# Navigation Instruments - Pelorus



## CaptKermie (Nov 24, 2006)

So I enrolled in a Power Squadron course, 'Seamanship I' (formerely Piloting) and one of the instuments they will teach us to use is a *Pelorus*. So I googled it to see exactly what it is. Seems like a decent instrument, but I wonder how many sailors actually use one of these on a regular basis. Is it something I should buy for the course or should I make do with a cardboard mock-up that the instructor showed me. After a few google searches I got the sense that a Pelorus is rather obsolete and mostly a vintage curiosity for collectors, is this so? They are not cheap to buy and I wonder if it is money not well spent or perhaps it may be something I will use on future trips. I sail in the PNW and I doubt I will get any further north than the Broughtons and any further south than the Puget Sound will allow, still a fairly large area. So does anyone out there use a Pelorus and if so how often and for what circumstances. thanks in advance.


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## captbillc (Jul 31, 2008)

a friend of mine had a pelorus & i used it to make him a deviation card for his 40ft steel ketch a few years ago. i don't have one of my own.


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## celenoglu (Dec 13, 2008)

I never used one and do not think it will be needed. The only good possible usage might be navigating with the stars without any compass or GPS.


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## AdamLein (Nov 6, 2007)

My understanding is that a pelorus is used to sight horizontal angles. I don't have one, and I've played with using my sextant (turned on its side) for the same purpose. Seems equivalent to me.

I think the difference is that the pelorus can be set to two different bearings (e.g. 065 and 090) whereas the sextant can only show a fixed angle (025 would be the equivalent of the above example), so if you want to use a sextant in place of a pelorus then a subtraction might be expected of you. Also sextants have a largest angle they can measure, whereas I believe a pelorus can be oriented to two arbitrary points on a full circle.


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## JomsViking (Apr 28, 2007)

It's for taking a relative bearing. E.g. the helmsman says 112 degrees, and you measure 153 degrees starboard, then the sighting is 112+153 = 265 degrees..
You won't need it - a hand bearing compass is what you need.


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## JohnRPollard (Mar 26, 2007)

My father used them extensively for aviation navigation in the high Arctic where the magnetic north was too far removed from true north. I don't think he got much use out of them in lower latitudes.

Still, it would be cool to have one.


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## btrayfors (Aug 25, 2006)

Don't know why the Power Squadron is teaching the use of the pelorus, other than to get students familiar with the concept of RELATIVE bearings (i.e., the horizontal arc as measured from the bow of the vessel). 

I doubt that you'd find even one pelorus in a fleet of 100 cruising sailboats.

The hand-bearing compass, however, is a very useful tool for piloting. A good set of binoculars with integral compass (e.g., the Fujinon Polaris or Steiner) obviates the need for a separate HB compass, IMO. In fact, the binocs w/compass are far superior in actual use.

Bill


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## floatsome (Jul 5, 2008)

GPS has made the pelorus obsolete, not that it was all that practical on small recreational boats anyway. It had its place on larger boats at a time when dead reckoning and manual fixes were all one had, and might still have a role on ships. Hand bearing compass is good enough, and well worth having, as others have said.


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## AdamLein (Nov 6, 2007)

floatsome said:


> GPS has made the pelorus obsolete,


I'm guessing that anybody who's asking about the use of a pelorus is probably not concerned about that sort of thing.

Also airplanes have made sailboats obsolete and now this thread will return to its topic before it turns into yet another GPS-vs-insert-traditional-navigational-tool-here. You'll excuse me while I calibrate my kamal.


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## floatsome (Jul 5, 2008)

AdamLein said:


> I'm guessing that anybody who's asking about the use of a pelorus is probably not concerned about that sort of thing. Also airplanes have made sailboats obsolete and now this thread will return to its topic before it turns into yet another GPS-vs-insert-traditional-navigational-tool-here. You'll excuse me while I calibrate my kamal.


No need for sarcasm. I am an old-fashioned navigator who blends old with new and tries to keep up old skills. I teach these courses, and this is a common question every year from students trying to understand how to properly equip their boats. I went through that thinking myself. My conclusion is that pelorus is taught because it was left in the package from an earlier era and because it helps to understand relative bearings. I don't think it is taught because it has practical value on a smaller power or sail boat in today's world, if other navigation tools are working properly and one knows how to use them in all situations likely to be encountered. Enjoy calibrating your kamal, they are fascinating reminders of brilliant navigators down through the ages. Wish I had one, but I wouldn't use it on the boat.


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## CaptainForce (Jan 1, 2006)

I use my pelorus regularly for taking bearings. You should note that a pair of binoculars with a built in compass does the same job and is what more people are using now for the same function. 'take care and joy, Aythya crew


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## PalmettoSailor (Mar 7, 2006)

CaptainForce said:


> I use my pelorus regularly for taking bearings. You should note that a pair of binoculars with a built in compass does the same job and is what more people are using now for the same function. 'take care and joy, Aythya crew


So a Pelorus requires you to be able to visually identify two known points to determine your position?

