TURNING CIRCLE n  The Figure shows the turning circle to starboard and to port of an

average vessel of abt. 10.000 tons and 20 knots speed, in the open sea with a draught of about 5 meters and a depth of water of about 100 metres.

© 2007 David House, Ship Handling, Elsevier Ltd.

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TURNING CIRCLE DEFINITIONS

Advance The advance is the distance the vessel has travelled in the direction of the original course from commencing the turn to completing the turn. The heading of the vessel has passed through 90° to starboard (starboard turn) or to port (port turn).

Transfer The transfer is the distance the vessel has moved perpendicular to the fore and aft line from the commencement of the turn (either to starboard or port).

Tactical diameter The tactical diameter is the greatest diameter sailed by the vessel from commencing to completing the turning circle.

Final diameter The final diameter is the internal diameter of the turning circle where no allowance has been made for the decreasing curvature as experienced with the tactical diameter.

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Turning circles off: Full bodied vessel and Slender ship

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TURNING CIRCLE GENERAL REMARKS

n  The turning circle conducted in shallow waters will be considerably increased compared with a turning circle conducted in deep water.

n  Turning a vessel with her helm hard over will cause the vessel’s speed to decrease considerably.

n  A deep laden vessel performing a turning circle (e.g. in case of a man overboard) will experience less effect from the wind or sea condition than in light ballast condition.

n  A vessel trimmed by the stern will generally steer more easily but the tactical diameter of the turn will be expected to decrease.

n  A vessel trimmed by the head will decrease the size of the turn but will be more difficult to steer.

n  A vessel conducting a turning circle with a list could normally be delayed.

n  Turning towards a list would normally generate a larger turning circle.

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TURNING CIRCLE GENERAL REMARKS

n  Turning away from a list would normally generate a smaller turning circle.

n  A vessel tends to heel towards the direction of turn once helm is applied.

n  A vessel turning with an existing list and not being in an upright position could in shallow waters experience an increase in draught.

n  The type of rudder can have an influence on the turning circle of a vessel.

n  A narrow beam vessel will normally make a tighter turning circle than a wide beam circle.

n  A vessel equipped with a right hand fixed propeller would normally turn tighter to port than to starboard.

n  Elements such as the draught, trim, length of the vessel, power of the engines, type of rudder, the propeller system, distribution and stowage of the cargo, available depth of water can also be influencing factors on the turning circle of the vessel.

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TURNING CIRCLE

n  The diameter of the turning circle is equal to about 4 ship’s lengths (4L).

n  In position 1, the helm is put hard to starboard and the vessel will first move to port of her initial course. The vessel also starts to turn to starboard. Due to the position of her turning point (at about ¼ from the bow), the bow will hardly be moving inside the initial course but the rear of the vessel will swing to port. Only in position 4, after 4 ship’s lengths on the initial course, the rear of the vessel will start to move to the inside of the initial course. In position 5 the ship’s course will have changed about 90° to starboard.

Conclusion If there is an obstacle straight ahead of the vessel at a distance of less than 4 ship’s lengths, this obstacle can not be avoided by a helm action only. The port quarter of the vessel will hit the obstacle.

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Rule 6 - Safe speed By all vessels

Background Lights Rule 6, § (a) (iv)

© layoutsparks.com

−  Reduces range of visibility of lights

−  Navigational lights may be mistaken for shore lights

Shore lights

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Rule 6 - Safe speed By all vessels

Background lights Rule 6, § (a) (iv)

Back scatter lights

−  Back scatter of vessel’s own lights reduces look-out’s keenness of sight

−  Portholes and windows must be blacked out

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Rule 6 - Safe speed By all vessels

The State of Wind, Sea and Current Rule 6, § (a) (iv)

Reducing speed: −  Greater drift by wind −  Greater drift by current Adverse weather conditions −  Vessel is less manoeuvrable −  Fishing boats, yachts, buoys, etc are less

visible

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Rule 6 - Safe speed By all vessels

Navigational hazards Rule 6, §(a) (v)

Navigational hazards may require the vessel to maintain a safe speed

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Rule 6 - Safe speed By all vessels

©svcrystalblues.blogspot.com

Navigational hazards Rule 6, §(a) (v)

AIS can give early warning of navigational hazards

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Rule 6 - Safe speed By all vessels

Navigational hazards Rule 6, §(a) (v)

Small iceberg are usually detected at a range of 4 miles. The smaller the icebergs (like growlers) the more difficult they will be detected

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Rule 6 - Safe speed

n  Insufficient depth of water

•  Must reduce speed •  Difficult to steer •  Reduced manoeuvrability •  Interaction •  Bank suction

By all vessels

Draught in relation to the available depth of water Rule 6, § (a) (vi)

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INTERACTION Introduction

According to Bernouilli’s Principle, there is a relation between the pressure and the velocity at different parts of a moving incompressible fluid. To express this principle in its simplest form, we can state that: “at points in a moving fluid where the velocity is high, the pressure is low and where the velocity is low, the pressure is high”. 33

Interaction on a single vessel

The streamlines passing near the body of the vessel are being diverted into slightly curved paths. It can thus be seen that the paths of water are longer near the body and therefore their speed increased and their pressure decreased, creating a suction along the body of the vessel.

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Interaction with two vessels

Amidships, between both vessels, the streamlines are crowded together. The velocity of the water is thus increased and the pressure decreased, MORE than on the outer parts of the vessels. Between both vessels a zone of reduced pressure is created and consequently an attraction or suction between them.

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Interaction Interaction can and does exist in many different situations and disregard of it has caused many accidents. Interaction is more liable to occur:

─ when a vessel enters shallow waters; ─ in narrow waters such as canals and rivers; ─ in close proximity of land or shoals; ─ in vessels meeting end-on passing too close; ─ in a vessel overtaking another vessel too

close; ─ in vessels negotiating a bend in a canal or

fairway.

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Vessel entering shallow waters – Squat

A vessel sailing with forward propulsion will push the water forward and create waves. When it moves at a rather high speed, a crest is formed close to the bow, a through in the midship and a stern wave at the stern. Close in front and at the back of the midship, some smaller waves

are also formed A vessel will most probably touch the bottom when sailing at full speed over a shoal, where apparently there is sufficient water, and remain clearly afloat when at rest. Reducing the speed will also reduce the extra sinking.

In shallow waters the distance between the bottom and the keel is considerably smaller, the flow of water beneath the keel of the vessel will, as we know, increase in velocity and decrease in pressure. This may (will) reduce the UKC with the risk for the vessel to touch the bottom. 37

SQUAT

This increase of draught or reduction in UKC is called SQUAT. In considering the squat, following points should be kept in mind: −  The position of the longitudinal centre of buoyancy

(LCB) determines the trimming effect and has a direct relation to the squat value.

−  The value of squat is influenced by the square of the speed of the vessel (speed2).

−  The greater the speed, the greater the squat value. −  The type of bow influences the wave making and

consequently also the pressure distribution of the under water volume.

−  Vessels with a large block coefficient Cb are more inclined to the effects from squat.

−  Vessels trimmed by the bow are more influenced by squat than vessels trimmed by the stern

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