Navigation and Logbooks in the Age of Sail
Introduction
During the late 18th century, there were two complementary methods of sea
navigation: “Coastal Navigation” and “Deep-Sea Navigation.” A ship engaged in
the coastal trade might use coastal navigation most of the time. But a ship
engaged in deep-sea voyages between different countries might use coastal
navigation within 20 to 30 miles of its port of departure, then deep-sea
navigation, and finally coastal navigation again when within 20 to 30 miles of
its port of destination.
Coastal Navigation
Coastal navigation was used when a ship sailing between ports remained within
5 miles to 20 miles offshore, laying a course well offshore but still keeping in
sight headlands, lighthouses, seaports, or large noticeable features like church
towers or steeples on the coast. Such landmarks were shown on the coastal
charts, as well as “soundings” (water depth at low water), known shoals, the
direction of the flood tide, safe anchorages in five to ten fathoms of water, and
“leading-lines,” or safe approaches to harbors (Figure 1).
Figure 1a. A 1794 navigation
chart showing soundings and
landmarks near Flamborough Head.
Figure 1b. Enlargement showing
the lighthouse on Flamborough Head,
soundings, and the shoal in Bridlington
Bay.
Coastal charts also showed the type of seabed, for example, mud, sand, shells,
or shingle (small stones). When the “sounding-lead” (Figure 2) was cast and its
base hit the seabed, some material would stick to the soft tallow inset into the
end. The lead-line also showed the depth of the water, assisting the captain, or
Sailing Master on a ship-of-war, to check his position on the chart. The “handlead-line” could be used while the ship maintained slow headway in 20
fathoms or less. The line was marked at different depths by colored pieces of
cloth or leather to provide the leadsman with quick indications of the water
depth, which he immediately shouted to the quarterdeck officer. The deep-sea
lead-line required the ship to be hove-to, and could measure up to 100
fathoms, being marked with two knots at 20 fathoms, three knots at 30
fathoms, etc. The deep-sea lead-line also had a tallow inset to take a sample of
the seabed.
Figure 2. A "sounding-lead." Image: P.
Reaveley
The coastal chart could be supplemented by even more details in the annuallypublished, “Coasting Pilot” for that area, which would provide highly detailed
information on buoys, marks in channels, and the best approaches to harbors
to remain clear of shoals.
Let us take a coastal voyage between two ports.
The correct title for the Captain of a merchant ship was “Master,” as in “Master
Mariner,” but in practice the courtesy term “Captain” was used. On merchant
ships, the Master-Captain and his First Mate were responsible for navigating the
ship. The Captain of a ship-of-war, although theoretically responsible for
navigation, usually delegated this to his “Sailing Master”, shortened to
“Master.” The Sailing Master (today's Navigation Officer) ranked as an officer
and messed with them in the wardroom. The Captain and the other naval
officers could navigate, but did not see it as their primary duty.
As the ship
proceeded the Master or Master’s Mate would take regular bearings on passing
landmarks with a “bearing compass,” which was a simple, portable, wood-box
compass fitted with two thin brass vanes (Figure 7). The Master would enter
into his log book the time, the name of the landmark, and the compass bearing.
He would then apply magnetic variation to derive the bearing of the landmark
with respect to true north, the reference used on all charts. The reciprocal of
this true bearing was a position-line, which would be drawn on the chart in
pencil from the landmark out to sea. The ship lay somewhere along this
position-line at the time the bearing was taken. In theory, the Master would
then take a bearing on another landmark, repeat the procedure, and the
intersection of the two position-lines would be the position of the ship (sailing
ships typically made only 6 knots to 7 knots, so plenty of time was available to
perform these simple calculations). In practice, Masters rarely took two
bearings, since all officers could estimate distance offshore fairly accurately
using the formula: “the observer’s horizon in nautical miles equals the square
root of the observer’s height in feet above sea level multiplied by 1.17.” In
practice, seamen used a multiplication factor of 1.2. The Master standing on the
quarterdeck 25 feet above sea level had a horizon of √ 25 x 1.2 = 6 miles, and
could clearly see the waterline of a passing ship, or breakers along the coastline
at this distance. A 100-foot high steeple or lighthouse would add: √ 100 = 10 x
1.2 = 12 miles for a total of 18 miles, or even farther if the church or
lighthouse was on higher ground.
