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|10-05-2013 06:23 PM|
Originally Posted by bfloyd4445 View Post
|10-05-2013 05:48 PM|
[QUOTE=Uricanejack;1099466]In practice most small wood or GRP boats can virtually ignore deviation unless someone has mounted or left a ferrous object
If you fail to ascertain the accuracy of the handheld compass your results will be skewed. What your saying is like comparing one cars speedometer reading against another car to determine if one of the cars speedo is correct. Does no good without first making sure the instrument used to calibrate with is accurate. One cannot assume accuracy
|10-05-2013 05:20 PM|
It would vary too much depending on how I was sitting and which eye I was using. We also traded off with one of my friends taking sites and we just couldnt remember who did which. We actually did try to correct for it because the computer cave mapping program will adjust shots to force a closure dividing the "error" up between all the shots or between shots you select but it made no difference, the last shot didnt come within 90 degrees of closing and many feet off.
AT that time, I could push my glasses up and look under them at the compass by my eye cuz my glasses were often so fogged I couldnt see and I think it was how much I pushed em up that caused so much difference between shots.
|10-05-2013 05:15 PM|
In practice most small wood or GRP boats can virtually ignore deviation unless someone has mounted or left a ferrous object near the compass.
How to swing a compass and create a deviation card.
Preferably on a calm day in a location with little or no set.
Head out in your boat with your compass and a means of taking a bearing or azimuth. A good hand compass will do
A transit is good but not necessary.
Pick a conspicuous object at least 2 miles away if possible. ( the further away the less error if boats position changes slightly)
Starting with North
Turn slowly to steer North or 000 deg on compass. Compare heading with HBC and take azimuth or bearing of object and make a note of bearing.
Turn Slowly to E or 090 deg compass and repeat the tacking of bearing of objectí
Turn slowly South. 180
Turn slowly West 270
Repeat the process turning slowly for NE 045
SW 225 deg
NW 315 deg
For example create a table
N 000 Deg Brg 150 deg Comp
NE 045 Deg brg 148 deg comp
E 090 deg Brg 146 deg comp
SE 135 deg Brg 144 deg comp
S 180 deg Brg 146 deg comp
SW 225 deg Beg 148 deg comp
W 270 deg Brg 150 deg com
NW 315 deg Brg 152 deg comp
Add all your bearings together and divide by 8
In this example it will equal 148 this is the compass bearing of the object
For this method variation is not required.
By applying the variation you would have the true bearing of the object.
You could do the same process with a known transit slowly turning and crossing the transit on each of the cardinal and inter cardinal points tacking the bearing each time you cross
It would be a very unusual vessel not to have symmetrical deviation card requiring no symmetrical metal or an compass off set to one side.
It is important to turn slowly between bearings so the compass does not lag or you will induce an apparent error into your deviation card.
Plot the results on a graph and join the dots and you have a deviation card.
|10-05-2013 05:13 PM|
Originally Posted by Frogwatch View Post
|10-05-2013 05:07 PM|
Once spent an entire day laying face down in the mud of a particularly horrible cave passage slowly mapping it with compass, tape and clinometer. The shots (point to point) were short and involved crawling over awful stuff. Got home and began drawing on paper before we entered it in the computer (old punch card entry) and one series of shots that was supposed to go around in a circle didn't even come close to closing no matter how much we manipulated the data. Suddenly had this really awful realization that it had been the first time I had gone cave mapping with my glasses as I normally wore contact lenses. I placed my glasses adjacent to the compass and saw it move. We threw the data away. To this day, over 30 years later, nobody has mapped that passage.
There is still no way to do cave mapping but compass and clinometer although the tape has often been replaced with a infra-red gadget and there are apps that plot data as it is entered on a waterproof and shock resistant keyboard. This helps see such errors as they occur.
On that trip, theyguy who was doing sketch and data recording in the "map book" (waterproof and tear resistant paper) fell asleep it took so long on one shot. He awoke when the flame from his carbide lamp burned hole in the map book cover.
|10-05-2013 04:08 PM|
Originally Posted by knuterikt View Post
Dosen't take much to learn the basics and correct your compass for error. Then you must remember to keep electrical things like portable radios etc. away from the binnacle or compass location. I set a portable radio close to the compass once and wondered why I just passed what appeared to be a duck blind in the fog. Turned out the radio was affecting the compass. No harm done
|10-05-2013 03:04 PM|
Originally Posted by Frogwatch View Post
|10-05-2013 12:45 PM|
This is not rocket science. It is easy to do with another hand bearing compass.
1. Look around your compass and check to see if there are any significant steel or iron items nearby (consider pots and pans under the bulkhead where the compass is mounted.) If so, move them and note whether or not the compass needle moves at all and how much.
2. Take a bearing on a distant object with the boats compass (head the boat directly toward the object and check bearing).
3. Get as far as possible from your iron keel by standing on your boom and take the same reading with your hand compass. Try leaning way back off your stern rails and do it. Any Diff tween that bearing taken in step 2 and this one? If you cannot see any difference, then your deviation is too small to worry about.
4. IF you see a diff (say > 5 degrees) then you need to take addtl steps. Note on your chart the actual bearing from one known nav aid buoy to another, compare to to what your boats compass says. Subtract (or add) known variation printed on your chart and compare again, the diff is your deviation (for that nearing). In general, at low latitudes, the diff in deviation between any bearings will be small.
In almost all cases for a fiberglass sailboat with no ferromagnetic items near the compass, the deviation will be small, even with an iron keel. The VARIATION can be significant. In the Abacos, it is about -10 degrees meaning you have to add 10to what your compass says.
|10-05-2013 12:09 PM|
Compass light should be wired with twisted wire to null the magnetic effect of current (Kirchoff's Law??) It's called twisted sisters in the old telephone world..Imagine yourself in the fog , rocks all around, big current,the radar is a blurr because of a rapid 180 and you are depending on GPS to show you heading because you don't have a trusted compass I crewed on a big cat to India without a compass once. Duh!!Depending on GPS only tells you where you are and course shown is based on where you have been. Good to know where the boat pointing.
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