The mariner's compass does not, however, give the true
direction of the various points of the horizon. The needle points to
the magnetic North and not to the true North, the difference between
them being called the variation of the compass, which differs widely in
various parts of the world, being sometimes easterly and sometimes
westerly, and constantly changing. The amount is generally marked on
the charts. In New York the variation for 1894 was 8° 26' West, or
three-quarters of a point to the West of the true North. Thus, to make
good a true North course, the vessel would have to steer North three
quarters West. A rule easy to remember is that westerly variation is
allowed to the left of the compass course, or bearing , and that
easterly variation is allowed to the right of the compass course or
bearing.
To convert true courses and bearings into compass courses
and bearings with variation westerly, allow it to the right of the true
course or bearing, and with variation easterly allow it to the left of
the true course or bearing.
Deviation is another error of the compass caused by local
attraction, such as the ironwork and iron ballast in a boat, or the
proximity of a marlinespike to the binnacle. In a wooden boat, if
proper care is taken, there should be no appreciable deviation of the
compass. Deviation can be discovered by swinging the boat as she lies
at her moorings, having first obtained the true magnetic bearing of
some distant object, such as a lighthouse or a church steeple. As the
vessel's head comes to each point of the compass, a compass bearing is
taken of the object, and the difference between that bearing and the
true magnetic bearing is observed and noted, and afterward tabulated.
It will often be found that the deviation differs not only in amount,
but in name, for different directions of the ship's head, being
easterly at certain points and westerly at others.
The rule is to allow westerly deviation to the
left to get the correct magnetic course, and easterly deviation to the
right to get the correct magnetic course.
To find out the error of the compass in order to steer a
true course, the sum of the deviation and the variation when both are
of the same name, and their difference when they have different names,
must be ascertained. For instance, deviation 20° West and variation
25° West, would give an error of compass 45° West, which should
be applied to the left.
If the deviation was 20° East and the variation
10° West, the difference between them would be 10° East, which
compass error should be applied to the right to steer a true course.
In order to find the compass course or course to
steer, proceed as follows, the true course being North 40° East,
the variation being 38° West and the deviation 18° East :
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Deviation, 18° E.
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|
|
Variation, 38° W.
|
|
|
being of contrary names, take their difference.
|
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Correction, 20°
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apply to the right, being westerly.
|
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True course N. 40° E.
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Compass course N. 60° E.
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Another example is given where the variation and
deviation are both easterly and the true course is S., 75° West.
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Variation, 24° W.
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|
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Deviation, 16° W.
|
|
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being of same name, add together.
|
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Correction, 40°
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apply to the left, being easterly.
|
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True course, S. 75° W.
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Compass course, S. 35° W.
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A volume might be written on the mariner's compass. It is
a fascinating study, but unfortunately my space is limited.
There is another correction to the compass that the
amateur should have cognizance of. It is called leeway, and is, in
untechnical language, the drift that the ship makes sideways through
the water because of the force of the wind or the impulsive heave of
the sea. Some craft, because of deficiency in the element of lateral
resistance, such as in the case of a shallow, "skimming-dish" sort of a
boat, with the centerboard hoisted up, will go to leeward like a crab.
Others of a different type, such as the "plank-on-edge" variety, with a
lead mine attached, will hang on to windward in a wonderful manner. It
requires, therefore, a certain amount of judgment as well as of
knowledge in this particular section of nautical lore to be able to
estimate with any degree of approximate certainty the leeway a vessel
may happen to make. It should not be forgotten that build has much to
do with this, and that trim and draught of water are also two powerful
elements in this connection. For instance, a boat with outside lead and
a centerboard in a strong breeze and a lumpy sea, so long as the wind
permitted her to carry a commanding spread of sail, might make no
appreciable leeway, but, on the contrary, might "eat up" into the wind.
