Transverse thrust for a right handed propeller is a tendency to give a small push to starboard of the stern part of vessel, when running ahead. On astern propulsion, it is a push felt towards the port side by the stern part. This tendency is clearly noticeable in calm and near perfect conditions. It can be influenced by other likely factors such as wind, ship’s speed, shallow water, etc. The effect of transverse thrust, whilst making an ahead movement is less pronounced than when making an astern movement. Various theories such as lower part of sea being more dense than the upper layer of water, have been ruled out. This effect may though be felt in shallow water areas, where the lower layers of water have suspended sand particles. If the propeller is half submerged, it is clearly the paddling effect. As if the width of propeller blades (pitch of the propeller) must be rowing through the water. This effect is also called propeller walk. On the ‘Ahead movement mode’ of a right handed propeller.
a) It is experimentally proven fact that the speed or flow of water into the propeller area is uneven in magnitude.
b) The angular thrusts on port and starboard side can be resolved in athwartship and fore and aft components. Fore and aft components add up to give speed to ship. The athwartship components are unequal and hence, create a bias. The hull shape forward of the propeller is not same, considering the upper and the lower halves. This means the blades, while going up will throw water aft with different intensity as compared to the throw when coming down. The effect is maximum, when starting from rest because at this time the pivot point is right forward as the ship tends to break in to the water ahead. The pivot point moves aft to about 1/3rd the length from forward when she is making way.
On the ‘Astern Movement mode’, 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. A stronger flow to the starboard side is responsible for slewing the stern to port as a reaction to the flow of water. When starting from rest the pivot point may be assumed to be amidships providing reasonable lever to turn the vessel. The lever becomes much smaller as the pivot moves aft close to stern. Thus, it is seen in both viz. ahead and astern propulsion that the transverse thrust is maximum when starting from rest. Suppose a ship of 60,000 dwt with a full ahead has 15,000 SHP and in full astern mode SHP is only 50% of this, then it only has a maximum of 7500 SHP when going astern. For all 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 750 SHP or 7.5 tonnes is available. (100 SHP is approx 1 tonne). The position of the pivot point is an important consideration as the lever will increase or decrease due to the shifting of pivot point.
Transverse thrust! Good or bad?
- Transverse thrust is often used by Master and pilots to their advantage. A commonly used procedure while anchoring on port anchor is to stop the ahead motion of ship by giving a stern movement, getting the bows to swing to starboard at the same time. This helps the laying of cable gently on the seabed as the vessel drops astern.
- Transverse thrust is often used to help bring the ship’s stern alongside during berthing. When a propeller is put astern on a ship moving forward at speed, the initial effect of transverse thrust is slight. However, as the ship’s forward motion decreases, the effect of transverse thrust will increase slightly.
- It is essential for a Master to understand, how much effect, the transverse thrust has on his ship. He should also be aware on how the traverse effect can vary or change due to the current and depth of water.
- At berth, the pilots use the property of transverse thrust to haul the bow or the stern in, without causing a fore and aft movement. A pilot thus, may give a stern movement but the moment the engines are heard, a stop movement is usually given. This would cause a quick swing without fore and aft shift in position. The stop movement however, must not be delayed.
- In ‘turning short round’, a series of dead slow astern movements with rudder amidships and dead slow ahead movements with rudder hard on starboard, turns the vessel to starboard in a limited basin.
- In case of twin screw propellers, the transverse thrust effects of the two propellers add up with the off centre effect to give a sharp turn.
The propellers of a twin screw vessel turn outward as seen from astern of the vessel, as she goes ahead. This means on the port side there is left handed whereas on the starboard side there is right handed propeller. This is because of two reasons:
- In ahead or astern mode, the transverse thrusts will cancel. This will happen even if the placement of propellers was other way round.
- Each propeller being not on the centerline has off-centre turning effect when operating. Thus, while turning to starboard the left handed propeller is on the ahead mode whereas the right handed propeller is on the astern mode. This causes the transverse thrust effects and offcentre effects to add up. The result being a brisk turn to starboard.
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