Stopping Distance
Stopping distance is the distance, which, a ship will continue to move after action is taken to stop engines and till the ship comes to rest. Stopping distance details may be provided for sea speed, harbour speed, half speed etc. It may be provided for loaded & light conditions.
Nowadays the precise course & speed over ground may be read directly on a GPS receiver. A navigator makes the best use of this facility. A time speed graph may be prepared for ‘stop manoeuvre. During an ‘Inertia or simple stop’ manoeuvre carried out in open sea it may be necessary to take into account the currents, if any, as the current might continue to carry the ship.
Thus a student might Wonder as to ‘why GPS is still showing, say ‘2 knot speed the while Water alongside may appear to be still with respect to the ship. Out at sea. ship is considered stopped when she is stopped w.r.t water in which she is floating. The direction and the motion shown by GPS at that moment are the set and drift the current. In harbour, a stop as desired by a pilot might mean stop w.r.t ground wharf, or w.r.t a ship alongside Which your ship is to be double banked. A student must understand that there is no standard stopping distance, which is true for all ships.Following observations are important in relations to stopping distance.
1. A ship in loaded condition will take longer to stop then in light condition.
2. A smooth hull vessel will take longer to stop than a fouled hull vessel for a given speed.
3. A loaded tanker or bulk carrier might have to be given stopping manoeuvre well in advance both time wise as well as distance wise. it may not be required on a medium sized general cargo vessel in light condition.
4. A light draft vessel, with lot of windage area is influenced a lot by prevailing wind conditions.
5. A vessel of fine form will take longer to stop as compared with a buff-formed vessel of similar draft length.
Inertia Stop
Headway of a ship going ahead is reduced to a stop merely by cutting of the fuel without reversing the propulsion. Stopping Distance may be associated with the speed at which the vessel was advancing at the time of the ‘stop’ order & varies from 10% to 50% of speed in knots represented in nautical miles.
Table showing stopping distance for different type of vessels.
| Type Of Ship | Speed | Stopping Distance | Influence of Wind |
|---|---|---|---|
|
Fine line and fast ship |
10 |
1 nm |
Strong |
|
Cargo Vessel 145 meters |
10 |
2 nm |
Strong Loaded |
|
Loaded tanker, Displacement 2,20,000 tons |
10 |
4.5 nm |
Less as compared to fine line and fast ship |
Crash Stop
In crash stop manoeuvre the ship is stopped by applying astern power. On a diesel ship it will be done in 3 stages.
a) Fuel is cut off to main engines.
b) In spite of fuel cut off, propeller continues to turn due to headway of the ship. Hence, compressed air is used to actually stop the engines.
c) Engines are reversed using maximum power. Rudder normally being kept amidships.
Head Reach: Is the total distance travelled in original direction.
Track Reach: Is the total distance travelled along the actual path followed by the ship. An idea of distance travelled & time taken can be achieved by studying following cases:
| Ship Type | Track reach | Time to Stop |
|---|---|---|
|
Fine and fast ship |
5 Ship's length |
1.5 to 2 minutes |
|
Cargo ships loaded, displacement 28000 tonnes L 145 m |
11-13 ship's length |
9-11 minutes |
|
Loaded bulk carrier and tanker |
15-20 ship's length |
20-25 minutes |
Turning Circles
Turning Circles When a rudder is put hard over (35 degrees normally) to port or starboard side, after a short interval the vessel begins to follow a curved path towards the side on which the helm is applied.
Radius of curve keeps reducing & by the time ship’s head is 90 degrees away from original, a steady radius of turn is reached.
Following terms must be understood in the turning circle.
Advance is the distance travelled by a ship’s centre of gravity in original direction a Navigation a measured from the point where the helm was applied.
Transfer is the lateral distance of the centre of gravity from original course line or it is the effective distance travelled by centre of gravity of the ship, in a direction 90° from original direction measured when the ship’s head is 90 from original heading.
Tactical Diameter is the lateral distance as above, if measured while the ship’s head is 180 from original heading
Drift Angle is the angle between ships fore and aft line & the tangent to turning circle at any given moment.
Pivot Point is a point about which a ship pivots in a turning circle. This point is approximately 30% of length from forward when steaming ahead and about 20% – 25% of length from the stern when the ship is going astern.
Observations (Turning Circle)
a) Ships head is inwards by an angle equal to drift angle.
b) Tactical diameter is less than & not equal to maximum transfer.
c) Advance is approximately equal to 3 to 4 ship lengths but may be 5 ship lengths for faster ships.
d) Tactical diameter is approximately equal to 4-5 ship lengths.
e) Quicker turn is possible at higher speeds time wise but diameter of turn will not vary in that proportion.
f) Turning circle to port may be slightly smaller as compared to starboard turning circle.
g) In a turning circle manoeuvre the ship loses 25% of original speed in first quarter.
h) In the next quarter, there is further loss of speed, reaching 55% to 65% of original speed. There after the speed remains steady. Time taken to complete a turn is 8 to 10 minutes for average sized cargo vessel & 14 to 26 minutes for large vessels