Chapter 6: Effect of the Current
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
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
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
Ch6. Effect of transverse current
In beam current: ship must compensate the effect of
the current
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
Current Ballast Loaded
/ 7m
14m
1 knot
30t
60t
2 knots
120t
240t
3 knots
270t
540t
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
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
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
Ch6. Effect of the current / Working across a tide
1. 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
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
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)
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).
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:
a) S = 2’ → V = 2’3 / R = 0’8
b) S = 3’ → V = 3’3 / R = 1’2
For α = 45°:
a) S = 2’ → V = 2’8 / R = 2’
b) S = 3’ → V = 4’3 / R = 3’
Ch6. Effect of current on manoeuvrability
Turning in the current
a) With the current, the ship makes a wide swing
b) Against the current it makes a tight swing
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
Ch6. Effect of the current / Swinging on an anchor
This manoeuvre depends 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
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
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
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.
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
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
Ch6. Effect of current / Rapid changes in tidal direction
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.
Ch6. Effect of current / Rapid changes in tidal direction
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.
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.
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
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
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)
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.
Longitudinal forces on tanker at anchor
Wind 50knots / Tide: 5knots
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
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