However, the Master would need to
identify the landmark, so he would need to see at least a part of it. He would
estimate distances as 12 to 15 miles, or 15 to 18 miles offshore, depending
upon the height of the landmark and would record these distances as “4 to 5
leagues,” or “5 to 6 leagues,” a "league" being 3 nautical miles. The Master
would then simply mark off on his plotted position-line the estimated distance,
which he could measure with his dividers from the vertical latitude scale at the
sides of the chart, since one minute of latitude equals 1 nautical mile. This
simple “bearing and distance” position-fixing would be repeated every hour if
possible and the results entered into the log book (Figure 3), and the positions
plotted on the chart. Figure 3. Page from the log of Winchelsea, part of the British squadron trying to overtake John Paul Jones
after the Battle of Flamborough Head.
Click here for a closer view of the logbook.
The Master would use his telescope to identify the landmarks if the ship was
well offshore. All officers of merchant and naval ships provided their own
navigational equipment, including day and night telescopes, a quadrant, a
Gunter’s scale, parallel rulers, and a fitted case of navigational instruments
including protractors, dividers, pens, and pencils.
Our Master would own two telescopes: a “long-glass”, which was a one-piece
wooden tube approximately 4 feet long, sometimes covered in leather, with the
object lens mounted in a brass ring at one end, and the eyepiece lens mounted
in a small brass tube at the other end, which could be adjusted to the
observer’s eyesight. A long-glass was heavy and awkward to use on a pitching
rolling ship, but it had a magnification factor of 15x to 20x. The Master's “night
glass” was only 2 to 3 feet long, and had a wider-aperture object lens to
permit more light to enter the telescope at twilight-dawn or twilight-dusk, or at
night by the light of the moon or stars. Although a night-glass only had a
magnification factor of 10x to 12x it was widely used by day, since it was so
convenient to handle on a ship.
Using his charts and these simple navigational methods and equipment, the
Captain or Master of a sailing ship could safely navigate round-trip coastal
voyages of several hundred miles.
The Ship’s Log
The ship’s log, or "journal" was the daily record of the
ship’s course, speed, and significant events. The log was a large hard-cover
book, about 11” x 17”, since each page had to contain a lot of information vital
to the ship’s safety. A typical ship’s log was ruled in vertical columns on each
page (Figure 4).
Figure 4. Excerpt from the log of Bonhomme Richard. Source: National Archives
Click here for a closer view of the logbook.
The Ship’s Log played a vital role in both coastal navigation and deep-sea
navigation, and when properly kept, enabled the Master to calculate the ship’s
position at any time, and especially at 12 noon each day. The ship’s log ran for
the 24 hours of the ship’s day, from 12 noon on Day 1 to 12 noon on Day 2,
with the calendar date of Day 2. Thus the log in Figure 4 ran from 12 noon on
Wednesday, 18th August to 12 noon on Thursday, 19th August 1779.
• The “H” column was for each hour of the ship’s 24-hour day. The first 12
hours would be the PM hours on the day before, and the second 12 hours
the AM hours of the calendar day.
• The “K” and “HK” columns recorded the ship’s headway, or speed through the
water, in Knots and Half-Knots as measured by casting the log-line every
hour.
• The “F” was for the depth of the water in “Fathoms” as measured by casting
the lead-line. The Fathoms column was sometimes used in the open seas
to indicate the number of fathoms in-between the knots run out on the log-
line, to serve as an indication of fractions of a knot more accurate than
“half a knot.”
