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The Voith Turbo Fin (VTF) A New System To Improve The Performance Of Escort Tractor Voith Tugs

Published online by Cambridge University Press:  23 August 2006

Santiago Iglesias Baniela  and
Enrique García Melón
Santiago Iglesias Baniela
Affiliation:
Universidad de La Coruña Email: sbaniela@udc.es
Enrique García Melón
Affiliation:
Universidad de La Laguna
Rights & Permissions [Opens in a new window]

Abstract

The geometry of the skeg in the Escort Tractor Voith tugs is the result of a series of intense investigations in the forms of the tug and its fins, oriented to get a significant improvement in the forces on the towing line when the indirect method is used in the escort towing. For that, and with the aim of getting the best behaviour of this fin, a variety of options have been investigated for years, evaluating its merits in terms of lift force and complexity to reach the present designs, which are adapted to the functions which the tug is destined to carry out. With the object of optimizing the lift force in the skeg when the indirect method is used in the escort towing, and after long investigations, the Voith Turbo Marine has incorporated a rotating cylinder at the leading edge to its design in escort towing for the first time at the beginning of 2005. The leading edge is the part over which the water flow first falls upon in normal escort operation conditions, calling this new development Voith Turbo Fin (VTF) to the system as a whole (skeg and rotating cylinder). This fin is analyzed in this article especially with regard to its basis, ways of operation and efficiency of the novel joining rotating cylinder.

Keywords

Escort Tractor Voith tugsSkegVTFMagnus Effect.
Type
Research Article
Information
The Journal of Navigation , Volume 59 , Issue 3 , September 2006 , pp. 505 - 519
Copyright
Copyright © The Royal Institute of Navigation 2006

1. INTRODUCTION

In this section we explain the basis used to provide the efficiency of the rotating cylinder used at the stern of the escort Tractor Voith tugs.

1.1. A brief history of studies on the “Magnus Effect” applicable to the VTF

The lift generated by a cylinder that spins in the middle of a fluid is called Magnus Effect Footnote 1. The Magnus Effect is a phenomenon discovered by the German physicochemical scientist Heinrich Gustav Magnus (1802–1870), an expert in aerodynamics who made experimental studies in 1852 on the aerodynamic forces generated by spinning spheres and cylindersFootnote 2. When the flow of a fluid such as air or water fell upon a static cylinder (also upon a sphere) perpendicularly, Magnus noticed that such flow was deflected in a steady way over both sides of the cylinder; but if the cylinder spun, the flow of the fluid on one side was helped by the cylinder’s spinning, while such flow decreased its speed on the other side. He proved that this effect created a difference of pressures that, putting into practice Bernoulli theorem, resulted in a thrust force in one directionFootnote 3.

Thus, in Figure 1, a cylinder can be viewed from above, spinning in the right side clockwise in a fluid like for example water, which falls perpendicularly upon it. Due to the cylinder spinning and to the friction of the water molecules on its surface, the laminar water flow which surrounds the profile near the cylinder becomes asymmetric. In this figure, it can be noticed that the laminar flow and the cylinder’s rotation act in the opposite direction at point A and in the same direction at point B; this gives rise to a speed difference between both points of the cylinder and, according to Bernoulli theorem, this means a difference of pressures, which creates a force perpendicular to the direction the water flow falls on (FM), from the highest pressure area (A) to the lowest one (B) due to the higher speed which the spinning cylinder confers it (B), this being called Magnus force “F M”.

Figure 1. Magnus Effect. Drawing: author.

All these studies related to the Magnus Effect suffered a radical change since the introduction of a new concept by Ludwig PrandtlFootnote 4 in 1904, which had to do with the behaviour of the laminar system of the boundary layer, applicable to the spinning object in this case. Thus, the most recent studies about this subject agree that the magnus force results from the asymmetric distortion of the boundary layer thickness caused by the combined effect of a spinning object and the flow incidence around it.