If that's the case I can't see the need for one in addition to a hand bearing compass or binocular/compass combination.

I also don't see what use understanding relative bearing is to a sailor. I know it from using ADF equipment in airplanes to navigate specified courses from non directional radio beacons. I suppose there may have been a similar system for maritime navigation in the past but I see little use for that knowledge on a boat today. (RB+MH=MB)

I'd much rather learn to use a sextant.


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## AdamLein (Nov 6, 2007)

midlifesailor said:


> I also don't see what use understanding relative bearing is to a sailor.


The relative bearing (i.e. horizontal angle between) two points on the circumference of a circle is constant, when viewed from any other point on the circumference of the same circle. Given two points and an angle, you can find a circle of position. A three-point fix is done by finding two circles of position (two relative bearings with one point in common) and finding their intersection.


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## bloodhunter (May 5, 2009)

AdamLein said:


> I'm guessing that anybody who's asking about the use of a pelorus is probably not concerned about that sort of thing.
> 
> Also airplanes have made sailboats obsolete and now this thread will return to its topic before it turns into yet another GPS-vs-insert-traditional-navigational-tool-here. You'll excuse me while I calibrate my kamal.


Didn't know you had to calibrate a kamal. Guess that's my calculations were always off -- but I guess if you have stretchy string...


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## PalmettoSailor (Mar 7, 2006)

AdamLein said:


> The relative bearing (i.e. horizontal angle between) two points on the circumference of a circle is constant, when viewed from any other point on the circumference of the same circle. Given two points and an angle, you can find a circle of position. A three-point fix is done by finding two circles of position (two relative bearings with one point in common) and finding their intersection.


I"m not sure I understand the methodology but if I have to KNOW the location of the points, this just seems like an arguably more complicated way to find your position using re-section.


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## tommays (Sep 9, 2008)




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## AdamLein (Nov 6, 2007)

bloodhunter said:


> Didn't know you had to calibrate a kamal. Guess that's my calculations were always off -- but I guess if you have stretchy string...


Board can heat up and cause the air around it to act like a lens (refraction error). String might not be perpendicular to board (perpendicularity error). The knot that ties the string to the board could have tightened up so that the string is now longer (index error). Board's edges can get worn down or nicked when the navigator dropped it on deck (erosion error). Over time, excessive kamal usage can cause you to become buck-toothed (dental error).

You can measure these errors and subtract them off, or you can recalibrate your kamal


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## AdamLein (Nov 6, 2007)

midlifesailor said:


> I"m not sure I understand the methodology but if I have to KNOW the location of the points, this just seems like an arguably more complicated way to find your position using re-section.


Your argument seems to be that you'd like to do some piloting without using any known reference points, and since the pelorus can't do that, it's not useful. Doesn't all piloting (in a sense, all navigation) involve finding your position relative to objects whose location you (or some device of yours) know? If you use compass bearings to get lines of positions to fixed objects like channel markers or recognizable points on shore, these are points you can locate on your chart. Ditto for circles of position found using relative bearings.

As an example, earlier this year we did a trip to Friday Harbor on San Juan island. On the way we only used compass bearings to take fixes. I found the bearing to recognizable objects: the breakwater of my marina (just about due north of my position most of the time), lighthouses on Patos and Saturna islands (to the south), smoke stacks near Bellingham, WA (more or less east), etc. I used bearings to these objects to draw lines on the chart, and where these lines intersected was approximately where I was. From there I knew what course to steer to avoid shoals, tide rips, and other hazards and get to my next destination (entrance to President's Channel). As the trip progressed I had to switch to new landmarks like Flattop Island (SW), radio towers on Orcas Island (NE), O'Neil Island in Rocky Bay (SW), etc. All stuff that's marked on charts and that I think I can recognize from the boat.

Using relative bearings to find circles of position is exactly the same thing, except that instead of drawing lines on your chart, you draw circles. I didn't really know about circles of position on that cruise, so I didn't try to use them, and yes they are more work than lines of position.

However I can see situations in which they might be more useful. Just as you use danger bearings to avoid hazards, you can use a danger circle. For example, imagine you can find two points with a circle through them such that some shoal water just fits inside the circle. If the angle between the two points that gives you that circle is thirty degrees, then you need to make sure the angle you measure with your pelorus or sextant never gets bigger than thirty degrees, which means you have gotten inside the circle.

Basically the compass and pelorus both provide some sort of line of position. Sometimes one is more useful than another. If nothing else, having access to both will add redundancy to your piloting and give you more confidence in your fix.


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