• The “Courses” column showed the compass course steered by the helmsman
during that hour, with the compass course being expressed in terms of the
32 “points” of the Mariner’s Compass (Figure 5): “N” (North), “NbE (North
by East), “NNE” (North-North-East). Each ‘point’ of the compass is 11¼
degrees, so in modern terms that would be 360°, 011°, 022°, etc. The log
always recorded the course to that direction in terms of compass north, not
true north.
• The “Winds” column recorded the direction from which the wind was blowing,
and the winds shown in the log were also expressed in terms of compass
north.
• The “Remarks” column always began at 1 p.m. on the new ship’s day with a
comment on the strength of the wind. This would usually be expressed in
seaman’s terms such as “A light breeze,” “A moderate breeze,” “A fresh
breeze,” etc. It was important for the officer-of-the-watch to record the
strength of the wind, because the Master would later use this information
to estimate the ship’s windage-leeway, or drift off the line of the keel, due
to the effect of the wind upon the hull and sails. Seamen’s descriptions of
wind strengths were codified by Captain Francis Beaufort in 1838 (Table
1):
Figure 5. A Mariner’s compass. Image: P.
Reaveley
Table 1. Beaufort Scale
Seaman’s Description
Wind Velocity
Beaufort Scale
Light breeze
7-10 kts
3
Moderate breeze
11-16 kts
4
Fresh breeze
17-21 kts
5
Strong breeze
22-27 kts
6
The wind strength and its trends were very important, since as wind velocity
increases the strength of the wind increases almost as the square of the wind
velocity. The winds at the height of the top-sails and top-gallant sails 100 feet
to 150 feet above the sea would be up to 10 knots stronger than the wind at
sea level. This is the reason a ship’s sails are made smaller in area higher on
the masts (Figure 6).
Figure 6. The Bonhomme Richard under sail.
Note the smaller sails on the higher parts of the mast.
Image: William Gilkerson
The “Remarks” column also included any significant sail changes and
significant events. If Magnetic Variation or windage leeway had been specifically
observed, these values also would be recorded.
We can now readily
interpret this section of the ship’s log, type scripted from Figure 4:
Thursday, 19th August 1779
H
K
1
4
2
3
HK
F
Courses
Wind
s
Remarks
NWbN
SbW
This day begins with
pleasant weather.
1
At 1:00 p.m. on Wednesday, 18th August 1779, the ship was steering NWbN
Compass (326°), with a breeze from SbW Compass (191°), making 4 knots.
Sailing ships of that period generally cruised at 5 to7 knots, and had to take
into account winds varying in both strength and direction. Ships therefore
constantly needed to tack, since they could basically only sail with the wind one
point or more abaft of their beam without losing too much headway and gaining
too much leeway, and therefore generally averaged only approximately 100
miles per 24-hour day in actual “distance-made.”
Deep-Sea
Navigation
Deep-sea navigation was used when the ship was sailing out of
sight of land. It relied on accurately recording the ship’s compass course each
hour, the ship’s speed as measured by the log-line, and any significant changes
in the wind which might affect the ship’s leeway due to windage. Windage
leeway was a significant problem since ships had very little keel area, and
would be driven off course downwind by the wind pressure on the sails and
hull. The ship’s course steered by the helmsman was rarely the ship’s actual
Course-Made-Good.
Generally speaking, under any given wind conditions a
sailing ship made more leeway at lower speeds than at higher speeds. A ship
also made more leeway with the wind directly on her beam, or with the wind
one point either side of her beam, than with the wind on her quarter. The ship’s
leeway could be observed if a Midshipman or Mate took a Bearing-Compass
(Figure 7) to measure the bearing of the ship’s wake. The reciprocal of that
bearing compared with the ship’s compass course would be its leeway, but
there are very few log entries indicating that this was regularly practiced. Most
Masters probably just estimated leeway from the winds in the log.
Figure 7. Bearing Compass. Image: J. B
Let us now briefly describe the Compass used for steering, and the Log-Line
used for measuring the speed of a ship.
