Wheel over
position calculation:
Wheel over
positions should be determined from the ship’s manoeuvring data and marked on
the chart. Suitable visual & radar cues should then be chosen to determine
when the ship is at the wheel over position. The best cues for large
alterations of course consist of Parallel Indexes or visual bearings parallel
to the new track, whereas for small alterations a near beam bearing is often
better. Even when the pilot has the con the wheel over position should be shown
on the chart so that the OOW will be aware of its imminence & importance.
It is also part of the ship’s officer’s monitoring of the pilot.
FACTORS
THAT AFFECT A TURN:
-
Structural design & length
of the vessel.
-
Draught & trim of the
vessel.
-
Size & motive power of the
main machinery.
-
Amount of helm used.
-
Available depth of water.
A vessel trimmed by stern will steer more
easily, but the tactical diameter of the turn is increased.
Trim by head will decrease the diameter of
the turning circle but it will be difficult to steer the vessel.
Listed vessel will be subject to delay in
turn. A larger turn will be experienced when turning into the list.
External forces, wind & current will
affect the turn.
CONSTANT
RUDDER
ANGLE
TURN
|
CONSTANT RADIUS TURN
|
1.
Larger drift angle with a corresponding loss of
speed
|
Lesser drift angle & hence lesser loss of
speed
|
2.
A large rudder angle is needed to steady the
vessel on new course
|
At the end of the turn, the new course can be
steadied with lesser rudder angle
|
3.
Uncertainty of ship’s position during the turn
|
Proper control of ship’s posn. during the turn
|
4.
Higher fuel consumption due to zigzagging with
excessive use of helm.
|
Lesser fuel consumption, with reserve rudder and
engine power available
|
Constant radius
turn technique is based on the following formula:


60 R
where V= Ship’s
speed over ground, in knots and,
R= Radius of the turn in nautical miles.
The distance of
wheel over point from the point where the turn is to become effective is
usually taken as one ship’s length but it is recommended to find it out by some
practice turns on the type of ship one is serving.
The distance of
wheel over point from the point where the turn is to become effective is usually
taken as one ship’s length but it is recommended to find it out by some
practice turns on the type of ship one is serving.
Following
formula can be used to find the distance of wheel over line from the new course
line and the same can be used to set the parallel indexing line or the line of
turn for giving the wheel over order:
Distance of
wheel over line from the new course line
= F sin q + R (1 – cos q) where,
F = one ship’s length (usually),
R = Radius of the turn
WHEEL
OVER POSITION:
![]() |
F = Head Reach – Distance traveled by
vessel after giving wheel over & before commencing turn, i.e. distance to
overcome inertia.
P = Perpendicular distance from wheel over
point to new course extension.
D + P = Parallel Index distance at W/O
position.
D = Parallel Index distance from new
course, as obtained from chart.
R = Radius of turn.
Ө = Change of course angle.
TO FIND & MARK APPROX. W/O POSITION:
F = 0.1 to 0.15 NM
P = R(1-Cos Ө) = F Sin Ө
Rate of turn = (V / R) x 0.96 where
V = speed of vessel
Distance to new course = Radius x Tan Ө/2
First the Master of the vessel has to
decide the radius of turn required, depending on the manoeuvring
characteristics of the vessel & available sea room.
A reference point such as an island, buoy,
etc. can be also used to decide the radius of turn.
Wheel over point = F + Distance to new
course
Distance to new course can be found by the
above formula.
Once the Master has decided on the radius
of turn, a tabular ready reconer may be prepared for various course change
angles.
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