Rule 6 - Safe speed

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Rule 6 - Safe speed Every vessel shall at all times proceed at a safe speed so that she can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions.

In determining a safe speed the following factors shall be among those taken into account:

(a) By all vessels:

i.  the state of visibility. ii.  the traffic density including concentrations of fishing vessels or

any other vessels; iii.  the manoeuvrability of the vessel with special reference to

stopping distance and turning ability in the prevailing conditions; (iv) at night the presence of background light such as from shore lights or from back scatter of her own lights;

iv.  the state of wind, sea and current, and the proximity of navigational hazards;

v.  the draught in relation to the available depth of water. 2

Rule 6 - Safe speed (b) Additionally, by vessels with operational radar:

i.  the characteristics, efficiency and limitations of the radar equipment;

ii.  any constraints imposed by the radar range scale in use; iii.  the effect on radar detection of the sea state, weather and other

sources of interference; iv.  the possibility that small vessels, ice and other floating objects

may not be detected by radar at an adequate range; v.  the number, location and movement of vessels detected by radar; vi.  the more exact assessment of the visibility that may be possible

when radar is used to determine the range of vessels or other objects in the vicinity.

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Rule 6 - Safe speed

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Rule 6 - Safe speed n  Safe speed

Difficult to define. In practice:

• A speed whereby proper and effective action can be taken to avoid collision

• A speed whereby the vessel can be stopped within a distance appropriate to the prevailing circumstances and conditions

It applies to: n  All vessels and at all times

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Rule 6 - Safe speed All vessels Power-driven vessels, sailing vessels, large and small vessels, fast and slow vessels, etc.

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Rule 6 - Safe speed At all times By day as well as at night −  In clear weather −  In restricted visibility −  In open seas −  In local waters −  Etc.

Thus: WHENEVER NECESSARY but ESPECIALLY −  In restricted visibility −  In areas of high traffic density −  Where frequent manoeuvres are required In accordance with the local prevailing circumstances and conditions

If speed is not reduced, put at least engines on “stand-by” © fimr.fi © fimr.fi

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Rule 6 - Safe speed By all vessels Rule 6 (a)

•  The state of visibility •  The traffic density •  Manoeuvrability of the vessel

n  Stopping distance n  Turning ability

•  Background lights •  The state of wind, sea and current •  The proximity of navigational hazards •  The draught in relation to the available depth of

water n  Interaction n  Squat

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Rule 6 - Safe speed Rule 6 - Safe speedRule 6 - Safe speedRule 6 - Safe speedRule 6 - Safe speedn  By all vessels

The State of Visibility Rule 6, § (a) (i)

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A rough rule for maintaining a safe speed could be the visibility of the sidelights:

§  Vessels of 50 m or more in length: 3 miles

§  Vessels between 12 m and 50 m in length: 2 miles

§  Vessels of less than 12 m in length: 1 mile Safe speed will also be influenced by the density of the visibility and the manoeuvrability of the vessel

The more the visibility is restricted, the more your speed should be restricted

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The more the visibility is restricted, the more your speed should be restricted 9

Rule 6 - Safe speed By all vessels

The state of visibility Rule 6, § (a) (i)

In certain circumstances the speed can be :

Too high: restricted visibility heavy traffic areas

Too low: steering may be difficult or impossible excessive drift, due to current and/or wind (especially in narrow channels)

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Rule 6 - Safe speed By all vessels

The Traffic Density Rule 6, § (a) (ii)

•  Light or heavy traffic •  Traffic separation schemes •  Number of fishing vessels in the vicinity •  The proximity of a convoy of warships •  Vessels at anchor •  Concentration of pleasure crafts •  Etc.

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Rule 6 - Safe speed By all vessels

The manoeuvrability of the vessel Rule 6, § (a) (iii)

•  The type of engine: steam, diesel, turbine, …. •  The power of the engine •  Fast or slow vessel •  Manoeuvrability of the vessel with regard to:

•  loading •  draught •  trim •  etc.

