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Chapter 6: Effect of the Current

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  1. Chapter 6: Effect of the Current

  2. Ch6. Effect of the current • A current (mass of water) is several hundred times denser than air / generates forces of great magnitude • In open sea a ship is handled in the same way with or without current • The ship in a current is being carried away relative to fixed objects (buoys, anchored vessels, obstructions…) • Prior to start any manœuvre: assess tidal strenght and direction in order to ascertain sufficient space

  3. Ch6. Effect of the current / Position of Pivot Point • Current acts on underwater portion of ship • When sailing with current: ground speed = speed in water + speed of current • When sailing against current: ground speed = speed in water – speed of current

  4. Ch6. Effect of the current / Position of Pivot Point Steering in the current: • Moving in the current: • Faster: pivot point near bow, good steering • Slower: same as backing , pivot point near stern , unstable ship • Moving against the current: • Pivot point in bow quarters, good steering, stable ship

  5. Ch6. Effect of transverse current In beam current: ship must compensate the effect of the current

  6. Ch6. Effect of transverse current Ship dead in the water: • Force exerted by the current is very large • Proportional to exposed underwater surface of ship (LxT) and to squared velocity of current V² • Force F = C x L x T x V² • Example: tanker of 260m Lenght

  7. Ch6. Effect of the current / Working in a tide When tide flows across a berth , it can be used to: • Improve slow speed control • Create lateral motion • Always better to stem the tide

  8. Ch6. Effect of the current / Tide from ahead • Short kicks ahead to maintain headway through the water and keep pivot point forward with little speed over ground • Good steering lever • Good control over the ship • - This is know as « stemming the thide »: ship’s speed bigger than current speed.

  9. Ch6. Effect of the current / Tide from astern • Most unsatisfactory situation • Extremely difficult to keep positive control of the ship • To keep pivot point ahead, ship must be running at a speed over ground much higher than speed of the tide: far too fast • To reduce speed: engine astern / pivot point moves after + transverse thrust

  10. Ch6. Effect of the current / Working across a tide • Balance ship’s speed through • the water and tidal stream • 2. Create sideway (lateral motion): • - use rudder angle alone or with kick ahead • - current comes onto opposite bow • - resultant of 2 vectors brings vessel towards the berth (crablike) • - to stop drift: bring ship’s head into the tide

  11. Ch6. Effect of the current / Working across a tide • When working across a tide: • Be patient • Never rush the manoeuvre • Always put the tide fine on the • bow • Don’t put the tide too far around • on the bow: • - good lateral motion • - difficult to bring ship back into • the tide

  12. Ch6.Effect of the current / Vectors • Ship’s head into the tide • Ship’s speed through the water = speed of tide • No sideways drift : • α = 0 (angle between ship’s heading and current)

  13. Ch6.Effect of the current / Vectors • α = 60° and V = S • V1 < S → ship is backing • V and S produce R (drift) • Conclusion: • - If V= S , the ship will berth astern of his • position • V has to be increased • in order to berth at the • good place (see next slide).

  14. Ch6.Effect of the current / Vectors • To keep position: V must be • increased until V1 = S • Then will R coincide with V2 • And vessel drifts perpendicular • towards the berth. • For α = 22°5: • S = 2’ → V = 2’3 / R = 0’8 • S = 3’ → V = 3’3 / R = 1’2 • For α =45°: • S = 2’ → V = 2’8 / R = 2’ • S = 3’ → V = 4’3 / R = 3’

  15. Ch6. Effect of current on manoeuvrability • Turning in the current • With the current, the ship makes a wide swing • Against the current it makes a tight swing

  16. Ch6. Swinging on an anchor A ship with a following tide swing through 180° to stem the tide prior to proceeding to its berth. In a narrow waterway the ship Swings on an anchor, keeping A tight control over the position. This is only possible if the bottom is clear of obstructions

  17. Ch6. Effect of the current / Swinging on an anchor This manoeuvredepends on experience and skill and: • Depth of water • UKC (Under Keel Clearance) • Strenght of the current • Type of bottom • Type of engine power available • Size of the ship • Amount of room available for turn

  18. Ch6. Effect of the current / Bends in a tidal River The tide may be of different strenghts : rapidly on the outside but weaker on the inside of the bend

  19. Ch6. Effect of the current / Following tide The strong tide is working on the stern with the pivot point forward: good turning lever and strong turning force. Anticipation + kick ahead and counter rudder

  20. Ch6. Effect of the rudder / Following Tide • A ship can react violently and rapidly to this force : never underestimate. • Better to keep more to the outside of the bend: the ship is always in the area of stronger following tide.

  21. Ch6. Effect of the rudder / Tide from ahead When a large ship negotiates a bend in a channel with current from ahead: Better to keep to the inside so that the bow does not enter the area of stronger current during the turn. This side is often the shallow side as well

  22. Ch6. Effect of the rudder / Tide from ahead In this position, the ship’s bow is influenced by the strong Tide : the turning moment opposes the intended turn. Risk of grounding Anticipation with helm and power

  23. Ch6. Effect of current / Rapid changes in tidal direction

  24. Ch6. Effect of current / Rapid changes in tidal direction A ship can pass close to shallow areas or man made structures where the tide changes rapidly in direction. If the ship proceeds at slow speed this can have serious Consequences for the handling.

  25. Ch6. Effect of current / Rapid changes in tidal direction

  26. Ch6. Effect of the current / Restricting the tidal flow Avoid acute angles with a jetty, even with weak tides The tide is forced to flow faster between ship and berth: a low pressure appears and the ship is sucked towards the quay.

  27. Ch6. Effect of the current / Tidal forces Force of the tide depends on: • Draft and depth of water • Ship’s bow configuration • Velocity of the tide • Under keel clearance Force of tide can vary with the depth: a tidal difference Of up to 2.5knots over a depth of 5 meter is possible. Published « Tidal Stream » is sometimes inaccurate.

  28. Ch6. Effect of the current / Tidal forces • Force of the tide: directly proportional to the square of the velocity • A small increase in speed means an enormous increase in force exerted upon a ship

  29. Ch6. Effect of the current / Tidal forces UKC: blocking effect of a vessel when the UKC is reduced / the tide cannot flow under and is forced to flow around

  30. Ch6. Effect of the current / Tidal force at anchor 50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio: 3.0: The tidal force = 19 tonnes (exerted on the windlass)

  31. Ch6. Effect of the current / Tidal force at anchor 50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio:1.1 due to falling tide: Tidal force increases three times to 64 tonnes and can exceed the Holding power of the anchor → ship will drag.

  32. Longitudinal forces on tanker at anchor Wind 50knots / Tide: 5knots

  33. Ch6. Effect of the current / Lateral tidal forces When ship moored or held with the tide on the beam with a small UKC: lateral force created can be enormous

  34. Ch6. Effect of the current / Lateral tidal forces Example 1: Ship of 280.000 dwt / draft 22m Current: 1knot on the beam Depth-draft ratio: 1.05 Total lateral force = 328 tonnes Example 2: Same ship Current: 1,5 knots on the beam Total lateral force = 700 tonnes