Voith Water Tractors for Making Fast Forward
Safety concept for ship handling vessels
By Captain Yves Le Chevalier, Voith Tug master
Connecting the towline forward is a maneuver that requires maximum concentration from the tug master. During this critical phase, several important factors must be taken into consideration: the concept of the tug, the ship’s speed, the size of the assisted vessel, the weather conditions and the skill of the tug master. Unfortunately, the majority of serious accidents occur when making fast forward.
Today, VWTs (Voith Water Tractors) and ASDs (azimuth stern drives) are the most frequently used tugs for harbor operations. During my 20 years of experience on board a VWT in Marseilles / Fos port, I have never seen an accident with a VWT while the towline was connected forward.
The main reason for this is the safety concept of the Voith Water Tractor, which was discovered in 1950 by the graduate engineer Wolfgang Baer (1913-1975). (1) While factors such as towline pull, speed and other features are important for a tug, the decisive starting points for Baer's considerations were maximum maneuverability and reliability in ship handling.
The Voith Water Tractor is a tug which, has the Voith Schneider Propeller (VSP) fitted in the forward section, rather than at the stern, while the towing hook is astern.
With this concept, the tug maintains its full steering capacity at all times and eliminates the risk of accidents that might occur with vessels that are steered from the stern, or when taking on the bow line of the incoming ship, which might lead to serious accidents.
The main concept developed by Wolfgang Baer in the early 1950s has remained unchanged. Due to changing requirements, the outputs have, however, risen from 550 kW for "Stier", the first VWT, to approximately 7.800 kW today.
The VWT is operated from the control stand in the wheelhouse. Depending on the size of the vessel, one or several control stands can be installed. A wheel adjusts the magnitude of the transverse thrust of both propellers, and two levers adjust the axial thrust of each propeller. It is therefore possible to accurately and easily control the direction and the magnitude of the thrust during ship handling operations in the harbor.
The Voith Water Tractor has this particularity, that is to say it is the one and only vessel which can be steered by moving the bow and not the stern like with a conventional vessel : the wheel moves the bow and the levers move the stern. This characteristic is very important especially for connecting the tow line forward of incoming vessel. Indeed, thanks to the concept of the tractor in case of emergency it is always possible to escape from the bow by turning the wheel without having the risk to touch the assisted ship because we move the bow and not the stern.
VWT control stand : human-engineered control
In such conditions, the tug master can fully concentrate on the maneuver and is able to operate safely over the bow, over the stern, or alongside for pull-push without any difficulties due to human-engineered control.
Make fast forward
When we make fast forward, we have to consider two main phases, firstly the approach to the bow for connecting the tow line, and secondly, as soon as the tow line is fastened, the application of a steering force when the ship is running ahead.
a) During the first step, the tug is approaching the bow of a container vessel, large tanker, gas carrier or war ship. (2) It is well known that the efficiency of a bow tug is small and decreases with rising speed. One reason for this is the power consumed to overcome the tug resistance by holding the speed. A further reason is the magnitude of lever between the attacking point of the towrope force and the attacking point of hydrodynamic transverse forces.
To take the heaving line, the tug comes close to the tow's bow. This results in a critical hydrodynamic interaction. Additionally, the towrope force may be critical with regard to the tug's stability. Extensive model analyses of the forces and moments between the tug and the tow have resulted in one of the basic features of the Voith Water Tractor, with direct control ahead of the pivot point of the vessel to overcome this classic safety risk in ship handling.
Interacting forces and moments
M = momentum
Q = transverse force
Making fast forward with a VWT in order to minimize the risk during this critical phase:
1 – Approach the vessel not too closely to the hull, in order to avoid turbulences, critical hydrodynamic interaction forces and low and high pressure areas. In such a position it is possible to safely overtake the vessel.
2 – Check the speed and determine the right position, in order to be ready for approaching the bow of the vessel and picking up the heaving line. In this position, the tractor should be able to hold the same speed and the same course as the ship on pitch levers ahead about 75 %.
3 – Carefully approach the bow parallel to the vessel, using port wheel and port lever, for picking up the heaving line and fasten the tow rope.
4 – Move further away from the ship, take the messenger line, make fast the tow line and adjust the length of the line.
Due to the forward position of the VSP, the propellers are always working in free water, outside of turbulences. The course of the tug can therefore always be perfectly stabilized. In case of an emergency, it is possible to quickly move away from the bow of the vessel by using the wheel in order to avoid the risk of collision.
b) During the second step, the tow line is fastened and the tug is ready to receive orders from pilot .
If it is necessary to pull on port side while the ship is running, the VWT is able to stabilize its position with the propellers operating, despite the hydrodynamic forces of the incoming water flow due to the location of the tow point, which is just above the fin and the VSP and fitted in the forward section. Thanks to this concept, when the tow line is fastened and the VWT is pulling, the tug can naturally turn around its pivoting point.
Depending on the ship’s speed, the VWT has to adjust its position, which is a stable equilibrium between the ship’s course and the direction of the tow line, in order to apply an efficient steering force.
If the hydraulic force of the incoming water flow becomes too large due to ship’s speed increasing or the ship’s course changing, it is necessary to pay out the tow line, turn the wheel to starboard and increase the speed, in order to prevent the ship from overtaking the tug.
Whatever the weather conditions, the VWT is always in the right position for picking up the heaving line from the bow. The speed safety limit is around 75% of the main engine power, i.e. there is a 25% safety margin.
For releasing the towline it is necessary to firstly decrease the speed slightly to slack the towline, and then hold position in front of the bow at same speed as the ship. When the towline is released and drops into the water, the levers must be pushed full ahead, and the wheel must be turned to ensure safe conditions for heaving the line. The safest speed is that used for making fast forward.
As far as the nautical training of tug masters is concerned, depending on the ship movements it is possible to train one or two trainee tug master in one week, including a VWT presentation, basic maneuvers and a ship assistance maneuver.
In view of the risks involved when connecting the towline to the bow of an incoming vessel, it is absolutely necessary to have a safe tug for such a maneuver, i.e. a Voith Water Tractor.
Considering the safety limits for making fast forward with a Voith Water Tractor, we can say the limit is only the ship’s speed of the incoming vessel without taking into account the weather conditions or the tug design. Indeed, for making fast forward the ship’s speed of the incoming vessel must not be above the safe speed of the VWT during the procedure of approaching the bow, i.e. 75 % of the main engine power.
The Voith Water Tractor is the true concept of the tractor tug, and it always operates safely and in right position. The VWT capabilities do not change, whatever the weather conditions, waves or wind, and it has received the classification "Best Available Technology" (BAT) by the Department of Environmental Conservation in Alaska, confirming the success of this design over many years.
(1) The Fascination of the Voith-Schneider Propeller, by Dr. Brigit Jürgens
(2) Voith Water Tractor – The Hallmark of Improved Ship Safety, by Dr. Jens-Erk Bartels
Voith Turbo, the specialist for hydrodynamic drive, coupling and braking systems for road, rail and industrial applications, as well as for ship propulsion systems, is a Group Division of Voith AG.
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