But given the same boat without the lead and without the adventitious
aid that the centerboard affords, she would be compelled to dowse her
muslin at the first puff, and as a purely physical consequence she
would retain no hold on the water and would drift off to leeward like
an irresponsible she-crab.
Thus leeway must be estimated by experience. It is often a
most disturbing quantity, especially when the weather is foggy and the
channel in which you are steering is perplexing on account of rocks or
shoals. I have already expatiated on the wisdom of anchoring in such a
contingency as this whenever the elements will permit. But, of course,
one is a slave of the winds and the waves, and "bringing up" is not
always possible. I should, therefore, advise the amateur to carefully
watch his boat and endeavor to find out approximately the amount of
leeway she makes when the first reef is taken in by comparing the
direction of the fore and aft line of the boat with that of her wake.
This method may also be pursued with advantage under all conditions of
wind and weather, and by this means a moderately correct and very
useful table may be made.
The old navigators like the Drakes and the Frobishers had
this matter arranged for them, so when they sailed forth on voyages of
great emprise and portent they were guided by certain tabulated formula
that gave them full and implicit directions for the allowance of
leeway. Thus the skipper of a ship with topgallant sails furled was
told to allow one point; when under double reefed topsails, one point
and a half; when under close-reefed topsails, two points; when the
topsails are furled, three points and a half; when the fore course is
furled, four points; when under the mainsail only, five points; when
under the balanced mizzen or mizzen staysail, six points; and when
under bare poles, seven points.
This antiquated method of computation answered very well,
for those sterling and sturdy navigators of the olden times seemed to
have had a rare faculty of achieving their adventurous purpose and of
gaining, too, both fame and fortune. But the commander of a clipper
ship, with whom I sailed as a youngster, undertook to demonstrate to me
the absurdity of any such hard-and-fast rule. We had carried away our
three topgallant masts, off Cape Agulhas, while threshing hard against
a westerly gale. They were whipped out of us like pipestems. It took
all hands a whole day to clear away the wreck. Next day the weather
moderated sufficiently for us to have carried every stitch of canvas
could we have set it. There were a number of vessels beating round the
Cape, and all took advantage of the cessation of the gale to spread all
their flying kites to the breeze. Our ship, under three topsails, inner
and outer jibs, foresail, mainsail, crossjack, spanker, foretopmast,
maintopmast and mizzentopmast staysails, beat all the fleet. When it
came on to blow again we were the first to reef, because some of, our
rigging had got badly strained in the squall that took our topgallant
masts away. Still we maintained our lead, although jogging along
comfortably while our opponents were driving at it, hugging their
topgallant sails and with lee rails under.
"Now," said our captain, coming on the poop after he had
worked up his dead reckoning at noontime, "you see all those ships dead
to leeward -- well they ought to be to windward of us unless all the
books on navigation are wrong. I have entered in my traverse table the
courses we were supposed to have made good under the old rule, and have
thus proved its falsity. The fact is the ships that were turned out in
the days when these nautical axioms were first propounded were built by
the mile and cut off in lengths to suit. They had no shape to speak of
below the waterline, and perhaps the rule applied to each alike. Times
are different now, and leeway must be determined by the model of the
ship."
The rule for reckoning leeway is as follows:
Wind on starboard side, allow leeway to the left.
Wind on port side, allow leeway to the right.
Or you may thus define it:
Vessel on starboard tack, allow leeway to the
left.
Vessel on port tack, allow leeway to the right.
In this connection it might be well to urge the young
mariner against keeping his boat all a-shiver and bucking against a
head sea, and all the while sagging off bodily to leeward. It is better
far to keep the wake right astern and keep way on the vessel -- unless,
of course, the weather is too violent.
The direction and rate of tides and currents have also to
be allowed for when correcting a compass course. Thus in crossing Long
Island Sound from Larchmont to Oyster Bay in thick weather, the
magnetic course as given in the Government chart would have to be
rectified and allowance made for the condition of the tide, whether ebb
or flood, or your boat might never reach her destination.
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