do interact with the flow then the attack angle is a function of the rudder angle, the drift angle at the rudder position and the propeller slipstream.

(Drift angle at the position of the rudder is the angle between the tangent to the ship's path at the rudder position and the ship plane of symmetry).




Fig. 9-6

In fact, the so-called flow straightening effect due to the propeller slipstream and the presence of the ship's hull, diminishes the influence of the drift angle. Finally : α  δ  β

α effective rudder angle (angle of attack)

δ geometrical rudder angle (rudder deflection angle)

β drift angle (kinematic, at the rudder position)

 flow straightening effect

2. Effect of ship's wake and propeller race on the rudder performance.

Rudder is located behind the hull and in the propeller slipstream. This is important from the point of view of the rudder lift force.

Effect of the wake:




Fig. 9-7

The flow velocity in the wake is considerably lower than the ship speed - so the lift force is reduced!

Effect of the propeller slipstream (race)

When the rudder is located behind the propeller, then it is in the propeller slipstream where the flow is accelerated




Fig. 9-8

The velocity in the propeller slipstream depends on the propeller thrust load coefficient C_T and on the distance x between the propeller and the rudder. This is taken into account by the coefficient k in the above formula:

x/D	0	0.25	0.50	0.75	1.0
k	0.50	0.79	0.88	0.94	0.96

Wake effect and the effect of the propeller slipstream act in opposite directions.

The hull wake w = 0.1 ~ 0.3 depending of the fullness of the hull. The velocity in the propeller slipstream V_ps=1.2 ~ 1.5 V_SHIP, that means that this effect is stronger. If the propeller is not working, then the effectiveness of the rudder is very small. At rest, if propeller is kicked ahead then the rudder lift force is large.

In twin-screw ships, the twin rudders positioned behind propellers are effective. If there is only a single rudder in ship's plane of symmetry, then such a rudder is much less effective.

3. Rudder in the propeller slipstream in ahead and astern condition

The sketches show how the rudder develops lift in different situations during manoeuvring with the propeller working ahead and astern .




Fig. 9-9

4. Different ways to develop lift - Special rudders





Fig. 9-10

Lift could be developed by the angle of attack or by the camber of the rudder profile (profile asymmetry). Combination of these two increases the lift available. Becker rudder is a practical application of this principle; the camber is achieved by the application of a flap at trailing edge.

The Schilling rudder employs a special form of the profile where the tail has sharp edges and in this way the separation point is fixed at the sharp edge and even at very large angles of attack there is no stalling effect (fig. 9-11). Schilling rudder may be deflected to very high angles (75 to 85 degrees) while still producing large lift force.




Fig. 9-11 Comparison of lift characteristics

Becker rudder and Schilling rudder are the most common high-lift rudders employed in practice. The advantage of the Schilling rudder is a lack of any moving parts; its disadvantage is slightly higher resistance and increased fuel consumption that is important in particular at long routes.

The disadvantage of the Becker rudder is the need to install a special mechanism to operate the flap.

The lift could be also developed by other physical mechanisms. One of these is an employment of so called Magnus effect. Magnus effect is illustrated in fig.9-12. If a rotating cylinder is put in the flow, then a lift develops. This principle could be used for the control of ship`s movement, but it is rather impractical and it was actually never used. The only practical applications known are those, where the rotating cylinder is used in combination with a conventional rudder (the cylinder is installed at the leading edge of the conventional rudder, as shown in fig. 9-12). The rotating cylinder could be activated in narrow passages, when manoeuvring in confined areas etc., otherwise the rudder is used as a conventional rudder.

In some ships (mainly tugs, coasters and fishing trawlers) a nozzle propeller is installed that considerably increases the thrust of the propeller in case it is highly loaded. The nozzle may be constructed as a rotatable nozzle, then the propeller race could be directed sideways which increases the effectiveness of the control.

Further development of this idea is an installation of a rotatable thruster, where the full propeller thrust could be directed to either side providing a very effective control.


Fig. 9-12

Fig. 9-13 shows the comparison of turning circles for the same ship fitted with different rudder arrangements. The data used for this diagram are from literature.

In the Iława Shiphandling Centre the turning characteristics of the model representing a Panamax size bulkcarrier (“SZCZECIN”), fitted with conventional rudder and with Schilling rudder were tested. The obtained results are shown in the table 9-1 and in figures 9-14 to 9- 16 for different rudder angles and for different ship approach speeds.




Fig. 9-13 Comparison of turning circles

Table 9-1. Panamax size bulkcarrier. Non dimensional advance and tactical diameter for different initial velocities and different rudder deflections (A/L_BP and D_T/L_BP)

Rudder deflection	Parameter	Conventional rudder		Schilling rudder
				5 knots	11 knots	5 knots	11 knots
	10^0	Advance	5.23	4.75	5.37	7.07
		Tactical Diameter	6.61	6.42	7.37	9.59
	35^0	Advance	2.63	2.57	2.45	2.75
		Tactical Diameter	3.28	3.05	2.85	3.18
	45^0	Advance	2.46	2.36	1.98	2.42
		Tactical Diameter	2.77	2.48	2.24	2.60
	70^0	Advance	3.46	3.23	2.49	2.58
		Tactical Diameter	5.81	3.70	2.47	2.20

Examples of trajectories realized by the model during turning trials are shown in figures 9-14 to 9- 16.





Fig. 9-14. Turning circles for conventional rudder and Schilling rudder. Velocity 5 knots







A/. B/.

