Parallel Indices -- Still Relevant?

[Dockhead]

Originally Posted by VanIsle

Parallel indices should only be used in N-up mode on the radar screen. They are not reliable in the Head-up mode.

Imagine you are headed North, passing an island on its west side. There is a submerged rock 1/2 mile to the south and west of the island. In N-up mode, you draw or place your parallel index line to keep your track 1 mile west of the island and 1/2 mile west of the hidden rock. Any drift to the east will be immediately apparent as the index line moves off the edge of the island. A change in course to the west brings the index line back on the edge of the island and puts you back into safe water, clear of the rock.

But, in Head-up mode, if you drift to the east, a change in course to the west brings the index line back on to the edge of the island, but not clear of the rock. Or at least not 1/2 mile clear.

Well, stabilized Course-up works too. But this is an excellent point, for those of us relying on the offset EBL rather than drawing it on the chart.

[S/V Alchemy]

Originally Posted by Dockhead

I do find nav mode useful for some situations, but your last point is a really good one. I don't use it much when sailing. The sometimes sudden course changes, the pilot makes to get back on the course line, can really play hell with sail trim.

Well, you can't ignore it with a wind vane steering the boat, but proper trim can really reduce the number of amps a powered autopilot requires, which has all sorts of benefits to the distance cruiser.

And good trim is easier on the rig and the sails and (my favourite) makes everything a little quieter. Who among us has not pulled a leech line a small distance, stepped back and said "ah, there it is!"

[Pete7]

Originally Posted by MartinR

In the last few months for example there have been a number of incidents with Russia interfering with GPS reception in Northern Norway. This had a lot of consequences, as GPS coordinates are not only used for navigation, but also for example for the planning and building of roads.

NATO ran a major exercise in Norway recently. The Norwegian destroyer that very nearly sank was part of the ex I think.

Now, with a major deployment of NATO countries, Russia wouldn't play about with GPS to see what the results were would they? Equally, since its a exercise, how about our (NATO) electronic warfare chaps testing their skills to measure the effects and add some realism to the ex.

[KJThomas]

Originally Posted by VanIsle

Parallel indices should only be used in N-up mode on the radar screen. They are not reliable in the Head-up mode.

Imagine you are headed North, passing an island on its west side. There is a submerged rock 1/2 mile to the south and west of the island. In N-up mode, you draw or place your parallel index line to keep your track 1 mile west of the island and 1/2 mile west of the hidden rock. Any drift to the east will be immediately apparent as the index line moves off the edge of the island. A change in course to the west brings the index line back on the edge of the island and puts you back into safe water, clear of the rock.

But, in Head-up mode, if you drift to the east, a change in course to the west brings the index line back on to the edge of the island, but not clear of the rock. Or at least not 1/2 mile clear.

That depends on the setting of your PI line, not whether your radar is NU or HU. What is happening is your radar is toggling the PI between relative to your vessel and relative to ground, when you change between RM, HU and TM, NU. You want the radar setting that gives RM(T). for the PI. Unfortunately this varies with different radars. However, if the PI direction changes when you alter course, you have the wrong setting.
The PI, to work as intended must stay a fixed distance from your vessel, but at the true course.

[Gruz]

Originally Posted by Pete7

Now, with a major deployment of NATO countries, Russia wouldn't play about with GPS to see what the results were would they?

Why they would not. What "NATO countries" are going to do, declare war on Russia if they start interfering with GPS reception.

[KJThomas]

Originally Posted by Pete7

NATO ran a major exercise in Norway recently. The Norwegian destroyer that very nearly sank was part of the ex I think.

Now, with a major deployment of NATO countries, Russia wouldn't play about with GPS to see what the results were would they? Equally, since its a exercise, how about our (NATO) electronic warfare chaps testing their skills to measure the effects and add some realism to the ex.

"The Russians done it" Eh!


Actually the Yanks simply failed to "look out the window".

[VanIsle]

Originally Posted by KJThomas

That depends on the setting of your PI line, not whether your radar is NU or HU. What is happening is your radar is toggling the PI between relative to your vessel and relative to ground, when you change between RM, HU and TM, NU. You want the radar setting that gives RM(T). for the PI. Unfortunately this varies with different radars. However, if the PI direction changes when you alter course, you have the wrong setting.
The PI, to work as intended must stay a fixed distance from your vessel, but at the true course.

You are correct. I'll have to check my settings. I'm from the days when we used to use a china marker to draw on the perspex screen.

