Rate of Turn Indicator
It indicates the instantaneous rate at which ship is turning.
Purpose - It is used to monitor the turn.
Requirement - All ships having GRT ≥ 50000
As per IMO recommendations, large course alteration to be planned along circular track with WOP ( wheel over point ) marked and the progress of the ship continuously monitor.
• Draw angle bisector ∠AOB
• Choose any point on angle bisector preferably a point which can be track by RADAR.
• Draw perpendicular from this refrence point to course line that meets at A & B.
Say P is refrence point for executing turn
∠A + ∠O + ∠B + ∠P = 360°
∠O + ∠P = 180°
∠P = Ө
• Draw an arc from P with PA radius.
• The ship while altering course will move along this arc.
Monitoring the Turn-
• Range remain same from refrence point.
• Refrence point is always abeam to ships fore and aft.
• Rate or turn indicator.
Distance traveled along the arc AB=R𝚹 ͨ
( Where R is radius & 𝚹 ͨ is radian speed V kts )
So the time taken to travel AB i.e.
t = AB/V = R𝚹 ͨ / V
{Note:- speed = dis./t }
t = R𝚹 ͨ x 60 / V min
ROT per min = 𝚹° / t = ( 𝚹° x V ) / ( R𝚹 ͨ x 60 )
= ( 𝚹° x V x 57.3 ) / ( R𝚹° x 60 ) {note :- 𝚹 ͨ = 𝚹°/57.3 }
= (57.3 x V )/ (60 x R )
ROT per min.=0.95 x V/ R
Advantages of ROTI :-
• The turn can be pre-planned and required ROT can be pre-calculated, Execution of Turn becomes simple.
• The turn can be easily monitored while it is being executed.
• Since the turn is pre-planned,the use of helm is minimising and thereby loss of speed is also reduced.
• Even the ship has drifted to port or starboard, the turn can be easily amended to bring her on track.
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