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  #1  
Old 09-16-2009
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Navigation Instruments - Pelorus

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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Old 09-16-2009
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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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Old 09-16-2009
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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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Old 09-16-2009
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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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Old 09-16-2009
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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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Old 09-16-2009
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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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Old 09-16-2009
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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.

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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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Quote:
Originally Posted by floatsome View Post
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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Old 09-17-2009
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Quote:
Originally Posted by AdamLein View Post
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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