The Compass
The helmsman
stood at the ship’s wheel, which was located directly behind the compass
binnacle. The binnacle was a large oak case with a steering compass mounted
on either side, so the helmsman could always see a compass. Under most
conditions the best a helmsman could do would be to steer within one point of a
given course. The center of the binnacle contained a fish-oil lamp, mounted on
gimbals, which illuminated the compasses at night. The helmsman’s
steering compass comprised a single magnetized steel needle mounted on a
compass card engraved with the 32 points of the Mariner’s Compass (Figure 5).
The compass card and pivot were mounted in a gimbaled wooden box that
could oscillate fore and aft, and from side to side (Figure 8). Figure 8. Steering compass. Like the bearing
compass, this would be mounted in the gimbaled
box so the needle could remain horizontal as the
ship heaved and rolled.
Image: J. Boudriot
The forward face of the inside of the inner wooden box had a thin black line
painted on a white background to permit the compass to be aligned with the
ship’s keel. The Captain also had an inverted “hanging compass” mounted on a
deck beam on the ceiling of his sleeping cabin, to check on the ship’s course
while he was resting.
The Log-Line
The ship’s headway, or speed
through the water, was measured in “knots,” and the term is derived from the
knots located on the log-line which was run out every hour. This measured the
distance the ship had run from a fixed point in the water during a fixed time,
both distance and time being the same relative fraction of a nautical mile and
an hour. For example, 50 feet is 1/120th of a 6,000 foot nautical mile, and 30
seconds is 1/120th of an hour.
The fixed point in the water was a wooden
“log-chip” which was approximately a quarter section of a circle, five to six
inches on its straight edges, and ¼" to ½" thick, and weighted on its curved
edge so that it would sit vertically in the water relatively motionless. The logchip, attached to approximately 150 fathoms of thin line on a reel held by a
seaman (Figure 9), was dropped over the lee side of the stern of the ship every
hour. As the line ran out a red rag appeared on the line, indicating that the logchip was now more than a ship’s length aft of the ship, and it should be
relatively stationary in the water. The log-line was now marked with knots
every 50 feet of its length. As the red rag appeared the reel-man shouted
“Turn!”, and a seaman holding a 30-second glass turned the glass. The reelman then counted the knots as they ran off the reel. When the seaman holding
the 30-second glass saw the glass was empty, he shouted “Stop!” and the reel-
man stopped the reel. The number of knots run out was then the speed of the
ship. Because of its importance, a Midshipman or Master’s Mate usually
supervised this operation.
Figure 9. Log-line, log-chip,
and log-reel.
Image: P. Reaveley
In practice, many ships used knots spaced 48 feet apart (8 fathoms), and also
used a sand-glass of 28.8 seconds, usually shortened to 28 seconds. Other
ships used knots spaced 42 feet apart (7 fathoms), with a 28-second glass,
depending upon the personal preferences of the Captain or Master.
All
officers knew that the log chip was never truly stationary, and that even a
second of error in turning the glass, or shouting “Stop!” could result in an error
in the speed logged. However, the ship’s Course-Made-Good also was probably
only accurate to plus or minus one point of the compass (11°), so hopefully
these errors cancelled out over the length of the 24-hour ship’s day, and during
the many weeks or months of long ocean voyages. The Noon Position
The hourly entries in the ship’s log were reviewed by
the Master shortly after 12 noon each day, as soon as the noon sighting of the
sun had been made. If you return to Figure 3, the log of the Winchelsea, at the
bottom right of the page you can read "Latitude Observation 54˚18' N". By
adding together with a simple “traverse table” all the courses steered
(corrected for leeway) and speeds run on each “tack,” the Master would now
have one single value of “Course-Made-Good” and one single value for
“Distance Run.” The Master would then correct this Compass Course-MadeGood for Magnetic Variation to derive his True "Course Made." The “Course
Made” and “Distance Made” would now form the isosceles of a right-angle
triangle, in which the North-South axis would be the difference in latitude in
miles, and the East-West axis would be difference in longitude in miles (Figure
10).