Fig. 9-15. Comparison of trajectories of the model during turning circle for 35⁰ rudder deflection. Velocity 5 knots ( fig. A) and 11 knots ( fig. B)




Fig. 9-16. Comparisons of turning circles for 70⁰ of rudder deflection. Schilling rudder and conventional rudder. Velocity 5 knots.

.

5. Active control devices: Thrusters

The most popular active control device is a lateral thruster. The thruster develops a force perpendicular to the ship centreplane. There are many different thruster arrangements, some of them are shown in fig. 9-17. The most common type consists of a tunnel with a propeller fitted inside, which is put across the underwater part of the ship at the bow or at the stern.




Fig. 9-17 Thruster types

The force developed by the thruster should be sufficient enough to peform certain kinds of standard manoeuvres for all ships or specific manoeuvres for special types of ships. Usually the thrusters force is estimated taking as the design pattern the unmooring off the quay manoeuvre in a windy day or turning the ship without wind action.

In the first case the required thrust could be calculated using the empirical formula:


[Newtons]

where: A_W = windage area

V_W = wind velocity (assumed)

The other manoeuvre could be a turning the ship dead in the water using the bow thruster only. The required thrust for this operation could be calculated by the formula:


[Newtons]

where: ω_e = angular velocity (assumed), [deg/s]

The angular velocity ω_e could be taken from the diagram shown in fig.9-18.

Lips company recommended the required thrust of the bow thruster to be calculated with the formula:


[Newtons]

where: A_U = area of underwater profile of the ship.




Fig. 9-18

The effectiveness of the bow thruster depends on the forward speed of the ship. The water jet ejected from the thruster opening is directed perpendicularly to the ship's centreplane when the ship is at rest. When the ship has some forward speed then the water jet is deflected by the incomig water toward the hull side aft of the thruster opening. The jet sucks the water along the ship's side and this reduces the pressure on the ship's side, which considerably reduces the thrust force. The higher the ship's forward speed the more the jet approaches the ship's hull and the area of lowered pressure increases.

The deflection of the jet from the direction perpendicular to the centreplane of the ship depends on the ratio of the jet velocity to the ship speed, w/V. Fig. 9-19 shows the relative deflection of the jet as a function of the relative distance of the thruster from the bow for different values of this ratio.

In order to increase the thruster effectiveness it was proposed to install so called ANTI-SUCTION tunnel (AST). This is shown in fig. 9-20. The anti-suction tunnel is a simple pipe installed parallel to the thruster tunnel, behind and very closely to it. When the ship moves forwars the difference of pressure on both sides of the ship causes the flow through the AST toward the thruster outlet side. This increases back the pressure close to the ship's side. It also pushes the deflected jet further from the ship's hull, which further decreases the suction of the jet on the ship's side. These two factors increase the available thrust. This suction effect depends also on the form of the entrance of this AST pipe. In the diagram results achieved with two forms of the entrance are shown.




Fig.9-19




Fig. 9-20 Transverse thruster with and without Anti Suction Tunnel (AST)

When the ship is very close to a solid quay and she is unmooring using thrusters, the work of the thruster is affected by the wall. When mooring, the suction side of the thruster tunnel is farther from the wall, because of the inclination of ship sides, and the flow into the tunnel on this side is undisturbed.

When unmooring, the water jet ejected against the wall flows in various directions between the wall and the hull, forming vortices and zones of reduced pressure along the ship side (fig. 9-21). This generates suction force on the ship's side that is opposing the thrust force of the thruster. The net force pushing the ship away from the berth is therefore reduced. (see fig. 9-21)





Fig. 9-21

Figure 9-22 shows the characteristics of the thrust force of thrusters T_T and the turning moment N_T at various modes of operation, i.e at traversing and turning manoeuvres (Brix). From the diagram it is seen that a pure traversing manoeuvre (case A) with T_T > 0 and N_T = 0 could be realized at zero speed only. When the ship makes headway or sternway then with the travesing motion there occur also drift angle and yaw rate.

From case D, pure turning motion with T_T =0 and N_T >0 is not practicable when the ship speed is not equal zero.

Case A - Traversing, twin thrusters

Case B - Turning, single bow thruster

Case C - Turning, single stern thruster

Case D - Turning, twin thrusters














Fig. 9-22

Literature:

Brix, J. Manoeuvring technical manual. (Fig. 9-20, 9-22))

Gofman, A.D.: Dviżitelno-rulevoy complex i manevrirovanye sudna. (figs. 9-18, 9-19, 9-21)

Lewis, E.V.: Principles of naval architecture.

Nowicki, J: Ship turning performance for large rudder deflection angles, HYDMAN'05 (Fig. 9-14, 9-15, 9-16)




9-10

Chapter 9- Control Devices

lift

0 4 8 12 16 20 24

Volumetric displacement m³x 10³

0.2

0.4

0.6

0.8

ω_e

deg/s

D

4 8 12 16

x/D

12

8

4

y/D

w/V=

8

6

4

2

T_E

T_N

0

-0.4

-0.8

-1.2

-1.6

0.4

0.8

2 4 6 8 y/D

y

(T_E-T_N)/T_E

T_T

N_T

0

0

(-)

(+)

(+)

(-)

0

0

V

V

(-)

(+)

(-)

(+)

A

B

D

C

D

B

A

C

A

B

C

D

Jet

direction

PP

PP

PP

PP

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