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.
Laying out the next turn? Tap the arrow between the two course fields to carry Leg 2 into Leg 1 and clear Leg 2.
Tap Formula & Help above for the formula, a diagram, and detailed instructions.
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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Formula
Distance to PI Line
Distance = Turn Radius × tan(Turn Angle ÷ 2)
Turn angle = the difference between the current and new courses (taken as ≤180°). This is the tangent distance from the intersection of the two course lines to the points where a turn of the given radius begins and ends — used as the VRM radius for placing the PI line.
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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.
The ← arrow between the two course fields carries Leg 2 into Leg 1 and leaves Leg 2 blank — so when you're steady on the course you just turned to, one tap sets you up for the next turn without retyping. The radius and any VRM addition stay as they are.
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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.