Figure 10. How a Master might determine his course. Image: P. Reaveley
Since he already had the latitude and longitude calculated at 12 noon the
previous day, he only needed to convert miles run N-S and E-W into degrees
and minutes of latitude and longitude, and apply the difference in latitude, and
the "departure," or difference in longitude, to yesterday’s lat/long position to
derive today’s’ lat/long position. By definition one nautical mile equals one
minute of latitude, and 60 nautical miles equals one degree of latitude. The
‘miles difference’ in latitude was therefore directly the ‘latitude-difference’ made
during the past 24 hours. Thus 93 miles of latitude = 93 minutes of latitude = 1
degree 33 minutes of latitude.
The conversion of ‘miles-run’ E-W into
longitude was more complex than the conversion of latitude N-S due to the
convergence of the meridians towards the poles. One degree of longitude
equals 60 miles at the equator, but only approximately 47 miles at 39˚N at
Annapolis. The Master solved a simple problem in trigonometry by assuming
that the ship’s course and distance formed the hypotenuse of a right-angled
triangle. The "miles-difference" east -west was divided by the cosine of the
mid-latitude. Thus, at 39˚ N: 93 miles / cos(39) (0.70772) = 120' = 2˚ of
longitude. A Nautical Table of sines, cosines, and tangents allowed him to
perform this operation, and he could also use his Gunter’s Scale.
The
Gunter’s Scale was a one-foot or two-foot-long hard wooden ruler engraved on
one edge with inches and tenths of inches for measuring (Figure 11). On each
side the scale had a set of finely engraved horizontal lines of logarithms, sines,
cosines, tangents and secants. Using his divider on the relevant scale, the
Master could quickly solve this series of simple trigonometrical problems. The
Gunter’s Scale was the precursor to the slide-rule.
Figure 11. One-foot-long Gunter’s Scale (top)
and a close-up version (bottom.)
Images: International Slide Rule Museum.
With the "difference" in latitude, and his "departure," or "difference" in
longitude the Master had now derived today's Dead Reckoning position at 12
noon. All of the ship’s officers knew that this “Dead Reckoning” or "D.R."
position was just an estimated position, based upon data of variable quality and
basic trigonometry. It was therefore important to take a sighting of the altitude
of the sun at its zenith above the horizon at local noon, if at all possible, since
this altitude is directly related to the observer’s latitude. However, he needed to
know the specific date since the earth rotates around the sun on an axis
inclined at approximately 23.5°, and in 1779 the sun was overhead at the
equator at the equinoxes on March 20 and September 23, and over the Tropic
of Cancer (23.5°N) or Capricorn (23.5°S) at the solstices on June 21 and
December 21. The master would consult the Nautical Almanac to get the sunhour-angle for that specific date, and several correction factors. The Master and
all officers would take their noon sun-sight using a quadrant (Figure 12), the
precursor to the sextant, which was just coming into use at that time. The
Master could now compare his “D.R. Latitude” with his more accurate
“Observed Latitude,” and if necessary correct his D.R. Latitude at the time he
plotted it on the chart.
Figure 12. Quadrant used in
18th century navigation. Image: J. Boudriot
The Master also needed to review his D.R. Longitude. However, because most
ships’ officers could not perform the more than one-hour of complex
astronomical navigation calculations required in celestial navigation using
moon-star hour-angles, nor readily identify many of the several thousand stars
then visible in the sky, the astronomical calculation of “observed longitude” was
usually not possible in the 18th century. An alternative would have been to
use the “Equation-of-Time,” based upon the fact that there are 360° of
longitude around the earth, and the earth rotates on its axis once every 24
hours. Therefore, every hour the earth rotates 15° of longitude, and every 4
minutes of time equals one degree of longitude. However, since accurate
watches were rare, extremely expensive, and difficult to keep safe in a sailing
ship environment, this method of determining longitude also was not available.
