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post #11 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.)
I haven't done this in thirty years because most my recent boats had compasses that matched so my guess is they had been corrected. To correct your ships compass you can use magnets or sometimes pieces of metal located to offset your compass reading to the direction u wish. Remember its the magnetic mass and its location of your vessel that will cause the deviation which can usually be easily corrected with strategicly positioned then permantly mounted, magnets or metal. I believe this is still covered in Chapmans
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post #12 of 29 Old 10-04-2013
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Re: Deviation

+1 on using ranges. It makes it certain that you are actually moving on a predicted course over ground. If you can find two sets of points at right angles to one another (or close) , it will get you close pretty quickly.

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post #13 of 29 Old 10-04-2013
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Re: Deviation

Hey,

A simple way would be to set your GPS to display magentic data, then motor around and see if it matches your compass. Pick a time of slack water to be more accurate.

Personally I have to state that I NEVER use my compass for navigation. I ALWAYS use my GPS to provide a bearing and then I follow that. If I can see something on land (which is most of the time because where I sail I can't ever be more than 10 miles from land) I aim for a point on land. if I can't see land than I try to keep the compass on a steady heading. The particular heading won't matter because my GPS is providing the bearing, so as long as I keep going in the right direction (keep the compass steady) I am going where I want to go.

Lastly, maybe it's just me, but I can't steer within 10 degrees at all. I go this way and that way all the time.

Caveats - I sail on the Long Island sound, so NAVIGATION is usually pretty easy. Go mostly east or west and maybe a little north or south until I get where I'm going. With GPS it's real easy. Without GPS I suppose I would just sail from buoy to buoy.

Barry
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Mt. Sinai, NY

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post #14 of 29 Old 10-04-2013
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Re: Deviation

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Originally Posted by jimkyle99 View Post
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.
Which is yet another reason that chartplotters belong under the dodger and instruments above the companionway. Stacks of electronics surrounding the compass on the binnacle can cause deviation to change in unpredictable ways in addition to the awkward positioning for watchkeeping, the distraction to the helmsman, and the impact on line of sight.

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post #15 of 29 Old 10-04-2013
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Re: Deviation

wow....I am surprised. I would think avid mariners would know it all even how to sextant your position. Can anyone out there use a sextant?
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post #16 of 29 Old 10-04-2013
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Re: Deviation

Disassociated tidbits I've learned over years ..Don't run the ammeter wires up to the dashboard under the compass... GPS heading (COG) has nothing to do with boat heading, it's where you've been. Flux gate compasses don't work at high latitudes. Making a deviation chart can determine if something is seriously wrong with compass . Likely you can't steer close enough to matter in todays cruising with updates from GPS. No cause to be envious of the Bishops balls ,.
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post #17 of 29 Old 10-05-2013
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Re: Deviation

Capt Len we had an issue once sailing to fiji from new zealand pre gps days over night we would seem to veer to port it turned out the compass light swung the compass off by 30 degrees when turned on,that took some figuring out.Back to the deviation its great you are trying to learn all the old ways but having been there done that i wouldnt go back too much like hard work,if you want to use your compass it doesnt really matter how far out it is just plot your dest on your gps sail towards it,once you are under way and on course follow your compass then i do this to save battery power
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post #18 of 29 Old 10-05-2013
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Re: Deviation

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Capt Len we had an issue once sailing to fiji from new zealand pre gps days over night we would seem to veer to port it turned out the compass light swung the compass off by 30 degrees when turned on,that took some figuring out.Back to the deviation its great you are trying to learn all the old ways but having been there done that i wouldnt go back too much like hard work,if you want to use your compass it doesnt really matter how far out it is just plot your dest on your gps sail towards it,once you are under way and on course follow your compass then i do this to save battery power
But it could be useful to know beforehand how far out compass is if you one day need to set a course without the chart plotter?

Yes you can load the boat down with spare GPS units running of small batteries as a backup.

But seems to be a cheap precaution to find the deviation?

So if you ever need the compass - some warnings about what could make it unreliable could be OK.
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post #19 of 29 Old 10-05-2013
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Re: Deviation

Don't know about anyone else but on a long, straight passage, I would much rather be steering by compass or a star at night than be staring at a freakin' computer screen with a little graphic image of a roadway.
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post #20 of 29 Old 10-05-2013
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Re: Deviation

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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