In the case of a sphere or a cylinder, the so-called whirlpool, or more accurately the circulation, does not consist of air set into rotation by friction with a spinning object. Actually, an object such as a sphere or a cylinder can impart a spinning motion to only a very thin layer and next to the surface. The really important thing is that the motion given to this layer affects the way the laminar flow separates from the surface in the rear. The separation of the fluid of this layer next to the cylinder is delayed on the side of the spinning object that is moving in the same direction as the free flow while the separation occurs too early on the side moving against the free flow. The creating wake then shifts towards the side of the cylinder moving against the free flow. As a result, flow past the object is deflected, and as a result of the change in the flow momentum, a force is created in the opposite direction upwards (shown in Figure 2).

Figure 2. Lift created by a spinning cylinder. Drawing: author.

1.2. The VTF and the Magnus Effect application

In the VTF, the rotating cylinder the water flow first falls on, develops a momentum in the boundary layer of the fluid increasing its speed in the active side of the same (let us remember that the Escort Tractor Voith tug always works skeg or stern first, that is, with its stern towards the escorted vessel). This higher speed will prevent or delay the stall from the skeg surface. Here, it is very important to emphasize that when the effect of a rotating cylinder is combined with a surface designed to create a high lift, as in the case of the skeg designed with this aim in the escort tractor Voith tugs, the fluid whirlpool has an effect, not only on the rotating cylinder, but also round the whole surface of the skeg and the cylinder; this has the result that the lift created increases while the drag decreases. This system may not be considered only due to the Magnus Effect, but also to the control of the flow movement in the whole VTF effect, as both one and the other create lift.

2. THE ROTATING CYLINDER AT THE LEADING EDGE OF THE SKEG (THE VOITH TURBO FIN)

2.1. Introduction

Traditional harbour towage requires Tons of Bollard Pull (TBP) for low speed operations (speed below 5 knots of the vessel to be assisted). The propeller thrust of each tug acts directly to place the assisted vessel, i.e. “Direct Mode”. At those slow speeds, in addition to the ship’s own rudder forces, the thrust of the propellers of the tug will be used for steering and braking the tow. With increased ship speed, the direct method cannot be used any more, as the side force necessary to keep the tug in position consumes a higher percentage of installed power and reduces the tow rope force and consequently, at a certain speed, the tow rope force drops to zero. For higher speeds the indirect method has been developed. Tug escort of ships requires Tons of Steering Pull (TSP) at higher speeds (5–12 knots). It is the underwater hull lines and special fin profile of the tug which are used to produce hydrodynamic forces, TSP, i.e. “Indirect Mode”. In this condition, the hydrodynamic lift from the tug’s hull and the highly efficient skeg will be used to develop a large force for transmitting through the rope into the tow. The thrust directly produced from the tug’s propeller is only used for manoeuvring. A definition of adequate tug escortFootnote 5 is therefore given as the tug’s ability to perform emergency steering of the escorted ship by certified TSP capabilities corresponding to that of the ship’s rudder force. In this case, the propulsion of the tug may be seen as a steering gear forcing the rudder, i.e. the tug’s hull and skeg, in the necessary angle of attack to generate the highest lift forces.

The use of the indirect method in the escort towing is therefore based on a combination between the Voith propellers (VSP) and the underwater hull design of the Voith tractor tug. The hydrodynamic forces created when the water flow falls on the underwater hull and the skeg Footnote 6, with the escort tug and the escorted vessel at a higher speed of about five knots, give rise to steering forces which can become twice its bollard pull at about ten knots of the escorted vessel. Not only does the skeg provide course stability and rolling damping effect to the escort tug, but it also causes the movement of the centre of lateral pressure towards the stern; thus, the arm between the VSP propellers and the said centre increases, and the hydrodynamic effect of the underwater hull also improves as a whole, when it is used as an “active rudder” in the indirect method of the escort towing. This helps to increase the tension over the towing line considerably. A special consideration in order to get the best tractor Voith tug designs for their specific working conditions has been focused in the improvement of lift generation but not drag. In order to improve its capability even more, Voith has developed the Voith Turbo Fin (VTF) with the aim of meeting the continuous demand of escort tug operators in order to get a versatile escort tug able to create higher steering forces by keeping the main dimensions of the Voith tractor tug as compact as possible, most preferably in the size of a typical harbour class tug.

