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Old 03-03-2009, 05:10   #1
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Deviation Tables

Dear all,

I wonder if I can get your view on something,

I sail with my husband around the Irish Sea coasts of the UK. Having done all the RYA Sailing courses practical and theory I keep saying to him that we need swing the compass and work out our deviation table.

I keep getting a reply that it is not necessary and that with GPS it does matter. I keep trying to point out that this is okay if everything is fine and the GPS are both working (One hand held and works of the 12volt supply but always has batteries in it. - just in case) but he thinks he is right even though I have sailed longer than him and more qualified that him!

What I am trying to find out is when sailing in water where you are no more than 60 or 70 miles from land is he right and that is not necessary?

Or is it good seamanship to have a deviation table. What do you think?

I have created a nifty spreadsheet that would create a graph and table once the data is entered and I am dying to use it!
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Old 03-03-2009, 06:57   #2
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Err on the Side of Caution

It would be wise to make up a deviation table as a matter of course. A 5º error in a 10 mile run may be of minor inconvenience--unless its dark, or foggy or stormy--as the consequent offset is a tad less than a mile. In 60 miles, however, a 5º error yields nearly 5-1/4 miles of error. If your darling chooses not to swing the ship, you can do so informally with an accurate hand bearing compass on your day sails. Simply align your hand bearing compass with the ship's heading while standing on the coach roof, well away from any magnetic influences and ask hubby the heading he sees on the binnacle compass—not the GPS (where course over the ground may bear no relationship to where the ship is “heading”). Done repeatedly, one can come up with a fair table. (Something else to remember is that a yacht generally cannot be steered to an accuracy of better than 5º, hence the old saw, “better to err on the side of caution”; particularly when approaching a coast. By choosing one’s error, one knows which direction to turn to reach one’s objective.) We keep our deviation table, on a little card laminated in plastic, hanging from a lanyard at the binnacle.

FWIW…

s/v HyLyte
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Old 03-03-2009, 07:04   #3
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Greetings from the US.

Well, in the US, if you were on an inspected vessel (one allowed to take more than 6 passengers for hire) it would be required.

I once helmed a warship (only for about an hour )and the requirement was to keep her within either 1/2 or 1 degree of course...not easy to do. Obviously, if the compass deviation were off 2 degrees the result of a long ocean crossing would not be good.

That said....are you on a sailboat? When you are out sailing, how on course are you? Do you account for set and drift? If you or your autopilot are keeping it to within a degree, you are doing quite well and the conditions must be quite settled.

And true enough, the GPS is unaffected by the elements on a boat that cause compass deviation. Further, generally, you simply need to take a good hand bearing compass forward, away from metals in the cockpit or wheelhouse and you can get a reading without deviation (generally).

BUT.. I am with YOU. What if....you got into a situation where you were slightly confused. The GPS gave one heading, your compass gave another and your autopilot perhaps another. What if there were some interference with the sat signal of the GPS and it was in error, how would you know? How would you reconsile the error?

What you are suggesting to do, build a deviation table, is a very prudent and...SIMPLE thing to do. It might be very illuminating...what if you found that you had a bad compass? Or that there was significant deviation on the boat? Or, what if you found your compass was near perfect, you woudl then have a standard on the boat that you could use for reference.

Get a very good hand bearing compass, and your GPS/plotter and tell your husband it's all about the girl! I am sure he is a smart guy and he will figure it out.

Best

John
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Old 03-03-2009, 07:04   #4
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Originally Posted by Georgiegirl View Post
is it good seamanship to have a deviation table. What do you think?
I think it all depends on how much you use your compass. There's less need to know deviation for a bulkhead-mounted compass that only gives you the ship's head than for a binnacle-mounted compass that can give azimuths. There's less need if you don't go far from shore. In your case the argument could be made that if both GPS go down while you're 60-70 miles from land, your DR back will be that much more accurate if you know the deviation. If you can take accurate visual fixes, then so much the better. If you really want a deviation table, then you should make one. If you're going to do all the work, then I can't see why your husband should object.


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Old 03-03-2009, 13:57   #5
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Agree pretty much with Lodesman.

Can add that on most sail boats the compass card is only graduated every 5 degrees and in any sail boat other than a quite large one or in relatively calm conditions most helmsmen will not be able to steer to better than 5 degrees anyway. So getting too carried away with a deviation table is fruitless (I have actually seen professional compass adjusters create a card supposedly accurate to 1/2 degree for small sail boats - a total waste of effort and money). Practices used on large vessels are irrelevant.

But it is an educational exercise to adjust a compass oneself and one should correct for deviation as much as is possible and note any significant (more than say a couple of degrees) uncorrected deviation.

By way of example, in my own boat I don't have a deviation card. I have an electronic compass (fluxgate) and a conventional steering compass and GPS. The electronic compass has a procedure one follows where it corrects itself, the conventional compass I corrected to be correct against the electronic one as best I could within the readability of the conventional compass (the card being graduated every 5 degrees so best readability accuracy 2-1/2 degrees). I then compared both to the GPS while motoring in still water on various headings and found all 3 agreed within a degree or two so a good correlation that could be relied upon.

When on passage I as a matter of nature periodically compare the electronic compass, the conventional compass and the GPS and if any discrepancy I need to explain that to myself. Over many years the only discrepancy has been able to be attributed to the GPS being course over ground or I have put something beside the compass that has affected it - is worth remembering that the latter problem would render any deviation card useless so one should always have a procedure such as the comparison one above for verifying the steering compass, that whether one has a deviation card or not .
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Old 03-03-2009, 18:09   #6
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...The electronic compass has a procedure one follows where it corrects itself, the conventional compass I corrected to be correct against the electronic one as best I could within the readability of the conventional compass (the card being graduated every 5 degrees so best readability accuracy 2-1/2 degrees). I then compared both to the GPS while motoring in still water on various headings and found all 3 agreed within a degree or two so a good correlation that could be relied upon...
I omitted but should have added that when doing the above I did some double checks with headings and bearings against reliably charted features eg lights, daymarks. I found all was consistent but is important that such checks be done.
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Old 04-03-2009, 01:41   #7
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Just another viewpoint - until you have swung your compass (created deviation table or not), you just DON"T know if it is correct. In my view, it has no place on board if you can't be sure of it.

Yes, you could sail around forever without a compass and just use the GPS but if you have a compass on board, then be sure that it is correct.

Hubby doesn't have to be involved if he doesn't want to but I bet he will get involved if you start to do it by yourself .
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Old 04-03-2009, 04:33   #8
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Georgiegirl,

Quote:
I have created a nifty spreadsheet that would create a graph and table once the data is entered and I am dying to use it!
I think that you should do it simply because you can, and because you would find it to be fun! Hubby be damned! You're the Admiral, you know, and he's just the Captain. You outrank him. Make a command decision!
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