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Rule 6 - Safe speed

STOPPING DISTANCE

and

STOPPING TIME

By all vessels Rule 6 § (a)

(Rule 6, § (a) (iii))

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STOPPING DISTANCE - STOPPING TIME

n  The stopping distance is the distance that a vessel with her rudder amidships and her engines full ahead, will run from the moment her engines are put to full astern until she comes to a complete rest over the ground. The time taken to accomplish this is called stopping time

n  Stopping distance and stopping time must: •  Be expressed in Ship’s lengths (L) or m. and in min. and

secs •  Be clearly exposed on the bridge

n  The water resistance, at a constant speed is equal to the power of the engines and, as a rough estimate, that the water resistance is proportional to the square of the speed (V2) •  If the speed of a vessel, equal to 16 knots, is reduced to

8 knots only ¼ of the power is necessary to maintain that speed 14

STOPPING DISTANCE - STOPPING TIME n  Suppose a vessel with a speed of 16 knots with her engines

at the average power of 100%. The water resistance in that case is also equal to 100%. The engines are stopped and the vessel is continuing to move on her own inertia with her helm at midships.

n  When the vessel has slowed down to 8 knots the water resistance will be equal to 25% of the initial water resistance. When she reaches a speed of 4 knots, the water resistance will be equal to only 0,0625 % and at a speed of 2 knots, 0,015625% of the initial water resistance.

n  In other words, since R = V2 where R is the resistance of the water and V the speed in knots,

n  162 = 256 n  82 = 64 n  42 = 16 n  22 = 4 n  12 = 1 n  02 = 0

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STOPPING DISTANCE - STOPPING TIME

n  The Figure shows how drastically the resistance of the water drops with a decrease in speed. At a speed less than 8 knots the water resistance is so small that the vessel will continue to move on her own inertia for quite a long time.

n  If the engines are put to full astern instead of simply being stopped, the speed pattern will be quite different.

n  Suppose that a ship with an average speed of 14 knots is temporary moving at a speed of 7 knots (adverse weather, bad visibility, etc.), thus to half her speed. Her stopping time will, in that case, also be reduced to half its value. 16

STOPPING DISTANCE - STOPPING TIME n  GENERAL REMARKS

n  The stopping distance depends for a great deal on the proportion between the propeller power ahead and astern.

n  The power of a turbine steam engine, working astern is about 70 % of its power working ahead.

n  When applying astern propulsion to stop a ship, the ship may be considered as being stopped when the wake reaches the middle of the ship. Another way to check when a vessel is stopped is to observe the cooling water of the engines which must produce a stationary pool.

n  When the stopping time and the speed of a vessel are known, it is quite easy to determine the stopping distance.

n  When considering the stopping distance, take into account the distance ran from the time the chadburn is put on full astern and that the propeller actually starts to turn astern. The engineer is not always close to the manoeuvring board and a whole minute can elapse before the propeller actually turns in reverse direction.

n  Elements such as the wind, the state of the sea, the depth of water should be taken into account when considering the stopping distance and the stopping time.

n  Keep in mind that when astern power is applied, the vessel will not stay on her original course but the bow will turn either to starboard or to port depending on the type of propeller used. For instance, with a right hand fixed propeller, the stern will move to port and the bow to starboard. When the vessel has come to a complete rest, the vessel may well have turned over 90°. 17

Rule 6 - Safe speed

By all vessels

TURNING CIRCLE

Rule 6, § (a) (iii)

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TURNING CIRCLE n  The turning circle of a vessel is the circle the vessel will

describe when her helm is put, hard over starboard or hard over port, usually with her engines full ahead.

n  The determination of the turning circle of a vessel is normally carried out during the sea trials of the vessel prior to handover from the builders to the owners.

n  The turning circle, together with the stopping distance, are placed on board of the vessel in the trial papers, so that they can be consulted by the ship’s Master, the watch officers and eventually the pilots.

n  With regard to the turning circle the following statements are usually stated in the trial papers:

─ the advance of the vessel; ─ the transfer of the vessel; ─ the tactical diameter that the vessel scribes; ─ the final diameter that that the vessel has scribed.

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