Titanic's Compass Deviation Card

John M. Feeney

Senior Member
Sep 20, 2000
1,072
0
0
Has anyone here encountered a reproduction of Titanic's compass deviation card during their research? It's specifically referred to during Benjamin Steele's testimony at the British Inquiry, and the comments allude to it having in fact been handed in to the Commission:

21945. Are you the Marine Superintendent at Southampton for the White Star Line? - Yes.
21946. Were you present on board the "Titanic" on the 10th of April when there was a boat drill or boat muster? - Yes.
...
21969. (Mr. Raymond Asquith.) I think you produce the compass deviation card, do you not? -Yes.
(The compass deviation card was handed in.)

Unfortunately, like several items referred to in the transcripts, this one doesn't actually seem to be *reproduced* there. Has anyone run across the card or its data separately?

Thanks!
John
 
L

lisagay harrod

Guest
John,

Of late, on the tech thread, I've been generously given (by Dave G. Brown, no less) a tutorial on binnacles, compass cards, and lubber lines. I'm still in school here...would you be so kind as to elaborate on a deviation card? Is it a card in the sense of a compass card, or does it refer simply to information recorded on headings, etc.?

Many Thanks & Cheers,
Lisa Harrod

ps...wanted you to know that appreciated your comments on the Gilded Age question I had...
 
Dec 4, 2000
3,239
483
213
The "deviation card" is more often called a "deviation table" to avoid confusion with the compass "card." One definition to a word.

Swill down a slug o' steamboat coffee 'cause what follows is pretty boring...we don't want to put too many people to sleep.

Deviation changes with the heading of the ship. At the time of adjustment, the compass adjustor (a skilled person) removes as much deviation from the compass as possible, but some always remains. This remaining deviation is carefully plotted using a Napier diagram. Such diagrams are intimidating and difficult for many people to understand. So a less complicated "deviation table" is usually constructed. This table lists the deviation for headings of every so many degrees--often every 15 degrees of change in the heading. The navigator must interpolate for intermediate headings not shown.

For example, the table may show 3 east deviation on a heading of 185; and 6 east at 200. That's 3 degrees of increase over 15 degrees of heading change. Or, 1 degree of increase for every 5 degrees of heading change from 185 to 200. By interpolation, a heading of 190 would have 4 degrees of easterly deviation, etc.

Every compass has its own unique deviation table. And, these tables are not "set in concrete." The causes of deviation are, well, "devious." For instance, if a steel ship is tied up for a long period of time it can take on a "magnetic signature" from the Earth's magnetic field. So, it is standard practice to continually verify the accuracy of the deviation table.

The function of the standard compass was to be just that--the "standard" against which the steering compasses could be checked. That's why so much care was given to protecting it from stray magnetic influences. The textbook says you are supposed to compare the steering compass to the standard once every half hour.

Titanic had four compasses: the standard compass on that platform between funnels #2 and #3; one in the wheelhouse; one on the forebridge; and one on the docking bridge. Each compass should have been provided with its own deviation table. I have noted that the singlar "table" and not the plural "tables" is used when talking about Titanic's deviation card. Hmmm. What happened to the other three cards? Were all four compasses combined on one card?

Deviation is just one of many compass errors. The other major error is variation (see below). Because courses are referenced true north, all errors are "east" or "west." When expressing a compass error you must use both the number of degrees and the name--east or west.

Navigators traditionally plot courses in "True" which is relative to the north pole. They must then apply variation, which is the difference between the true direction and the direction of the Earth's magnetic field at that particular spot on the planet. Variation changes...or "varies"...from place to place. Fortunately, it has been measured and the amount for each geographic area is printed on navigational charts.

Applying the variation to the true course gives a "magnetic" course. This is the heading to which deviation is applied to obtain the compass course. The combination of variation and deviation is known as "compass error."

Going from a true course to a compass course is known as "uncorrecting." Westerly errors are added and easterly subtracted.

Going from a compass course to a true is known as "correcting." Easterly errors are added and westerly subtracted.

OK, pretty dry and dull stuff. Not everything about seafaring is an adventure.

-- David G. Brown
 

John M. Feeney

Senior Member
Sep 20, 2000
1,072
0
0
David wrote: The causes of deviation are, well, "devious."

David: Laughed out loud at that one! :) And thank God you tackled this before me, because I just barely understand it myself. (I would have had a devil of a time trying to explain it.)

The only slight "layman's aid" I can even add here is to think of deviation itself as a tug or pull induced on a compass by its (usually, but not necessarily) metal surroundings.

A compass needle (in pocket compass parlance) is, of course, a little magnet that aligns with the earth's own magnetic field, *provided* it's isolated from other magnetic influences. So if you were standing out in the woods with a pocket compass, and there were no local magnetic sources around, it would correctly indicate magnetic north (but not true north, which is at the earth's axis of rotation).

Unfortunately, the first few explanations I ever saw of deviation deluded me into thinking it was some kind of *constant* error, sort of like a watch that keeps perfect time but sadly is always set a few minutes fast or slow. But (generally speaking), at any given time it's not the deviation that's constant, it's the magnetic *field* of those surroundings, as considered separately from the earth's magnetic field. (At least it can be visualized this way.)

