In order to negotiate a plan turn, a constant radius turn concept can be used. Theory and procedures of using constant radius turn are explained below.
Suppose the vessel has to take a turn in the path marked in above figure (from A to B). The total course change is Ø°
The distance AB= R Ø /57.3 mile (R in Miles)
Vessel’s speed, V Knots
Time taken to cover distance AB, T = R Ø/ 57.3 V
Therefore Rate of turn Ø/T = 57.3 V/R, (Deg/hour)
Rate of turn, Degree per minute = 57.3V/R 60 ~ V/R Degree/ minute
Rate of Turn = V/R (degree per minute)
Therefore, to have a turn of constant radius, relationship between ‘V’ & ‘ROT’ can be established.
For example: For having a turn of radius 1 Mile, V/ROT = 1, So the ‘ROT’ is to be monitored with the speed of the vessel during the turn.
Steps to mark the Wheel Over Line (WOL) and Wheel Over Point (WOP)
1. Mark off the arc of radius as required on both the courses.
2. Draw a line parallel to the initial course & final course, tangent to the arc already made as per point 1.
3. The point of intersection of both the line will give the centre of the circular path the vessel will be following while going from initial to final course.
4. Taking above as a center make an arc of the circular path meeting both the courses.
5. WOP is to be marked off at a distance equal to the reaction time of the vessel.
6. WOL is the dotted line shown on the above figure, which is parallel to the final course passing through WOP.
7. WOL is marked so that the vessel remains on the final course after alteration, irrespective of the vessel’s cross track error in the initial course.
8. Required helm to be given when vessel passes ‘WOL’.