The aim of the VTF design was thought not only to improve the Voith Tractor tug’s efficiency, but also to simplify the complete design of the tractor tug itself by reducing weights and building costs at the same timeFootnote 7. In order that the Voith Turbo Marine could get this improvement in the steering forces, numerous development steps were necessary to take the Voith Turbo Fin to market maturity. The research and development department employed modern numeric methods in this process, such as the Computational Fluid Dynamic (CFD) methods and the Finite Element Method (FEM). Furthermore, extensive model tests were carried out. With this aim, Voith adapted the well-known rotor or spinning cylinder to the typical skeg of the tractor Voith, which some conventional rudders have at the leading edge and that, in general, have been used to control the boundary layer on streamlined bodies with the result of considerably improved lift characteristics, since it gets its increase before the separation of the laminar flow. (See Figure 3).

Figure 3. Schematic flow conditions of a) a conventional fin b) a Voith Turbo Fin. VTF minimizes the turbulences of the incoming flow (b) in comparison with a tractor Voith tug without it (a). Source: Maritime Journal no 151, October, 2000, page 59, drawing: author.

In short, the VTF consists of a skeg typical of this kind of tugFootnote 8 able to generate a high lift when it presents an angle of attack related to the direction of the incoming flow and which has had a spinning cylinder set at its backFootnote 9. According to the German manufacturer Voith Scheneider Footnote 10, such a cylinder has a favourable influence on the incoming flow upon the tractor tug hull, in such a way that the VTF minimizes the negative influence of the water flow turbulences created as they fall on the skeg when the tug works with its stern towards the escorted vesselFootnote 11 “stern first” or “skeg first”, using the escort towing indirect method. By selectively altering the boundary layer of the laminar flow, the stall delays and, as a consequence, the steering force which the escort tug is able to create increases. As we know, when an escort towing uses this method, the forces over the towing line are created due to the incoming flow over the skeg and the rest of the underwater hull tug, while its propulsion system is only used to keep an effective relative position in relation with the water flow. In normal conditions, starting from about 32° in relation with the water flow, the separation of the fluid is produced, generating turbulences which limit the force generated over the towing line. With the VTF this separation of the fluid is produced at higher angles of attack. This supposes an increment of the hydrodynamic forces created and, as a consequence, of the forces over the towing line in such a way that, with similar dimensions to the typical harbour class tug, it can generate higher steering forces over the towing line when it carries out escort towing with the indirect method. Thus, a considerable versatility is achieved, which lets it carry out assisting manoeuvres both as a typical harbour class tug and a escort towing tug.

2.2. Design characteristics, operation mode and performance.Footnote 12

To drive the VTF we need an oil hydraulic circuit which can come from the oil hydraulic circuit of the vessel itself (in which case, that system controls both the pressure and the oil amount) or otherwise, Voith supplies a complete hydraulic circuit with the VTF, where an electric engine moves an electric pump, the one which moves the oil hydraulic engine. This pump can be activated from the main switch in the control desk after which, and in the case that the pitch is zero, both in the longitudinal and transversal direction, the oil pump spins without supplying oil to the circuit and, as a result, the VTF does not rotate. This spinning is produced during the escort towing manoeuvre only in the case that the angles of attack on the VTF are bigger than about 15° to 20° and the speed is higher than 4 knots. In order that this condition is fulfilled and the VTF works, the control levers must have a more than step 2 in “fin first” or “fin-ahead” longitudinal pitch and the transverse pitch of the wheel must also be bigger than 2 to port or starboard. The sense of rotation of the VTF depends on the turning of the direction of the wheel; that is, of the direction of the incoming flow over the skeg. (see Figure 4 where the same can be seen in each case). Thus, once the pump is activated from the main switch in the control desk, oil under pressure starts to flow through the circuit and the VTF starts to spin up to its full rpm Footnote 13 when the said condition is fulfilled. If the transverse pitch is changed (the steering wheel turns towards the other side for a bigger pitch than 2) the tug stern moves to the other side over the incoming flow, in which case, the oil pressure flow is reduced to zero and, as a consequence, the oil hydraulic circuit stops the VTF rotation and it starts again in the opposite direction until it reaches the full rpm in this new direction.

