Deviation?

[skipperpete]

For commercial vessels of any size a deviation card for ALL compasses on board is never an option , it’s mandatory as part of the survey...... and it needs to be re swung after steelwork and welding ........ and it must be done and signed off by a licensed compass adjuster. Class survey is very rigid about this requirement, AMSA less so and it’s an item on the USCG nav checklist for inspected vessels.
My first boat was steel and when I built it it was aligned North ,south... purely by accident and the limitations of the work site and a year of hammering and welding gave that hull a powerful magnetic signature that diminished over time but never really subsided so compass swings were a fairly regular event. Fortunately I had a Sestrel Moore compass with a shadow pin and azimuth ring so it was only a matter of doing a long slow 360° and watching the shadow on the compass card while simultaneously recording the deviation. Initially the error was 15- 30° and I corrected it to within about 5° on most headings, not without difficulty I can assure you and as time passed I got used to the errors as they became more settled and predictable. I met some cruisers who had huge problems steering to a compass course and we found that the timber spoked helm was fastened with iron bolts hidden under teak plugs, apparently it was meant as a feature at a yacht club or waterfront bar, not something meant to go to sea 4 inches away from the compass..... and the card followed the helm very gently then was released and stopped by the next hidden steel screw. I even saw one cruising yacht with a welded steel helm that was even worse. All this was pre Satnav and GPS so the accuracy of the compass was very important.

[Bowdrie]

Originally Posted by chrisr

be careful who you re calling a fool my friend because you know not to whom you speak
would you care to justify such a sweeping statement - or retract it ?
cheers,

It needs no justification, and I don't retract it.
Well, on second thought I'll retract it by half.
One who goes to sea without a compass is an ass, going to sea with a compass of unknown error(s) is probably only half-assed.

[chrisr]

Originally Posted by Bowdrie

It needs no justification, and I don't retract it.
Well, on second thought I'll retract it by half.
One who goes to sea without a compass is an ass, going to sea with a compass of unknown error(s) is probably only half-assed.

not amused mate...not even half

yes, such an accusation does need justification - or perhaps it's just something you read in a book and are parroting back ?

when you've done half as many sea miles as i have you might understand it a bit better

the deviation on a small non-steel construction vessel is negligible when compared to the other factors.

ship's heading adjusted for deviation adjusted for variation adjusted for current adjusted for leeway adjusted for tide equals COG.

added to this is the difficulty of reading a small yacht compass at sea (have you ever tried ?) and the complexity of course holding (try sailing to wind gusts/shifts instead of to a course and then at the end of a watch deciding what your average heading was)

experience and training tells me that anyone who thinks they know all the other corrections accurately enough to make knowing a couple of degrees of deviation is relevant is a fool

however i've long ago learnt not to waste time with fools so ask some other grown up to explain it to you

au revoir mon ami

cheers,

[TrentePieds]

ThreeSprings:

You've not told us what manner of boat you sail, or just where you sail it. But if it is a garden variety fiberglass cruising boat used in coastal cruising the deviation is not worth bothering with because you CANNOT steer such a vessel with enough accuracy to make it worthwhile.

If you are under power, the vessel will be thrown off her heading every few seconds by wind and wave and you need to "bring her past" her desired heading buy helming and letter her go as far "to the other side" of your desired course for as long a time as she was "thrown off" by wind and wave. That way you will steer a track undulating around your desired course.

If you are under sail, you can simply come to your desired course and trim accordingly, then keep her sailing like that REGARDLESS of wind shifts. If you do that, you need to "take a fix" every so often, say every half hour, in which time you will have gone about 3 NM, or the distance from where you are to your horizon, if you are standing up in cockpit of, say, a 30-footer. From the fix you've taken simply plot a new course to your destination. It's all common sense, so don't make a great science of it, and, above all, do NOT get involved in chasing spurious accuracy. That way lies dementia :-)!

