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post #1 of 29 Old 10-04-2013 Thread Starter
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Question Deviation

Good Afternoon,

This is a timely subject that perhaps many of you cannot help with. However I know some of you will probably remember some far back time when this was actually still done and can perhaps fill me in.

I am learning the art of navigation. I am pretty solid on charts, plotting, headings, yadda yadda yadda. However- Before I can feel solid I need to know how to figure out my boats deviation. I know in days of old you had to use a lead boxed compass on a dock or something, tie the boat to the dock and move it around tied in different directions. Then you see what the compass on the boat reads in relation to the one forin the lead box and you subtract the difference and figure out the boats deviation for that particular angle of sail. I should also like to do this while the engine is running to see how such will affect deviation.

Does anyone know how I can accurately determine my boats deviation without a lead compass and a bunch of extra hands?
Boat is a 1979 AMF Paceship PY26
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post #2 of 29 Old 10-04-2013
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Re: Deviation

Normally if you want it right you pay a compass guy to swing it, mine happens to be close enough

There also is a DIY option and much info on google

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post #3 of 29 Old 10-04-2013
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Re: Deviation

I am not an expert and I'm not sure if this is the "official" way to make a deviation chart, but it works: Go out on a calm day and drive. Your GPS will give you your true heading. Subtract out your variation to give you your magnetic compass heading. Now that you have an accurate magnetic compass heading, just subtract what your boat's compass says and that's your deviation. (Where we were doing there was no variation, magnetic north = true north, so we just had to subtract the GPS heading from the boat's compass.)
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post #4 of 29 Old 10-04-2013
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Re: Deviation

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Originally Posted by Minnesail View Post
I am not an expert and I'm not sure if this is the "official" way to make a deviation chart, but it works: Go out on a calm day and drive. Your GPS will give you your true heading. Subtract out your variation to give you your magnetic compass heading. Now that you have an accurate magnetic compass heading, just subtract what your boat's compass says and that's your deviation. (Where we were doing there was no variation, magnetic north = true north, so we just had to subtract the GPS heading from the boat's compass.)
This will only work if you are in a place with zero current. Otherwise, bearing and heading will not be the same.


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post #5 of 29 Old 10-04-2013
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Re: Deviation

"There also is a DIY option and much info on google"

If you're into paper there is a pretty good description of how to swing the compass in the two sailing bibles Chapman's Pilotage and Annapolis Book of Seamanship. On a plastic boat deviation is not normally a big deal if you can keep magnets (speakers), electronics, and large hunks of iron (engine) away from the compass.
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post #6 of 29 Old 10-04-2013
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Re: Deviation

Using range marks is one method of swinging the compass. In some ports (like SF) there was actually a "range" set up specifically for swinging a compass, and if the is one in your area, though out of date, perhaps it would still be usable.
Power directly on the ranges, in and out; that will give you two opposite courses. Do the same with other ranges to get as many as you can. Should be plenty of ranges in the St. Johns River?
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post #7 of 29 Old 10-04-2013
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Deviation

It's important to note that swinging a compass is extremely tedious and only half the job required. Once a table of deviations is developed, the helmsman must actually use it to be of any benefit. To me, it's too much work to continually look up the deviation for each course being piloted. I find it of little benefit anyway since I am typical of most helmsmen and am lucky if I can keep within 5 degrees of my intended course in any kind of seaway.

With all things considered, for short runs I don't think that deviation matters that much. I think that variation is more important to consider.
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post #8 of 29 Old 10-04-2013
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Re: Deviation

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Originally Posted by capta View Post
Using range marks is one method of swinging the compass. In some ports (like SF) there was actually a "range" set up specifically for swinging a compass, and if the is one in your area, though out of date, perhaps it would still be usable.
Power directly on the ranges, in and out; that will give you two opposite courses. Do the same with other ranges to get as many as you can. Should be plenty of ranges in the St. Johns River?
Fun, fun, fun?

Using fixed marks as range marks is the best way to find the deviation.
Another compass can also be affected by magnetism on the boat.

If you don't find range marks on the map you can find your own.

Identify two fixed objects on the map (or out on the water) that can be used as range mark.
Find the true bearing line between the to marks on the map.

And do the maths
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Re: Deviation

Note that if you have an autopilot, there is a built-in deviation table that it builds and stores. Go into "calibrate" mode and motor through couple slow circles. It stores the table internally, and uses the table (along with variation) to report your true heading on the display.

It doesn't calibrate your manual magnetic compass, but it does calibrate the autopilot's compass.


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Deviation

I forgot to mention that a more practical and useful exercise is to zero the compass. I never hear of anyone doing that but the impact can be enormous.

When we bought our current boat, the compass was off by up to 30 degrees on several bearings. It had nothing to do with deviation. I took the compass off the boat and away from cars and houses and spent an hour zeroing it. The result was profound.... And accurate.

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