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 14^th 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 18^th 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°.