In the Salish Sea, where I sail, you can never NOT see a landmark. Unless its foggy of course :-). Point your boat in the direction your destination lies according to your chart - you do have real honest-to-goodness paper charts, don't you :-)? - then pick a landmark on the shore way, way beyond your forestay. Then as you sail (power OR sail), you keep your forestay pointed at that landmark. You don't need to give a second thought to Can Dead Men Vote Twice or Tired Virgins Make Dull Company. Just head for your landmark - that's good enuff :-). But, anyway, there you have the old mnemonics for Compass, Deviation, Magnetic, True and the other way about. But there again, don't complicate things. In the Salish sea there is really no reason ever to bother with anything other than "Magnetic" because Deviation is trivial and Variation is invariable, at least during the time you are at sea. As always, there is profit in knowing when enuff is enuff:-)

Now in Ocean Nav, or even sailing along a shore where you have no landmarks to point at, things are a little different, but still easy, and you still can't steer accurately enuff to have to bother with the coupla degrees, or even five, of deviation you find in modern cruising boats. You do want to make sure you don't stow heavy ferrous object close to your steering compass, though.

All the best

TrentePieds

[Wotname]

Yes, adjusting the marine compass on a steel vessel is an exercise in black arts invoking all sorts of incantations and pacts with entities of the spirit world.

My first boat was an amateur built 30' steely. It had 3 previous owners before it was launched. Although I knew nothing of sailing, I did have a reasonable understanding of compass work. As this was well before the GPS era, a reliable magnetic compass was an essential bit of kit for the budding off-shore wannabe cruiser. The boat was fitted with a rather nice 6" Sestrel Moore binnacle compass along with the soft iron quadrantal correctors (Kelvin's Balls). It didn't take long before I realised the compass was all over the shop and after trying unsuccessfully to carry out the basic adjustments (A & C errors), I knelt before the local card carrying MCA (Marine Compass Adjuster), implored him to have pity, threw tokens of enticement in his direction and kissed the planks he trod on.

He lorded aboard, issued commands to me to steer towards various landmarks along the banks of the Brisbane river, he broke open his chest of dozens of little bar magnets and began sticking them (with blue tack) in and around the binnacle housing and what not. He scratched Kelvin's Balls, looked wizard-like and announced rather solemnly that castration would be necessary. I duly did the deed and the binnacle was free of the encumbrance.

More tacks and tracks were undertaken, much head scratching was present and even I could see he was calling on his reserve powers to satisfy the gods of the poles. Then in a flash of understanding he ordered one ball to be reattached. With additional adjusting, bar magnets getting chucked around, rotated and generally forced into service, it came to pass on the seventh traverse of the river, the Deviation Card came into being and without further ado, the MCA stepped ashore over my prostrated body with the hint of a smile and perhaps a slightly raised finger in salute to the god of steerage.

One lasting legacy was the question everyone asked when first stepping aboard 'Deumer' - What happened, why only one ball?

And a footnote - the MCA must have taken a kindly interest in me, he offered me a pelorus he had hand made in his younger years before his acceptance into hallowed halls of the magnetic fraternity. It fitted perfectly over the Sestrel compass and was also the subject many questions - how does it work and where did you get it, et al?

I don't suppose it is still doing service...but if anyone is ever in Moreton Bay and sees an old 30' steely with a one balled binnacle pelorus dressed Sestrel compass, do let me know!

[TrentePieds]

" 'Deumer' - What happened, why only one ball?"

Deumer has only got one ball...DUM,dummm. dum,DUM,dum, DUM,dum, dum,dum,dum...

Dimly remembered from long ago :-0)

But tell us, did the MCA entertain you at Flinder's bar? I'm told there is one in every port!