The Master's longitude derived from his D.R. calculations therefore probably
remained unchanged, and for the next century, after a couple of weeks at sea,
most deep-sea ships had no idea of their real longitude to within 50 to 100
miles. We mentioned that the Master needed to correct his compass
“Course-Made-Good” from compass, or magnetic north, into true north to plot
the course on his chart. Magnetic Variation is the angular difference between
True North and Magnetic North, and Variation differs over time at the same
location, and is different at different locations on the earth’s surface. Variation
can be significant, and in the seas around Britain and France in 1779 Magnetic
North was 22° (two points of the compass) west of True North. The simplest
method to determine Magnetic Variation was for the Master to sight Polaris with
a bearing compass, and directly read off the angular difference between True
North and Compass North. He could also use his bearing compass to sight the
angle at which the sun rose in the morning and then set in the evening, and
check those values against that date in the tables in the Nautical Almanac. That
would give him the true bearings of sunrise or sunset for that date, which he
could directly compare with his compass bearings to derive magnetic
variation.
“Magnetic Deviation,” or the difference between Compass North
and Magnetic North, was known but not understood. “Deviation” was caused by
the natural magnetic fields of the often several hundred tons of ships’ iron
ballast, cannons, shot, anchors, fixtures and fittings, etc. Since seamen of that
time knew about but did not understand this form of compass variation, they
just included “deviation” in “variation.” However, the officer of the watch knew
that when the ship was under way he was never to wear his sword or his pistols
near the compass binnacle.
Having derived the ship’s Dead Reckoning
position at noon that day the Master would now prick that position on the chart
with his dividers, and join today's position to the previous day’s position with a
pen-line, which represented the Course-Made and Distance-Made. He would
then place today’s date alongside today’s position at 12-noon as shown in
Figure 13.
Figure 13.
An illustration of
“Course Made” and
“Distance Made.”
Image: P. Reaveley.
The Captain could now observe the overall progress of the ship, and knowing
the present wind conditions decide what course to steer to best reach a position
near his destination at the same latitude as his destination. He would then
carefully tack back and forth along this latitude, taking soundings and
maintaining a good lookout until reaching the coast (Figure 14). The bottom of
Figure 3 includes these computations, which were much more important when
the ship was not in sight of a known landmark.
Friday, October 1, 1779 (calendar date)
H
K HK
4 (pm) 3
1
8 (pm) 3
11
(pm)
F
23
Courses Winds
SWbS
Remarks
WNW Moderate Weather
21
3
16
Saturday, October 2, 1779 (calendar date)
H
K HK
2 (am) 3
1
Sounded every quarter of an
hour until daylight
8 (am) 3
1
NEbE
2 (pm)
Made the land of Holland
near the Texel, bearing EbS
distant 5 leagues.
4 (pm)
A pilot came on board.
Figure 14. Excerpt from the log of the Serapis.
Conclusion
Although thousands of ships were lost at sea in storms, or ran
ashore due to navigation problems, tens of thousands of ships sailed the world
for more than 300 years using latitude, log, lead, and lookout.
As we’ve seen, the logs from the Bonhomme Richard and Serapis can be very
helpful in learning about how the ships were sailed, the crew’s actions, and
each ship’s sailing capabilities. The understandable but unfortunate lack of logkeeping during the hours immediately after the battle up until the time the
Bonhomme Richard sank only adds to the many challenges of locating its final
resting place.
Glossary:
• Altitude: the vertical angle of an object such as the sun above the horizon
• Azimuth: direction of an object, measured clockwise around the observer's
horizon from north. So an object due north has an azimuth of 360°, one
due east 090°, south 180° and west 270°.
• Fathom: 6 feet or 1.8288 meters
• Knot: speed of one nautical mile per hour
• League: 3 nautical miles, or 5.556 km
• Nautical mile: 1 minute of latitude, or 1.852 km