Figure 4. Tug in escort mode seen from above, showing the flow direction to the tug. Left side VTF anticlockwise turning; right side clockwise turning VTF to increase the steering forces. Source: Voith Turbo Marine, drawing: author.

Here, two effects are produced which facilitate the tension created over the towing line and, as a consequence, the steering forces created increase: on the one hand, an additional lift is created by virtue of the incoming flow cylinder’s rotation with the tug sailing at speed; and, on the other hand, there is a delay stalling, what means that the tug can present bigger angles of attack with regard to the incoming flow over its underwater hull.

Figure 5 provides a timing diagram basis needed for the hydraulic circuit design for the operation of the VTF when the tug carries out escort tasks, being necessary that the hydraulic motor has to be able to break the VTF from full rpm to still standing and to accelerate to full rpm in the opposite direction within 20 seconds.

Figure 5. Timing diagram for a typical operation of the VTF. Source: Voith Turbo Marine, drawing: author.

After exhaustive tests with models, the manufacturer Voith thinks that a tractor Voith tug, equipped with a VTF unit and at escort speed of 10 knots, is able to increase up to 18% the steering forces, comparing it with a tug which does not have itFootnote 14. (See Figure 6). Figure 7 represents a complete diagram of thrusts (what in the escort world slang is known as a “butterfly diagram”) at escort speeds of 8 and 10 knots in a Escort tractor Voith tug with a 70 tons bollard pull, where the increase in steering forces provided by VTF compared with a non-VTF equipped vessel can be seenFootnote 15.

Figure 6. Results of an escort indirect in the model tank with and without VTF. Source: Voith Turbo Marine, drawing: author.

Figure 7. Performance “butterfly diagram” of a 70 t Bollard Pull VWT escort tug for 8 and 10 kn with and without Voith Turbo Fin (VTF). Source: Voith Turbo Marine, drawing: author.

Although in theory there is the possibility of setting up a VTF, not only in new building units, but also by using a simple retrofitting modification, in tractor Voith tugs already built, in the last case, most existing escort tugs are inside the limits of their stability for the assisting manoeuvres they are to carry out especially when using the indirect method. Consequently, this means that if we add a tractor Voith tug of about 450 gts an extra weight of 20 tons, this theoretic possibility of setting up a VTF in this kind of tug which had not been initially designed to have it, becomes actually impossible. Figure 8 shows the rotating cylinder on VELOX.

Figure 8. Forefront, seen from stern, of the rotating cylinder of the Escort tractor tug VELOX recently built by Gondán Shipyards S.A. (Asturias-Spain) for the famous Norwegian operator Østensj⊘ Rederi AS. Photograph: author.

The gap between the VTF and the fin should be about 2·5% of the diameter of the VTF. And in this sense, some opinions have appeared which alert of the possible problems originated from the marine growth which could be created round it. With regard to this question, it is necessary to make two considerations:

  • ● There is certain flexibility in the higher and lower bearings of the cylinder axis to allow some movement there.

  • ● This risk would mainly come due to long intervals when the VTF had not started to work; that is why it is suggested that the cylinder starts rotating from time to time, supposing it is not frequently used for assisting manoeuvres.

