Originally Posted by Watermann
Parallel indexing (often abbreviated as PI) is a radar technique that is generally used to keep a vessel on a true course that will/would (eventually) result in it passing a certain point at a pre-determined distance. It's also useful in determining whether there is any set of the vessel (whether by wind
or both) off its trackline. Although it doesn't provide you with a true fix, it's very handy in quickly determining if you are on course, or about to reach a wheel-over point or getting set one way or another (except along the axis of the index line).
Basically, you determine how far off of a point (or rock, fixed ATON, pierhead, etc - any fixed object easily distinguished on radar) you want to be when you're abeam of it (its CPA). Using the Parallel Index function on the radar (or utilizing a true EBL that can be offset, combined with a VRM), you project
a line that is parallel
to your desired course (not necessarily your heading) at a distant offset from the vessel that is equal to the distance you want to stay off that point (the index line is tangent to the circle representing the distance). By comparing where the point of reference is to the parallel index line (on the radar screen), you can tell if you are right on course (index line intercepts reference point target tangentially), or off track (index line is well inside or outside of point of reference target).
In pilotage waters, you generally setup at least one PI for every leg of the passage. These are denoted on the chart and usually also present in the passage plan. It is a very powerful technique that is generally very reliable. Remember that the hazard you are trying to avoid (or TSS/channel you are staying in) doesn't have to be off of the point you are using as a reference.
All IMO compliant radars must have a dedicated PI function, but any radar with the capability to offset a true
EBL (meaning, you have to give it a heading input) and a VRM can do it. Frankly, the PI functions on some IMO-approved units are so poor that I prefer to just use the offset EBL/VRM anyways.
That's the basic jist to it and there are more techniques involving parallel indexing that can also be a big aid in piloting.
Hope that didn't sound too complicated... I could probably explain it a lot better in person with a radar in front of me.
Great explanation Watermann!
In case people think this is just for big ships, let me give you an example where small boaters can greatly benefit from knowing how to do this and prepare a paper chart ahead of time as a passage menu thru some tricky waters…WITH ZERO VISIBILITY!
: Seymour Narrows
is a 5 km (3 mile) section of Discovery Passage between Vancouver Island
at Menzies Bay, British Columbia
and Quadra Island
The section known as Seymour Narrows begins about 18 km (11.5 miles) from the south end of Discovery Passage where it enters the Georgia Strait
near Campbell River
. For most of the length of the narrows, the channel is about 750 meters wide. Through this narrow channel, currents can reach 15 knts.
Seymour Narrows was described by Captain George Vancouver
as "one of the vilest stretches of water
in the world. because the flowing current can be sufficiently turbulent to realize a Reynolds number
of about 109, i.e. one billion, which is possibly the largest Reynolds number regularly attained in natural water
channels on Earth (the current speed is about 8 m/s, the nominal depth
about 100 m).
Because most displacement
boats and ships can only transit the narrows near the turn of the tide, it get’s busy then …day and night… also as PNW boaters know…. you get a lot of Fog
in the summertime, lasting for weeks, so you need to know how to proceed with instrument piloting only using…. Mainly the Radar
As Watermann said, your Radar needs to be connected to a compass / fluxgate
or now sat-comp to give you true headings. It is best to train yourself to work your radar in North up
presentation, primarily so that it mimics the chart which you have prepared. You need to have on your radar... one, preferably two electronic bearing lines (EBL) that you float off the center to a precise distance.
For those, who say you can just use a Chart overlay on the radar, that is ok in wide open situations, but in this case you are using the radar to not only spot large and small boats but also in zero visibility
, tuning it to spot whirlpools and tidal overfalls so as to maintain control. The painted in chart is a distraction and you can easily miss floating logs
and debris moving out from a back eddy right into your path.
I show a sketch of a basic PI for the narrows. You would put true courses in and then at perpendiculars to good fixed radar targets you index off a safe distance as a guideline. (You actually also index off a minimum or danger
distance in case you are deviating because of traffic)
One EBL is already set on your present course and floated off boat’s position at the measured distance of your PI
Second EBI is offset on next course and when it lines up with Radar Fix, that is your turning point.
The Triangles are great for doing that chart work quickly and with better accuracy than any other instrument.
So the paper chart is basically a prepared record
of all the dangers, special notes and route
you use with the radar. In those current conditions you usually have someone on the helm
quickly counteracting the boils and another watching the index lines and any other targets to keep you safe
Hope this helps.