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
:
|
Deviation, 18° E.
|
|
|
Variation, 38° W.
|
|
|
|
being of contrary names, take their
difference.
|
|
Correction, 20°
|
apply to the right, being westerly.
|
|
True course N. 40° E.
|
|
Compass course N. 60° E.
|
|
Another example is given where the variation and
deviation are both easterly and the true course is
S., 75° West.
|
Variation, 24° W.
|
|
|
Deviation, 16° W.
|
|
|
|
being of same name, add together.
|
|
Correction, 40°
|
apply to the left, being easterly.
|
|
True course, S. 75° W.
|
|
Compass course, S. 35° W.
|
|
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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