‘Parallel Indexing’ or ‘Blind Pilotage’ involves navigating along a pre-planned track in waters where standard methods of navigating are impractical because the waters are so confined, or where co-ordinates are offset as per the chart title information. Usual methods of navigation, e.g. transferring radar ranges to the chart and transferring visual bearings to the chart, are impractical because it is not quick enough.
Parallel Indexing is where you use your radar’s VRM (Variable Range Marker) and an offset bearing line. (Some modern radars have an electronic indexing line that can be utilised anywhere on the display.) It is a line drawn or displayed that is parallel to your heading line.
First a note on radar operation: The radar display can be set up to show North-Up, Ship’s Head-Up (which is usually abbreviated to Heads-Up) or Course-Up picture. Some people prefer a North-Up presentation because the display is orientated to look the same as the chart, and the display is stabilised so only the heading marker moves and you do not lose the picture when altering course.
Others prefer to maintain Ship’s Head-Up (usually preferred on open waters), because the view outside the window is the same as the radar; however, care needs to be taken. This is an unstabilised display and when you alter course, the heading marker remains upward while the whole picture rotates in the opposite direction by the amount of course altered. During course alteration the whole radar picture rotates, making the radar difficult to read until the vessel settles on its new heading. Course-Up stabilised and North-Up stabilised works well, as yawing is eliminated.
How to do Parallel Indexing: You place a line at a chosen clearance range from the centre (your boat), parallel to your heading line and on the side where the obstruction/headland exists (using an offset EBL (Electronic Bearing Line) or a PI (Parallel Index) line if your radar provides one). Use the VRM (variable range marker) to set the EBL/PI line at the appropriate distance and make sure you do not breach that line. (See diagram below.)
If set correctly, this will keep you in safe water, away from navigation hazards.Your radar instruction book may not provide instructions specifically for Parallel Indexing. Ours doesn’t, we simply use the bearing line (EBL) set as an offset bearing line, as our parallel line.