Ahead Movement of the Propeller
The effect of transverse thrust whilst making an ahead movement is arguably less worrying than that of an astern movement, perhaps because the result is less noticeable.
(a) The helical discharge from the propeller creates a larger pressure on the port side of the rudder.
(b) A slight upward flow from the hull into the propeller area puts slightly more pressure onto the down sweeping propeller blades.
(c) It was evident during the tests that the speed or flow of water into the propeller area is uneven in velocity.
The net result is a tendency for a right handed propeller to give a small swing to port when running ahead. Whilst this may be noticeable in calm and near perfect conditions it is easily influenced by other likely factors such as wind, current, shallow water, tugs, rudder errors and so on.
Astern Movement of the Propeller
The importance of transverse thrust when using an astern movement is of much greater significance to the ship handler. The helical discharge, or flow, from a right handed propeller working astern splits and passes forward towards either side of the hull. In doing so it behaves quite differently. On the port quarter it is inclined down and away from the hull whilst on the starboard quarter it is directed up and on to the hull. This flow of water striking the starboard quarter can be a substantial force in tonnes that is capable of swinging the stern to port giving the classic ‘Kick Round’ or ‘Cut’ of the bow to starboard.
Force in Tonnes
Mainly a function of water flow, the transverse thrust can be increased or decreased by varying propeller rpm. This in turn varies the magnitude of the force in tonnes applied to the quarter and it can be viewed clinically as one of the forces available to the ship handler in much the same manner as rudder, tug or bow thruster forces. It is, however, a weak force and can be roughly calculated if the shp of a particular ship is known.
For example let us take a ship of 80,000 dwt with a full ahead of 20,000 shp. If full astern is only 50% of this then it only has a maximum of 10,000 shp astern.
For practical purposes it can be taken as a rough guide that transverse thrust is only 5 to 10% of the applied stern power therefore in this case at best a force of 1,000 shp or 10 tonnes. (100 shp appx 1 tonne)
Whilst shaft horsepower is an important factor in determining the magnitude of transverse thrust and how much a ship will cut when going astern a further consideration must be the position of the pivot point.
Pivot Point & Transverse Thrust
Vessel making Headway
Looking at another ship, this time of 26,000 dwt with a maximum of 6,000 shp astern, it can be seen that shp relates to approximately 6 tonnes of force on the starboard quarter. When the ship is making slow enough headway for the propeller wash to reach the hull, it is acting upon a pivot point that is forward and thus a turning lever of 110 metres. This creates a substantial turning moment of 660 tonne-metres.
The forward speed of the ship must be considered because at higher speeds the full force of propeller wash will not be striking the quarter. As the ship progressively comes down to lower speeds and with the pivot point still forward, the magnitude of transverse thrust will slowly increase reaching its peak just prior to the ship being completely stopped. It is an unfortunate fact of life that at the slower speeds approaching a berth, if stern power is applied, transverse thrust is likely to be at its maximum!
Vessel Making Sternway
With the same ship making sternway the pivot point will now move to a new position somewhere aft of amidships. With the propeller working astern the flow of water on to the starboard quarter is still maintaining its magnitude as a force of 6 tonnes but is now applied to a reduced turning lever of 40 metres. Unlike the situation with headway we now have a reduced turning moment of 240 tonne-metres with sternway.
In the first instance this may not seem strikingly important. It must be remembered, however, that transverse thrust may be a poor force in comparison to other forces such as wind and tide. With the example of sternway, a wind acting forward of the pivot force may be strong enough to overcome that of transverse thrust. This will be investigated more thoroughly in a later section concerning effect of wind.
Rudder Force and Pivot Point
We will start with a ship of 67,000 t displacement, stopped dead in the water assuming even keel, calm conditions and no tide. With the rudder hard to starboard, an ahead movement is now applied and for the moment it is academic whether it is dead slow, slow, half or full. This we can refer to simply as The Rudder Force’.
This will be attempting to both turn the ship and drive it forward.
Forward movement is initially resisted because of the inertia of the ship while the turn, which is working at the end of the ship on a good lever, sets in slightly earlier. This results in a pivot point which is initially well forward and approximately 1/8L (P) from the bow. The importance of this is absolutely vital because at this stage, with the ship just beginning to make headway and the pivot point well forward, we have the optimum rudder force.
It will never be better!
When the ship thereafter begins to build up speed, the water resistance ahead of the ship balances forward power and pushes the pivot point back a further 1/4 L . At a steady speed, whilst turning, the final position of the pivot point will now be approximately 1/3 L (PP) from the bow. With the turning lever thus reduced the rudder force has now become progressively less efficient.