Nevertheless, the main risk of damage in the VTF would come when touching some objects there, especially when the tug is pushing and touches some obstacles, that would provoke bending forces and impact forces that could damage it. It is perfectly natural that the hull forms aft over the waterline are designed in such a way that the possibility of contact decreases. However, it is beyond doubt that the risk has to be assumed, although it has to be taken into account that, at worst, the tug is still functional if the damage ever happens because it can make this kind of escort operation without the turbofinFootnote 16. The skeg dimensions depend on the particular design of each tug. The length of the fin profile varies from 20 to 37% of the length at the waterline of the tug, being higher, as natural, in the case of escort tugs. The maximum thickness of the fin should be of about 12% its average lengthFootnote 17. As a consequence of the systematic tests with models carried out by Voith, the diameter “d” of the cylinder, that was found to be more effective in order to create an increase of steering forces of about 18% at 10 knots ship speed, has to be of about 64% the maximum profile thickness over which it rotates, though it has been proved that a tolerance of +/−5% is acceptable without a great influence in its characteristics and capabilities. Model tests have proved that the lineal peripheral speed of the cylinder has to be twice the escort tug speed.Footnote 18 Taking this into consideration, the rpm at which the cylinder has to rotate, comes from the following formula: u=lineal speed of an exterior point of the cylinder=d∗π∗nr, where: “d” is the exterior diameter of the cylinder, and “nr” is the rotor rpm. As a consequence, knowing the relation u/v, the escort towing speed and the diameter of the cylinder, we can calculate the maximum rpm it has to rotate at in each case, in order to be more effective. Thus, the following rpm calculations were made in the escort tug “VELOX” Footnote 19, the first VTF equipped tug in the world, taking into account that the diameter of the cylinder is 0·8 metres (although not necessary for its calculation, it is 3.7 metres high):

u \equals 2 \ v \comma \ldquo v\rdquo\ {\rm being \ the \ vessel \ speed \ in} \ m \sol s {\rm \semi \ if \ we \ consider \ a} \ \ldquo v \rdquo \ {\rm of \ 10 \ knots \comma \ then \colon }
v \equals 10{{miles} \over {hour}} \cdot 1852 \cdot { {1} \ hour \over { {3600}\, s}} \equals 5\hskip-1\mdot\hskip-1 14\, m\sol\hskip-1 s
u \equals 2v \equals {\tf="Times-i" 10\hskip-1 \mdot\hskip-1 28} \equals \lpar external \ lineal \ speed \ of \ the \ cylinder\rpar
n_{r} \equals {{ 10\hskip-1 \mdot\hskip-1 28} \over { {0\hskip-1 \mdot\hskip-1 8}\pi }} \equals 4\hskip-1 \mdot\hskip-1 09 \ rps \times {{ {60} \ rpm}\over { 1}\ rps} \equals 245\hskip-1 \mdot\hskip-1 6\ rpm

The upper part of the leading edge with short transition to the hull is less effective and can be designed without the VTF. The typical length of the rotating cylinder is about 75% of its draft.

The hydraulic motor system with the coupling is installed after the rotor installation. The space above the VTF is connected to the inner part of the hull and the hydraulic motor can be inspected from the inner part of the vessel. The rotor is installed in the dry dock from the side by using a crane (See Figure 9). The hydraulic motor that is inside the hull is connected to the top of the shaft by means of a coupling and to the driving hydraulic circuit by means of a flexible pipe. The bearings can be either of a swivel joint roller type or of a friction type; in the first case, its lubrication is made by oil with a connection to a small oil tank placed high over the sea level. Each bearing has got its own oil tank. With regard to the inferior bearing, a pipe in the inner part of the rotor connects the tank with that bearing. In the second case, the lubrication of a friction type bearing is made by seawater. However, we do not get a complete hydrodynamic lubricating film which includes the whole rotor surface and this causes a steady abrasion in the bearings, which reduces its life cycle in such a way that it becomes necessary to change those bearings, applying preventive maintenance and after the working hours prearranged by the manufacturer. The rotor extremes are protected with a structural member like a “bumper”, designed in order to avoid a tangle of ropes to it.

