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
ground speed = speed in water + speed of current
ground speed = speed in water – speed of current
Steering in the current:
In beam current: ship must compensate the effect of
Ship dead in the water:
When tide flows across a berth , it can be used to:
A ship with a following tide
swing through 180° to stem the
tide prior to proceeding to its
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
This manoeuvredepends on experience and skill and:
The tide may be of different strenghts : rapidly on the
outside but weaker on the inside of the bend
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
When a large ship negotiates a bend in a channel with current
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
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
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.
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
Force of the tide depends on:
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.
UKC: blocking effect of a vessel when the UKC is reduced /
the tide cannot flow under and is forced to flow around
50.000DWT tanker at anchor / 5 knots tide / depth/draft ratio: 3.0:
The tidal force = 19 tonnes (exerted on the windlass)
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.
When ship moored or held with the tide on the beam
with a small UKC: lateral force created can be enormous
Ship of 280.000 dwt / draft 22m
Current: 1knot on the beam
Depth-draft ratio: 1.05
Total lateral force = 328 tonnes
Current: 1,5 knots on the beam
Total lateral force = 700 tonnes