Blewtooth

I have liked the idea of getting a pelorus ever since I discovered what they were and what they were used for. I remember seeing them on the bridge of Navy ships and in the movies whenever someone would shout "Torpedo! Bearing 36 degrees" I always wondered what instrument it was they were looking through to come up with that.

The Pelorus or "Dumb Compass" is a simple little device that is nothing more than a compass rose and a sighting scope. It can be used to get angles from a moving or stationary ship to do things like get the bearing of other ships or landmarks. Another important job of the pelorus is for compass correction and the making up of a compass deviation card or deviation tables. Nowadays it seems that GPS units are being used for making up deviation cards but still, I like the idea of a device I can get angles of other objects while I’m under way or sail without having to interpret it with a hand bearing compass.

I began to look around for an inexpensive pelorus……NONE TO BE HAD! A few on Ebay that were WWII era models and one or two on a charting instrument site. They even wanted seven hundred dollars for a brass and wood one! Wow! They are rare or possibly being offered with that art mystique thing that so many of these marine supply houses do such a good business in. Things like really cool, shipy looking brass bells and barometers and art work for hundreds to thousands of bucks. Well of course that rules it out for me.

So I decided to just make one. Cant be that hard. I found a web site that allows you to download and print a free compass rose so I did that first. I got this one from :
www.maptools.com



Next I needed to enlarge it and make it water proof since I had printed it off on standard printer paper with an ink jet printer. I took it down to Office Depot and had them enlarge it first and then laminate it for me. Cost about two bucks and change.


Next I purchased a clear plastic ruler from Wal Mart for 49 cents to use as a sighting tool. It already had the holes in the center and edge which lined up perfectly with the graduation marks on the compass rose. The clear plastic seems to amplify the light and make it easy to read.



The one thing I did not want to do is use any metal at all because of the corrosion problem with instruments on a boat. I have already had a pair of parallel rulers that had their aluminum pins get sticky from corrosion. I already had these little plastic bolts in my tool bin but I am sure you can purchase them from some place like Home Depot for less than a buck.


I used a plastic Folger’s coffee container for the platform because it has a nice little gripping handle and a plastic lid that fits tight but can still be spun in place. I also purchased a little suction cup hook that I got from the dollar store. There were six to a pack so prorated that would run about 18 cents.


The only other piece of material I needed was a small piece of foam board that I cut with a scissors to back the compass rose and a small piece of half inch PVC pipe that I cut cross ways and used for sighting rings. (Not really necessary; a piece of tape or push pin would have done just as well but I thought this added a nice touch. A couple of dabs of silicone sealer and that is all the material needed.

I drilled a sighting/aligning hole in the coffee container which was conveniently marked with a molded in dimple to mark it’s center. Not sure why it was there but it worked out perfectly for the pelorus.

So here it is after putting it all together:



The suction cup can be used to mount it to the top of the cabin hatch or any smooth surface that is eye level when it is needed. First line it up fore and aft of the boat center line with the sighting/aligning holes and some reference marks that you pick out on the boat. I broke the hook on the suction cup off leaving just enough of it for a little lever. It has a little cam that pulls air out of the suction cup and serves to tighten down the bottom of the Folgers container as well. This keeps it pointing straight but still a little flexible to account for the rocking motion of the boat. I really don’t think gimbals would be needed if the boat is not rocking too badly.

Because the lid can be spun you can use it to create a deviation card for your compass. All you should have to do is find a set of charted range markers and have at it with the "Swinging Ship or Swinging Compass" exercise described in Chapman’s or some of the other navigation books.

Or of course keep it pointing straight ahead and use it for getting your bearings.

Another thing I added was the option to remove the sighting scope and replace it with a shadow pin for use as a sundial. When no range markers are to be found (say at sea). Again, Chapmans has info on how to use the compass rose and shadow pin. I used the tip off of a broken fishing rod and one of my little plastic bolts cut and fitted in the hollow center of the rod end. You can screw it in the place where the center pin was for the sighting scope.



So, I know it looks a little amateurish (kind of like a Cub Scout project) but I think it should work and be accurate enough for what I want to do with it. Any comments or suggestions would be appreciated since I haven’t tried it out yet.

"Pirates! Bearing 28 degrees!"