The team at the bridge should be efficient enough to sail the ship in all kinds of waters and weather.
One of the natural factors about which every navigator should be very careful while steering a ship is – the wind.
Vessels such as Container and Ro-Ro ships have large freeboard and are thus more affected by winds. This exposed area of the ship is also known as windage area as the effect of wind is more prominent over it.
The wind effect on the same ship will be different at different places, depending upon the draught condition of the ship.
A wind with force of 3-4 on the Beaufort scale will have similar effect in light condition as with wind force of 7-8 when the ship is down to her marks.
When ship is at slow speeds during maneuvering or near to the coast, wind direction is easy to find; but this is not the case when out at high sea. The direction of the wind perceived when standing on deck is its relative direction. This is the resultant of the true direction of the wind and the course steered by the ship.
It is very important for the ship’s navigator to steer the ship considering the wind effects so that ship can be steered efficiently without any difficulty. Following are the techniques a navigator must master related to wind effects.
Ship underway with wind from right astern
When the wind is blowing from the right astern, steering the ship becomes easy; however, in the case of head wind, the stern part of the ship has the tendency to pay off on either side. This is a difficult situation to tackle and getting the ship back on course is no piece of cake.
Such effect is more often seen on ships where the accommodation area is at the aft region. Moreover, the wind in such case has no braking effect.
Note: Given a choice between head wind & wind from right astern, the head wind is preferred for berthing.
Ship underway with wind from abeam
When the ship is underway with the wind flowing from abeam, the steering of the ship is not affected. However, depending on the strength of the wind, the ship drifts sideways due to leeway and this has to be accounted for while handling the ship.
Ship underway with wind on the bow
Here again in lighter conditions, the effect on the ship’s stem is larger and this tends the ship’s head to swing away from the wind (leeward). This requires the weather helm (helm on the side of the wind) to be steered continuously.
Ship underway with wind on quarter
When the wind is pushing the ship’s stern away to leeward, the stern tends to swing towards the leeward. The ship is therefore steered towards the wind and the ship is required to be given a lee helm.
Vessel under sternway
When the ship is going astern, it rarely goes at a great speed. When going astern most ships also tend to swing to the starboard. The effect of the wind is therefore a little more complex.
In ballast condition where the wind catches the bow, which it often does, the stern is pulled into the wind. This effect is quite definite & rapid.
Note – This effect must be remembered while maneuvering for anchoring, berthing etc.
All ships turn around a pivoting point. This point is an imaginary reference and is fixed from observations of the ship turning around. It is known that when going astern the pivoting point moves aft.
Conclusion
Navigators can use the wind:
1. As a good brake
2. As a device for making a tight turn.
3. To maneuver comparatively easily as long as the wind remains about two to three points on the bow
The effects of ocean current
Ocean currents play a very important role in ensuring the stability of the ship.
The effect of currents therefore must also be considered when handling ships in waters.
Effects of current are important especially when the ship is under the effect of on shore winds, near off shore platforms, while maneuvering in narrow channels and open seas, or in inland waters or harbors. When the ship is in harbors or inland waters and the current is at constant strength and direction, the ship’s handling becomes considerably easier.
Such conditions exist only in comparatively narrow channels of the rivers.
However, navigational officers should take into account different current streams that can exist over a small area, within which the vessel has to maneuver.
The main different between currents and winds is that currents affect the ship in definite and predictable ways, unlike the wind does.
Even in open waters, when the ship is approaching a rig or a mooring buoy, due allowance should be made for the effect of the current for a safer maneuver.
Current from ship’s ahead will reduce the ship’s speed over ground, improve ships response to the rudder, and also give more time to assess and correct developing situations.
Shallow Water Effects on Ships – Ship Squat
When a ship proceeds through water, it pushes the water ahead. This volume of water returns down the sides and under the bottom of the ship. The streamlines of return flow are speeded up under the ship, causing a drop in the pressure and resulting in the ship dropping vertically in the water.
When the ship drops vertically in the water, it trims both forward and aft. This overall decrease in the static under keel clearance, both forward and aft, is called Ship’s Squat. Learn more about Ship’s Squat here.
If the ship moves forward at a greater speed in shallow water, where the keel clearance is 1.0 to 1.5 metres, then there are high chances of grounding at the bow or stern due to excessive squat.
What are the factors that govern Ship’s Squat?
The main factor on which the ship’s squat depends is the ship’s speed. Squat varies approximately with the speed squared.
The blockage factor “S” is another factor to be considered while understanding ship squat. The blockage factor is defined as the immersed cross-section of the ship’s mid-ship section divided by the cross-section of water within the canal or river.
The blockage factor ranges from about 8.25b for super tankers, to about 9.50b for general cargo ships, to about 11.25 ship-breadths for container ships.
The presence of another ship in a narrow river will also affect squat, so much so that squats can double in value as the ship pass or cross the other vessel.
How to find out if a ship has entered shallow water?
1. Wave generation from the bottom of the ship increases, especially at the forward end of the ship.
2. Ship becomes more sluggish to manoeuvre.
3. Draught indicators or echo-sounders will indicate changes in the end draughts
4. Propeller rpm indicator will show a decrease. If the ship is in “open water” conditions i.e. without breadth restrictions, this decrease may be up to 15% of the service rpm in deep water. If the ship is in confined channel, this decrease in rpm can be up to 20% of the service rpm.
