rule 06 - safe speed_part b

7/24/2019 Rule 06 - Safe Speed_Part B

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TURNING CIRCLE

! The Figure shows the turning circle to starboard and to port of anaverage 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 waterof about 100 metres.

2007 David House, Ship Handling, Elsevier Ltd.20


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TURNING CIRCLE

DEFINITIONS

Advance

The advance is the distance the vessel has travelled in thedirection of the original course from commencing the turn tocompleting the turn. The heading of the vessel has passedthrough 90 to starboard (starboard turn) or to port (port turn).

TransferThe transfer is the distance the vessel has moved perpendicular tothe fore and aft line from the commencement of the turn (eitherto starboard or port).

Tactical diameter

The tactical diameter is the greatest diameter sailed by the vesselfrom commencing to completing the turning circle.

Final diameter

The final diameter is the internal diameter of the turning circlewhere no allowance has been made for the decreasing curvatureas experienced with the tactical diameter.

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Turning circles off:Full bodied vessel

andSlender ship

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TURNING CIRCLE

GENERAL REMARKS

! The turning circle conducted in shallow waters will be

considerably increased compared with a turning circleconducted in deep water.

! Turning a vessel with her helm hard over will cause thevessels speed to decrease considerably.

!

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

!

A vessel trimmed by the stern will generally steer moreeasily but the tactical diameter of the turn will be expectedto decrease.

!

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

!

A vessel conducting a turning circle with a list couldnormally be delayed.

! Turning towards a list would normally generate a largerturning circle.

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TURNING CIRCLE

GENERAL REMARKS

! Turning away from a list would normally generate a smaller

turning circle.!

A vessel tends to heel towards the direction of turn oncehelm is applied.

!

A vessel turning with an existing list and not being in anupright position could in shallow waters experience anincrease in draught.

!

The type of rudder can have an influence on the turningcircle of a vessel.

! A narrow beam vessel will normally make a tighter turningcircle than a wide beam circle.

!

A vessel equipped with a right hand fixed propeller wouldnormally turn tighter to port than to starboard.! 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 ofwater can also be influencing factors on the turning circle ofthe vessel.

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TURNING CIRCLE

! The diameter of the turningcircle is equal to about 4 shipslengths (4L).

! In position 1, the helm is puthard to starboard and the vesselwill first move to port of herinitial course. The vessel alsostarts to turn to starboard. Dueto the position of her turningpoint (at about !from thebow), the bow will hardly bemoving inside the initial course

but the rear of the vessel willswing to port. Only in position 4,after 4 ships lengths on theinitial course, the rear of thevessel will start to move to theinside of the initial course. Inposition 5 the ships course will

have changed about 90 tostarboard.

Conclusion

If there is an obstacle straight aheadof the vessel at a distance of lessthan 4 ships lengths, this obstaclecan not be avoided by a helm action

only. The port quarter of the vesselwill hit the obstacle. 25


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Rule 6 - Safe speedBy 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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Background lights Rule 6, (a) (iv)

Back scatter lights

! Back scatter of vessels own lights reduceslook-outs keenness of sight

! Portholes and windows must be blacked out

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

Reducing speed:! Greater drift by wind

! Greater drift by currentAdverse weather conditions! Vessel is less manoeuvrable! Fishing boats, yachts, buoys, etc are less

visible

treehugger.com

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Navigational hazards Rule 6, (a) (v)

Navigational hazards may require the vessel to

maintain a safe speed

khaolak.net

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AIS can give early warning of navigational hazards

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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

! Insufficient depth of water

Must reduce speed

Difficult to steer

Reduced manoeuvrability

Interaction

Bank suction

By all vesselsDraught in relation to the available depth of waterRule 6, (a) (vi)

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INTERACTIONIntroduction

According to Bernouillis Principle, there is a relationbetween the pressure and the velocity at different parts ofa moving incompressible fluid. To express this principle inits simplest form, we can state that:

at points in a moving fluid where the velocity is high, thepressure is low and where the velocity is low, the pressure

is high. 33


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Interaction on a single vessel

The streamlines passing near the body of thevessel are being diverted into slightly curved

paths. It can thus be seen that the paths of waterare longer near the body and therefore theirspeed 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 streamlinesare 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 pressureis created and consequently an attraction orsuction between them.

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Interaction

Interaction can and does exist in many differentsituations and disregard of it has caused manyaccidents. 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 tooclose;

" in vessels negotiating a bend in a canal orfairway.

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Vessel entering shallow waters Squat

A vessel sailing with forward propulsion will push the water forward andcreate waves. When it moves at a rather high speed, a crestis formed

close to the bow, a throughin the midship and a stern waveat thestern. 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 remainclearly 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. Thismay (will) reduce the UKC with the risk for the vessel to touch the

bottom. 37


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SQUAT

This increase of draught or reduction in UKC is calledSQUAT. In considering the squat, following points should bekept in mind:

! The position of the longitudinal centre of buoyancy(LCB) determines the trimming effect and has a directrelation to the squat value.

! The value of squat is influenced by the square of thespeed 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 underwater 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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rule 06 - safe speed_part b

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