TP

[skipperpete]

That sounds delightfully familiar Wottie, my Sestrel had a little magnet carrier plate and a box of rod magnets, 4 horizontal magnets, one in each direction and one vertical.... no balls though. The chart shop owner in Townsville was a commonwealth adjuster and repairer, he refilled my Sestrel with his magic elixir after it developed a leak and it did 10 years of service until I sold the boat. GPS was the death knell for magnetic compasses and now for paper charts too.
Where’s the OP?

[Wotname]

Originally Posted by TrentePieds

...........

But tell us, did the MCA entertain you at Flinder's bar? I'm told there is one in every port!

TP

A card carrying MCA does not entertain - at least they do not entertain the likes of me.
They allow themselves to be entertained when the mood suits...
But if I was entertaining say perhaps yourself TP, it would be at the Flinder's bar!

[Jim Cate]

It pleases me to see so many old salts refuting the need for super accurate mag compass corrections, folks who have voyaged offshore for years and lived to tell of it. Why does this please me? Because it agrees with my reasonably long term experience, that's why!

Jim

[Bowdrie]

Originally Posted by skipperpete

GPS was the death knell for magnetic compasses and now for paper charts too.

You're probably more right than I care to admit.
A previous poster considered me as the fool, well, at least I can be an "old fool".
I've considered myself as fortunate to have had three main sailing/yachting mentors over the last 30>50 years, and they all were active sailors with a lot of miles under the keels of their own boats.
All three grew up in yachting families that had medium>large boats.
All three were licensed Merchant Marine officers.
Two were graduates of maritime academies.
One became a Naval Architect after the Merchant Marine.
Another had been a Navy officer who served at sea in three wars, WWII, Korea, Vietnam.
All three were in agreement on many things pertaining to seamanship.
That no matter how many electrical devices were on board, having a good compass with an up-to-date deviation card was "something to be expected" on a well-found vessel, so to speak.

[GordMay]

The year is 13,000 BCE. The first human migrants to America are about to cross the land bridge, between Eastern Russia and Alaska. The navigator seems a bit lost.

"You alright?" They ask him, waiting eagerly at the shore, with a distant view, of the new lands, that awaited them.

"Yeah, I think so," the navigator replies, staring intently at some kind of primitive sundial-compass, and peering with hand to brow, at the distant mountains, the sun, and the sea ...
"I just need to get my Bering Strait."

[SailingHarmonie]

Every boat is one lightning strike away from the 19th century. Not having an accurate steering compass is to me is either being ignorant of the risk or hoping it doesn't happen to you!

If you have an accurate electronic compass for you AP, that is calibrated and checked, making a "pretty good" deviation table is pretty simple. Swing the boat using the e-compass as the magnetic data source. not PERFECT, but pretty good.

Especially if you have radar overlay on your chartplotter, you ought ot have really good confidence that your e-compass is right. (Although about half of the deliveries I do have a hopelessly miscalibrated fluxgate compass)

[SailingHarmonie]

Originally Posted by chrisr

be careful who you re calling a fool my friend because you know not to whom you speak

would you care to justify such a sweeping statement - or retract it ?

cheers,

I'll double down on his statement, but change it a bit,

Only a fool or an ignorant landlubber sails a boat with an uncalibrated compass outside the sight of land.

Nothing wrong with being ignorant--if you are willing to learn.

[Benz]

Originally Posted by GordMay

The year is 13,000 BCE. The first human migrants to America are about to cross the land bridge, between Eastern Russia and Alaska. The navigator seems a bit lost.

"You alright?" They ask him, waiting eagerly at the shore, with a distant view, of the new lands, that awaited them.

"Yeah, I think so," the navigator replies, staring intently at some kind of primitive sundial-compass, and peering with hand to brow, at the distant mountains, the sun, and the sea ...
"I just need to get my Bering Strait."

Priceless!

[GordMay]

In the early days of pioneer life, a compass was essential. One of the more common varieties was called the "Tates" compass.
Unfortunately, it was a very low quality compass; from which, comes the expression: “He, who has a "Tates, is lost.”