[ameehan]

Greetings al,
Just to say as regards relevancy, a parallel index would have saved the Costa Concordia.
Ciao,
APWM

[Dockhead]

Originally Posted by VanIsle

You are correct. I'll have to check my settings. I'm from the days when we used to use a china marker to draw on the perspex screen.

My radar, and I suspect, all small radars, holds the shifted EBL at a constant angle to your heading line, when the radar is in Head up mode. So the problem described is really valid.


This is fixed in my radar (and probably all others) by shifting to stabilized Course up mode, if you don't want to use North up.

[KJThomas]

Originally Posted by ameehan

Greetings al,
Just to say as regards relevancy, a parallel index would have saved the Costa Concordia.
Ciao,
APWM

Unfortunately no. If they had been on a course parallel to the rock, as they were on previous occasions, the PI would have worked fine.

In fact, they were altering course as they approached the rock, and underestimated the bodily advance/sideways transfer, on the turn radius.

[KJThomas]

Originally Posted by ameehan

Greetings al,
Just to say as regards relevancy, a parallel index would have saved the Costa Concordia.
Ciao,
APWM

Originally Posted by VanIsle

You are correct. I'll have to check my settings. I'm from the days when we used to use a china marker to draw on the perspex screen.

Me too. But even the young people I train have problems with the endless, and to my mind superfluous, endless options on electronics.
I can't imagine a circumstance where you want a PI that doesn't stay on your desired course. For example.

[KJThomas]

The real advantage of the PI, to my mind, is it is akin to looking out the window. And using the compass, transits and clearing bearings, and predictive heading lines. It tells you where you are going to be. Unlike GPS and position fixing, which tell you where you were, in the past.
It focuses you mind ahead!

[KJThomas]

Originally Posted by Dockhead

My radar, and I suspect, all small radars, holds the shifted EBL at a constant angle to your heading line, when the radar is in Head up mode. So the problem described is really valid.


This is fixed in my radar (and probably all others) by shifting to stabilized Course up mode, if you don't want to use North up.

Different manufacturers use different settings.


Luckily, the basic radar settings were set in concrete by IMO, decades ago. So on every radar, gain, tune, clutter, pulse length etc, do the same things.


PI is one where the manufacturer got to decide.

[Lodesman]

Originally Posted by VanIsle

Parallel indices should only be used in N-up mode on the radar screen. They are not reliable in the Head-up mode.

You have to be aware of the limitations, and compensate accordingly, but they can still be useful in H-up. When suitable reference points will be on or near the beam, such as in a canal they are still useful, but then a VRM ring will do the trick too. If gyro/compass input is lost, H-up is default - you can still use the PIs, but must be aware of the issues described by Vanisle - remembering if the ship's head is on the planned course, the PI is valid.

[Pelagic]

Originally Posted by Dockhead

It may be little known that it is possible to use PI's without doing any actual chart work, on modern MFD's --


Just use the horribly misnamed "set offset EBL" tool. Once offset, of course, it is no longer an EBL . But it can be used to set a perfectly usable PI.


If you use the radar in HU, as I do, then this "offset EBL" is automatically created parallel to your heading.



PI's are incredibly useful -- they show graphically and at a glance, whether you are being set towards some obstacle and whether you are correctly following your track.


They show graphically, the relative motion of everything your radar sees, in relation to your HEADING (and if you use Course Up orientation, then in relation to your course).



Do we need this anymore? After all, we can set a waypoint and have a track line represented on the screen, so that we immediately see if we are drifting off the track. We can even read out XTE in real time from our instruments.


Being oriented to a safe track line is OK for most purposes, I guess, but I find that when sailing hard on the wind, for example, a PI is incredibly useful to show the limit. I often use them to determine where to tack, for example.

As you know, I am a strong advocate to using PI on radar.

Where I differ from you is that I stay on North up rather than Heads up as the slewing of the coastline as your course wavers makes for a fuzzy coastal return.
I'm in the same camp as your father and keep my chart and radar presentation both North up. I know heading southbound you seem inverted, but that's how other traffic sees you. weather reports relate to you and it trains the mind to 'Port-Stbd' protocols regardless of orientation.

Radar and compass are my primary navigation tools as they can constantly be verified by the watch keeper and are not subject to terrorist attacks or clock updates.

GPS once verified gives you a much more accurate detailed and useful tool...but it needs to pass the radar test.

In dense fog, I depend on PI to navigate through the fjords and inside passages.
SOP is actually 2 PI's plotted.
One is on my planned track, Second with wheel over position to next course from the opposing coast, or when the corner lines up to new offset.

Once settled on new course and especially if aware of other traffic, the 2nd offset EBL is set to a danger limit from which I can deviate from my course without running aground