This calculates the distance you need to place your parallel index (PI) line ahead of the ship. For larger / faster moving ships, you may need to add up to 0.15 nm to the VRM distance.
Tap ? above for a diagram and detailed instructions.
Distance to PI Line
--nm
Turn Angle--
Turn Calculator Help
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What This Calculator Does
The Turn Calculator helps you determine where to place a Parallel Index (PI) line on your radar to execute a defined radius turn at the correct location.
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Input Fields
1
Radius of Turn
The desired turning radius in nautical miles. This is determined by your vessel's maneuvering characteristics, speed, desired rate of turn and available sea room.
2
Leg 1 (Current CMG)
Your current Course Made Good (CMG) in degrees. This is your vessel's current true course.
3
Leg 2 (New Course)
Your intended new course in degrees. This is the course you want to steady up on after completing the turn.
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How to Use the Results
1
Set your VRM (Variable Range Marker) to the calculated distance
2
Create a PI line set to the new course
3
Position the PI line at the point where your current CMG / True Vector intersects the VRM
4
Begin your turn just before the PI intersects your new track or touches a distance off indicator
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Visual Example
Example: CMG 217°, next course 174°, VRM set at 1.2 nm
The blue circle is your VRM set to the calculated distance. The orange dashed line is a PI set to the new course (174°), positioned where your current CMG vector intersects the VRM.
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Important Notes
For larger or faster vessels, add 0.1–0.15 nm to the calculated VRM distance. For example, on a 163K GT azipod cruise ship, adding 0.15 to the 1.2 nm VRM — resulting in placing the PI at 1.35 nm — gives more reliable results.
Verify this VRM distance with other sources (e.g. TrackPilot next-course curved heading line, fixed VRM set to turn radius) before relying solely on this method.
Always account for wind, current, and sea conditions.