Figure 9. Four phases of rotating cylinder installation at the leading edge of the fin of the escort Voith tractor tug “VELOX” at the Spanish shipyard Gondán, S.A. following the Voith Turbo Marine instructions. Photographs: author.

With regard to the escort tug “VELOX”, (See Figure 10) the VTF only needs 45 kW of added power to generate an additional increment of 30 t of steering forces over the towing line at 10 escort speed knots using the indirect method. With the cylinder turning, a stable flow is guaranteed. This, on the one hand, allows it to increase the lift generated by the incoming flow over the “skeg”; and on the other, it also allows it to delay the stall,Footnote 20 letting the angle of attack increase in relation to the water flow. This increase of the steering forces lets tugs be built cheaper and with smaller dimensions.

Figure 10. Photo during sea trials of the escort Voith tractor tug “VELOX”, the first tug in the world with VTF. Photograph: author.

3. CONCLUSIONS

The VTF provides versatility for escort tractor Voith tugs in such a way that it is possible to build tugs with smaller dimensions and consequently at a smaller cost. However, they are able to carry out escort towing with the increment of steering forces which they generate, whilst at the same time keeping the typical dimensions of the harbour class tug. This means that a tug itself may be effective for both functions and this option meets a demand from lots of tug operators. The significant improvement in the steering forces, the Tonnes of Steering Pull (TSP), which the VTF provides (about 18% when it carries out an escort towing using the indirect method at 10 knots) is an important contribution to the security increase when oil tankers are escorted on restricted waters. The assembly of a VTF in escort tractor Voith tugs already built is technically possible and relatively simple, but in most cases, the tug stability will probably limit this possibility of improvement in its services. Although the model tests have shown that to be effective, the peripheral speed of the rotor of the VTF “u” is in direct function to the escort speed “v” and Voith having empirically proved that this relation must be double (u=2v) in order to be more effective, it is plain that the cylinder should vary its rpm according to the escort speed. However, for the moment, Voith has decided that it is more suitable for those full rpm to be constant, fixing them to the corresponding 10 knots escort speed and consequently, as we have already seen, we only need the exterior diameter of the cylinder to calculate them, taking into account the known data. This decision is mainly based on the following reasoning:

  • ● 10 knots is the most usual escort speed in normal conditions.

  • ● It has been proved that there is not an important variation in the effectiveness of the VTF if the rpm of the cylinder are varied in order to adapt them to what experience has shown to be more effective in terms of speed (relation u=2v).

  • ● It is important that the VTF is as simple as possible and that the tug captain does not have his/her working tasks increased; that is why its functioning is thought to work automatically and with constant rpm. Thus, in no situation does he have to pay any attention to the VTF for rpm settings or adjustments, which will be a very important aspect considering the human factor.

The advantages that the VTF provides have their fundament in the Magnus effect although, as it deals with a system formed by a fin (the skeg) and the rotating cylinder, both designed to create high lift forces, the effect they produce may not be considered only due to the Magnus effect produced when the cylinder is rotating.

References

BIBLIOGRAPHY, REFERENCES AND END NOTES

1 However, before 1742, Robins showed that a rotating sphere was able to create a transverse aerodynamic force (that is why this phenomenon is also known as “Robin’s effect”).

2 The effect had already been mentioned by Isaac Newton in 1672 (apparently referred to the course of a ball and the effects which that course had when given a rotating movement) and investigated by Robins in 1742. Nevertheless, the first explanation to the work that Magnus carried out in 1852 about the lateral deflection of a spinning object in the middle of a fluid is due to Lord Rayleigh (1842–1919), one of the few aristocratic British figures who became an outstanding scientist. He proved that the force was proportional to the speed a fluid fell upon the sphere (or cylinder) and to its spinning speed.

