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Old 01-06-2010, 13:32   #16
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Right. I was just curious to know what people used elsewhere to plot routes manually (since we're required to be able to do this without GPS to pass).

Thanks everyone for the great help.
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Old 01-06-2010, 18:36   #17
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What is "parallel indexing"?

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 current 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.
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Old 01-06-2010, 20:44   #18
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I'm an old Navy Pilot and we used this in the cockpit: Ultimate Fixed Plotter - MyPilotStore.com. Small enough to use on a cramped nav. table yet big enough to work on a full sized chart if you should be so lucky to have the space for that big a chart. Used one to navigate for a lot of miles before GPS came along. Still find it an invaluable tool in backing up the electronics.
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Old 01-06-2010, 23:01   #19
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I've seen those things but Google didn't return videos on how they're used while sailing to know what direction to take.

What is "parallel indexing"?
Theres a compass overlay application.

Draw a path and move it onto the overlay.

Have a look 'ere

http://bbs.keyhole.com/ubb/ubbthreads.php?ubb=showflat&Number=785959#Post7859 59
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Old 02-06-2010, 02:27   #20
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G'Day all,

A thought: perhaps the reason the French are concerned about this, is that some French charts (like many of the ones for New Caledonia) DO NOT have compass roses on them. Thus the need for a plotting tool with a pair of protractors on it.

Big PITA.

Cheers,

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Old 02-06-2010, 03:42   #21
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... or maybe, because everyone here is taught to use the Cras ruler, there's no real need to have a rosace on maps, except to show the difference between true North and magnetic North :-)

Thanks for the great explanation about parallel indexing.

BTW, are protactor triangles also used by navigators, or are they mostly used by pilots?
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Old 02-06-2010, 05:35   #22
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G'Day all,

A thought: perhaps the reason the French are concerned about this, is that some French charts (like many of the ones for New Caledonia) DO NOT have compass roses on them. Thus the need for a plotting tool with a pair of protractors on it.

Big PITA.

Cheers,

Jim

I've heard that some older French charts have the prime meridian running through Paris instead of the Greenwich observatory.
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Old 02-06-2010, 05:58   #23
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"The Prime Meridian is ultimately arbitrary — a matter of convention — and various conventions have been used or advocated throughout history: [...]

The modern Greenwich Meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851. By 1884, over two-thirds of all ships and tonnage used it as the reference meridian on their maps. In October of that year, at the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA, for the International Meridian Conference. This conference selected the Greenwich Meridian as the official Prime Meridian due to its popularity. However, France abstained from the vote and French maps continued to use the Paris Meridian for several decades.

Prime Meridian - Wikipedia, the free encyclopedia
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Old 02-06-2010, 07:34   #24
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On the big nav table on the motoryacht I Captain we use the parallel rulers with the compass engraved on them.

On my sailboat I got better use out of a pair of 8" triangles with handles. With a bit of practice you can really scoot them across the chart.
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Old 02-06-2010, 07:47   #25
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On my sailboat, I use parallel rules if using the saloon table for chart work, if at the nav station, I use either Portland rule or a pair of triangular protractors. On my tug boat, I use the 2nd Mate
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Old 02-06-2010, 07:58   #26
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How are the triangular protractors used?
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Old 02-06-2010, 08:00   #27
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BTW, does someone know why they are called Portland Plotter? Is Portland, Oregon a famous sailing zone in the US, or is it after another place/person?
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Old 02-06-2010, 08:37   #28
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How are the triangular protractors used?
The triangles are 90 degree isosceles triangles, with the hypotenuses about 8 inches long.
By holding the triangles with the hypotenuses together, the two traingles can be slid about to transfer bearing lines. Some are graduated in degrees, so its easy to read of a course or beaaring using the scale, others do not, and you need to move them along to line up with a compass rose.

Most Brits dont use them, but Germans and Dutch use them a lot
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Old 02-06-2010, 09:18   #29
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Portland Plotter
  • It can move easily over the creases in charts.
  • You can pick it up to read it.
  • It does not have to be walked over the chart, slipping along the way.
  • The variation scale makes conversions between true and magnetic much faster. (In the Gulf Islands some folks mark an arrow on the plotter at at 18E.)
  • It is easier to use in a pitching boat.

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Old 02-06-2010, 09:55   #30
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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 current 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!

Position: 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).

Visibility: 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.

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.

PI: 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.
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