5. There will be a drop in ship’s speed. If the ship is in open water conditions this decrease may be up to 35%. If the ship is in a confined channel such as a river or a canal then this decrease can be up to 75%.
6. The ship may start to vibrate suddenly. This is because of the water effects causing the natural hull frequency to become resonant with another frequency associated with the vessel.
7. Any rolling, pitching and heaving motions will be reduced as ship moves from deep water to shallow water conditions. This is because of the cushioning effects produced by the narrow layer of water under the bottom shell of the vessel.
8. The appearance of mud cloud will be visible in the water around the ship’s hull when the ship is passing over a raised shelf or a submerged wreck.
9. Turning Circle Diameter (TCD) increases. TCD in shallow water could increase 100%.
10. Stopping distances and stopping times increase, as compared to when a vessel is in deep waters.
11. Effectiveness of the rudder helm decreases.
Every vessel shows different characteristics when it comes to the distance covered when stop signal is given due to difference in dimensions, loading and ballast condition.
It is very important for a navigating officer to learn the principles of passage planning and understand his ship’s characteristics even as a small mistake in understanding may lead to collision, grounding or other kind of mishaps.
Stopping distance of ships
As we all know, ship like any other transport utility does not have brakes to make them stop immediately. When the engine is given stop order, the ship will continue moving in the same direction due to inertia and will come to stop after moving for some distance.
Every ship has two different stopping distances depending on:
- Inertia Stop
- Crash stop
As described above, when the engine of the ship is stopped, the ship will continue moving in the same direction for some more distance due to inertia. Here no astern command is given (used to produce “braking effect” for ships), and hence ship will travel more distance in the inertia stop method.
Crash Stop
Crash stop is usually the term used when the ship has to sudden stop in emergency situation. Here the engine, which is moving in an ahead direction is given an order for full astern, leaving the rudder in the mid ship position to stop the ship within minimum distance and shortest possible time. To know the complete procedure read for crash stopping read – crash manoeuvring.
In general operation i.e. berthing or departure of the ship from port or manoeuvring through channel or narrow passage, the above two methods are combined for a swift navigation of the ship i.e. in between giving an astern kick to stop and slowing down the ship’s speed for better manoeuvring.
The stopping distance data and chart is given in sea trials of the ship and made handy on bridge for reference. Every deck officer must refer this data to master the navigation of the ship.
The data may differ when used due to variation in weather condition, ships loading, stability and other factors; however, deck officers can compare the trail data and make use of it in practical situations.
Few Practical Examples
Depending upon the loading condition and the speed of the ship, the stopping time will be different when these two conditions are changed.
Also Ships fitted with Diesel machinery will have stopping distances approximately 70% of those fitted with Steam Turbine machinery.
When the ship’s hull has been due cleaning (dry dock) for longer time, the stopping distance and time will be less as compared to when the ship is just out of dry dock. This is because the hull resistance is more in ships with dry-dock done long ago.
The wind direction and sea condition also plays an important role as wind and waves acting from behind the ship will increase the stopping distance and vice versa.
It is important for a navigation officer to know the surrounding of its ship and how the ship will react to change in speed and loading condition. Only then he/she will be able to sail safe through all kinds of seas.
Berthing the ship using bow-thruster
Bow Thrusters are fitted normally in the fore and aft parts of the ship. These points of location help to create a turning effect and assist
the ship in changing the lateral direction during berthing or departing the jettyThe bow thruster is solely introduced in ships to avoid, or in better words, to minimize the use of expensive tug boats as most of the port state authorities around the world have compulsory requirements to use tug boats for safety purpose.
It is extremely important that the navigating officer on bridge understands the significance of assisting machinery such as bow thrusters during the most critical operation performed by ship and its staff – The berthing of the ship.
Following points must be considered while berthing the ship using bow-thruster:
- While using thrusters ensure that the ship’s speed in not more then 4 knots as above this the effect caused by thrusters would reduce. This happens because of the merging of the thruster stream with the general water flow on the side of the ship’s hull due to its forward movement.
- When using a single forward thruster, it is important to concentrate more on the astern as the bow can be controlled by the forward thruster. In such situation, always prioritize to berth or bring the astern of the ship alongside first and then control the ship’s bow.
- When turning the ship with two thrusters located at fore and astern, the pitch of the thrusters must be opposing each other, creating a turning moment. Massive cargo ships must be assisted by the tugs at the astern part to control the stern movement.
- When using the bow thruster while the ship is at stop, the astern part will act as a pivotal point. If the thruster is put in the port side, the ship will turn in the same direction.- When using the thruster with ship running headway, the thruster’s effect will be slightly less as both the pivotal and thrusting points are now in forward position.
- When using bow thruster with ship travelling sternway, the pivotal point will be the ship’s stern, which will turn in the same direction as that of the thruster and act as a rudder.
- It is very important to have an efficient steering gear system as the ship turning by use of thruster is highly dependent on how responsive the rudder (steering gear system) is.
- Thrusters are used while anchoring the ship. They assist in turning the bow of the ship away from falling anchor to avoid damage by the anchor chains.
Ship Navigation is more of an art which is developed through experience rather than knowledge. When you merge both experience and knowledge, you can definitely Master the Ship’s navigation technique.
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