3 SENGUPTA, Tapan K., TALL, Srikanth B. Talla. Robins-Magnus effect: a continuing saga. Current Science, vol. 86, NO.7, 10 April 2004, pp. 1033–1036.

4 (1874–1953), a lecturer at Gottingen University, considered the modern Fluid Mechanics’ father. According to his theory, a fluid with a high Reynolds number falling upon a rigid object has to subdivide into two different regions. The main part of the flow field may be considered non-viscous. However, there is always a narrow region near the rigid object where the fluid is mainly viscous. Prandtl called this region “boundary layer” and suggested that the stall is due to the behaviour of this layer.

5 In our opinion perhaps the best definition of the genuine concept of escort towing which establishes the basis criteria of it, is the one adopted by BANKS, GERRY: “… for active escort towage (tethered, i.e. with the towline made fast) it is the magnitude of the force that can be exerted in the towline al certain speeds of advance, and the ability to control the towline’s angular direction to offer effective steering or braking components of the towline force, to the escorted vessel …” [“Escort tug performance comparisons”, Ship & Boat International, Diciembre 1996, pp. 28–37 in p. 28 and Escort tug performance comparisons, ITS’96. The 14th International Tug & Salvage Convention and Exhibition (Seatle, USA). Complete papers and discussions. Thomas Reed Publications, Wiltshire, UK, 1996, pp. 139–162 in p. 139].

6 In the Anglo-Saxon terminology this fin is known as “skeg”; a literal Spanish translation adapted to the sea language could be “orza”. This is considered so, both by the form of the fin as by its function; however, in the tug slang and also in consulted works and reports in Spain this fin is called “quillón”, a word that does not appear in the Spanish Academy. But with the purpose of being consequent with the unstoppable tendency towards the adaptation of this word around the tug world in Spain, this term is adopted in Spanish to refer to the Anglo-Saxon word “skeg”, with the discussed reserves and always writing this word in italics. In our opinion, the most suitable definition is the one adopted in the publication of the American Marine “U.S. Towing Manual”, where the fin is defined as “A portion of the underwater hull with significant longitudinal and vertical dimensions but without appreciable transverse dimensions. Its purpose is to give directional stability to the hull” (this publication can be found at: www.supsalv.org/pdf/towman.pdf) and we should have to add, specially when we talk about escort tugs, the possibility of increasing its hull wet area which this fin gives them and as a consequence of creating hydrodynamic forces over the underwater hull transmitted to the towing line. On the other hand, it also moves the Centre of Lateral Pressure (CLP) aft.

7 BARTELS, JENS-ERK, JÜRGENS, DIRK. The Voith Scheneider Propeller. Current Applications and New Developments p. 17–20 www.dmkn.de/1779/technologie.nsf/7E4DD9BB54C9C462C1256FD90035E905/File/handbuch_der _werften_englisch.pdf

8 Although usually of smaller dimensions, the Tractor Z tugs also carry it (the name “Z” comes from the form the shafting line of the tug propellers adopts) and its shapes and way of working are similar to those of the Tractor Voith, but instead of having cycloidal Voith propellers, they have acimutal thrusters which some manufacturers like Schottel, Rolls-Royce, etc. commercialize.

9 Actually, the original idea of assembling a cylinder able to rotate at the leading edge of a conventional rudder with the aim of improving the manoeuvring capabilities of ships at slow speed, was considered and developed in the United Kingdom for the first time by the Ship Division of the National Physical Laboratory (N.P.L.). The fundament lies in succeeding to make the water flow speed created by the propeller and the peripheral speed of the cylinder rotation be always constant.

10 ANDERSON, NEAL, “Novel Propulsion Products Improve Safety”, Maritime Journal, October 2000, p. 59–60.Google Scholar

11 This relative position is not only the one adopted by this kind of tugs when using the indirect method, but also when using any other assistance method, either as a typical harbour classtug or as a escort tug.