So if a compass is fixed in place, as on a ship, and its surroundings (the ship) are rotated, the *interaction* between the two magnetic fields (earth's and the ship's) -- is what produces the deviation -- anything from zero, when the fields are perfectly aligned with each other such that the compass does correctly indicate magnetic north, to +/- n degrees, when the fields are fully at odds with each other.

Considered this way, deviation is sort of a score differential in a little tug-of-war. The difference depends on the heading of the ship, loosely analogous to how a tug-of-war score between equally matched teams would differ on various slopes (where one team has the downhill advantage).

The best way to actually see this -- a friend advises -- is to cut out a paper boat shape, put a compass in the middle and some magnetic metallic object on the back (such that it does visibly influence the compass needle), and then rotate the paper boat, keeping your eye on the needle. You'd think it should always point "north" (magnetic), so it should stay in the same position. But it will "deviate" to the left and right as you swing your ship. How much it does so is the ... [drum roll please] ... deviation.

Does this help? (I hope.) David, please correct me if I'm "off". It could only lead to greater enlightenment on my part.

And thanks, Lisa! (But was that me in Gilded Age, or are you thinking of John Meeks?) Thanks either way, of course. :)

Cheers,
John
 
Dec 4, 2000
3,239
483
213
I forgot..we have two mnemonics for remembering how to correct and uncorrect a compass course. The first letter of each word stands for the first letters of the various parts of the problem: Compass, True, Deviation, Variation, and Magnetic.

Correcting:

Can Dead Men Vote Twice (At Elections)?

Compass--Deviation-- Magnetic--Variation- -True
(Add East)


Uncorrecting:

True Virgins Make Dull Company (After Weddings)

True--Variation--Mag netic--Deviation--Co mpass
(Add West)


--David G. Brown
 
L

lisagay harrod

Guest
David & John,

Whew! How DO sailors ever manage to get where they're going?! Complicated stuff, but I'm such a novice that I really do find it interesting. My appreciation for the art of sailing is growing by leaps and bounds!

It also helps me understand how dangerous the sea can be...in the context of being the Master of a liner like the Titanic or the Californian. I understand better the questions here on the board like:
What time did she actually sink...
Why Boxhall's position was wrong...
They go on & on.

I appreciation you gentlemen taking the time to answer my questions. Actually John, it was Mr. Meeks that had responded to the Gilded Age post. Opps... : }

I'm going to be taking a break from the ET forum for a while...I've ordered some books and videos, and I want to study them in hopes of improving my posts here. I'll continue to lurk of course...too much to miss!

Thanks again for sharing your time and expertise so graciously!

Cheers,
Lisa Harrod

ps...I almost forgot! One last question, (I promise)...what exactly would the deviation card tell us?
 
Dec 4, 2000
3,239
483
213
It occurs to me that the deviation card might only show Lord Mersey's address. He certainly deviated from the truth when convenient.

But seriously, having a copy of the deviations for the ship's compasses would allow easier reconstruction of Titanic's ded reckoning. This might...might...give us information on what happened between say 11:00 p.m. and crunch time.

--David G. Brown
 
Dec 2, 2000
58,588
378
283
Easley South Carolina
David Brown said; "Can Dead Men Vote Twice (At Elections)?"

Why not? They seem to vote at least once in Chicago!
grin.gif


(Sorry all, but that one was just too good to pass up!)
wink.gif


We now return you to your regularly scheduled discussion, already in progress!

Cordially,
Michael H. Standart
 
Dec 4, 2000
3,239
483
213
Michael...in my classes I always told my students that the answer to that question was, "yes, but only in Chicago." Today, it has been nearly a half century after the Kennedy/Nixon election during which Chicgo's deceased population provided the winning edge. So, I've had to change my patter to, "Yes, but only in Palm Beach County."

-- David G. Brown
 

Jim Currie

Member
Apr 16, 2008
4,941
607
183
Funchal. Madeira
C.E.R.T.E.L.M.E.L.C...Certelmelc!
C.ompass to T..rue : E..asterly to the R..ight.
T..rue to M..agnetic: E..asterly to the L..left.
M.agnetic to C..ompass: E..asterly to the left.

There's a True Pole and a Magnetic one. The position of the Magnetic one VARIES at a constant rate so is easy to determine.(Variation)The earth is a giant magnet with invisible lines of force emanating from the poles - much like your school bar magnet. The lines of force are more or les in a north south direction. A ship, is just a bar of metal and like a bar of metal can be temporarily magnetised from another magnet (remember doing this as a Kid and making your own magnet?) In simple terms the ship becomes a magnet itself. This magnetism effects the magnets in the ships compasses.
If the ship travelled north and south all its life, the induced magnetism would be fairly easy to determine. However, since this is not so, the strength of this induced magnetism is constantly changing.
When the ship is built, it lies on it's slipway in a certain direction for a long time. The the earth's field induces magnetism into the hull in a very precise way.
After the ship is ready to go to sea, a Compass Adjuster swings the ship round 360 degrees .. stopping it briefly at 'points'. He takes true bearings of known objects at each point and notes the ship's head by compass. By calculation he is then able to determine the difference between the true and compass bearing. Since he already knows the VARIATION - what's left is the DEVIATION. He can then note all the Deviations on every course. He plays with compensating magnets to remove as much of the deviation error as possible then from what's left, compiles a set of deviation curves and a deviation card. Simple!