Introduction
Stopping distance tells you how far a ship will travel after the order to stop is given. It is measured in the crash stop, or full astern, test. The officer of the watch must know it to avoid collisions and groundings and to plan an approach to a berth. It is a different subject from the turning characteristics of the ship, such as advance, transfer and tactical diameter.
Head Reach
Head reach is the distance covered from the point where the astern order is given to the point where the ship stops, measured along her initial course.
Track Reach
Track reach is the distance covered along the actual path from the point where the astern order is given to the position where the vessel stops.
Side Reach
Side reach (lateral deviation) is the sideways distance of the stopping point from the original course. It appears because the ship usually swings off her course while stopping, for example under the effect of the propeller.
If the ship stops in a straight line, head reach and track reach are the same. The more she swings, the longer the track reach is compared with the head reach.
Stages of a Crash Stop
After the order, the ship does not stop at once. The engine has to be stopped and then reversed, and the astern thrust takes time to build up. All this time the ship keeps moving ahead, and it all counts in the head reach.
Why Speed Matters
A simple model shows how strongly speed affects stopping distance. Suppose the astern thrust and resistance slow the ship at a steady rate a.
Step 1: For uniform deceleration from speed v to rest, v² = 2 × a × s.
Step 2: Solve for the distance s:
s = v² / (2a)
Step 3: The distance depends on v squared. Doubling the speed multiplies the distance by 4, and halving the speed divides it by 4.
Real ships do not slow at a perfectly steady rate, so the numbers are different. The trend is the same: stopping distance rises much faster than speed, so reduce speed early.
IMO Standard for Stopping
IMO Resolution MSC.137(76), Standards for Ship Manoeuvrability, says that in the full astern stopping test the track reach should not exceed 15 ship lengths. The Administration may modify this for ships of large displacement, but never above 20 ship lengths.
For a ship 200 m long, this means a track reach of at most 15 × 200 = 3000 m, or up to 20 × 200 = 4000 m where the larger limit is allowed.
What Changes the Stopping Distance
- Speed: the biggest factor, as shown above.
- Loading and displacement: a heavier ship carries more momentum and takes longer to stop.
- Engine and propeller: the time taken to stop the engine and develop astern thrust adds to the head reach. A controllable pitch propeller can usually change thrust faster than reversing a fixed-pitch propeller.
- Current: over the ground, the distance is longer with a following current and shorter against it.
- Wind and sea: a following wind and sea make stopping harder, and a head wind helps.
- Propeller effects: the stern tends to swing to one side when going astern, which creates side reach.
Practical Points
- Use the stopping figures on the wheelhouse poster and pilot card, and remember they apply to the trial condition shown on them.
- Start slowing down early and allow a margin on top of the figures.
- Remember the ship will not stop on her heading line, so allow for side reach.
- Check that the engine is ready for astern movements before entering confined waters.
Conclusion
Head reach and track reach tell you how much water your ship needs to come to a stop. They rise quickly with speed, depend on how fast the engine can go astern, and are never exactly along the heading line. Plan every approach with them in mind.
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