12 DESIGN of the VOITH-TURBO-FIN. Design criteria, construction and solutions. Voith paper (unpublished) 2.78-1339 seh HGr 2003-06-17 REV.A pp. 14.

13 As we will see later, in the case of the escort tractor tug “VELOX”, the first one in the world to incorporate a VTF, the cylinder rotates to its full 245 rpm. Although for its best effectiveness, the tangential speed of any exterior point of the cylinder and consequently the rpm have to be a direct function of the escort speed, the fact that it is foreseen that it spins up at these constant rpm regardless the tug speed, is based on that a great variation of the rpm it spins up, does not give a big additional benefit. Voith decided this due to simplicity reasons as well as based on the investigation results from the model tests in a tank. Of course, certain flexibility would anyway be possible in order to adjust the rpm a little in terms of the tug speed; but it turns out to be quite complicated as well as too expensive; besides, incorporating this option could alter the tug Captain’s working tasks in some way, because he/she would have to handle the functions of one more control, which in normal working conditions does not need to watch as it works automatically.

14 BARTELS JENS-ERK and JÜRGENS, DIRK “Latest Developments in Voith Schneider Propulsion Systems” in ITS 2004. The 18th International Tug & Salvage Convention and Exhibition (Miami-USA). Complete papers and discussions. The ABR Company Ltd., Wiltshire, UK, 2004, p. 155–157.

15 Voith brochure “Voith Water Tractor” [http://www.voithturbo.de/applications/documents (document file 792, p. 12 – Dynamic ship assistance and escort work-)]

16 To avoid any damage of the rotor, some basic design rules are selected. Thus, the rotor and the bearings can withstand the bollard pull of the tug boat; the deflection of the load with the bollard pull should be within the amount of the gap, the bearings have to withstand the bollard pull and the weight of the rotor in a dry dock as the buoyancy lift in the ship.

17 In most cases, the length of the fin differs between the upper end and the bottom line; that is the cause of the reference to the average length as a basis of its thickness.

18 This relationship between the lineal speed of the cylinder “u” and the vessel speed “v” differs, in the case of a Tractor Voith tug with a VTF, from the one shown in different publications when referring to conventional rudders with a spinning cylinder at the leading edge as, according to the results obtained in different tests, they stand out that the best lineal speed of the cylinder in m/s that provides a bigger increase of the lift has to be 3 to 4 times the vessel speed in m/s [vid. in this sense for example, Brix, J. (editor) (1993), Manoeuvring Technical Manual. Seehafen Verlag GmbH, Hamburh, p. 140–141]; even the Voith manufacturer himself considered this relationship factor right at first, surely starting from this studies, researches, etc. vid. in this sense BARTELS, JENS-ERK, JÜRGENS, DIRK. Voith. Current applications and new developments, where he states (p.19): “… to get the best results from the VTF, the velocity of the cylinder’s circumference should be amount 4 times the ship’s speed” (vid. file in web p: http://www.dmkn.de/1779/technologie.nsf/7E4DD9BB54C9C462C1256FD90035E905/$File/handbuch-der-werfen-englisch.pdf). However, as it was mentioned before, it is stated in the VTF manuals edited (not published) later by Voith “… The model tests have shown to be effective; the peripheral speed of the rotor U rotor of the VTF should be double of the ships escort speed Vship escort”.

19 It has been built in Spain by Gondán Shipyards, S.A., Castropol (Asturias) and designed by the famous Canadian naval architect firm Robert Allan Ltd. (It belongs to the kind AVT 37/65, where Carl Johan Amundsen, the shipowner’s technical representative and Voith, the German manufacturer of the propellers and the VTF rotating cylinder have cooperated). It was delivered to the famous Norwegian shipowner Østensj⊘ Rederi AS from Hausegund. Nowadays, the tug is working in the Norwegian Terminal of Norsk Hydro in Sture.

20 When he uses the indirect method in the escort towing, the stall in a Tractor Voith tug takes place when the angle of attack is of about 32°.