Naval Warfare in the American Revolution
Lesson Plan #7
OBJECTIVES: Students will be able to:
1. Determine speed and bearing
2. Calculate a position by dead reckoning
3. Modify this position using vectors for wind and current drift
I. It’s all about location.
A. In the eighteenth century, sailors did not have tools like global positioning to tell
them where they were.
B. They had to calculate their position hour by hour and day by day by keeping track
of four pieces of information while taking others into account.
Question: What information would a sailor need in order to determine his
position at sea?
Answer: A sailor needed to know his speed, course, the wind direction, and depth
Question: What other information would a sailor need to know in order to
determine his position?
Answer: Tidal and current directions and speeds, and local magnetic variation.
C. This kind of navigation, or reckoning of position, is referred to as “dead” reckoning.
1. “Dead” is a corruption of “ded,” from “deduced” reckoning.
2. Here, “deduced” means calculated.
D. By using a variety of tools such as the quadrant, sextant, or octant, a sailor could
also “fix” his position by “shooting” the sun at noon.
1. The sun is always shot at noon, when the sun is at its zenith.
2. A sailor could read his latitude either directly off the sextant arc or by
subtracting it from 90°.
3. Longitude could also be calculated during the American Revolution though this
was a more involved process.
a. A chronometer, a very accurate clock, was needed aboard ship, set to
Greenwich Mean Time (the time in Greenwich, England).
b. By shooting the sun, moon, or certain stars at various times and looking up
the angle in a Nautical Almanac, a sailor could find out the difference in
time between his current position and Greenwich, England.
c. This difference in time could then be translated into a difference in longitude
between the ship’s current position and Greenwich, England.
d. Greenwich, England, is still the home of the prime meridian, or 0° longitude.
4. This was generally used to verify his “dead” reckoning calculations.
A. Speed is measured in knots.
B. The term comes from the way speed was measured in the eighteenth century.
C. Usually, speed was measured with the chip log.
1. The chip log was a quarter of a circle, weighted in the bottom and secured to a
line, called a log line, with a bridle to keep it upright in the water.
2. In this way, the chip log stayed in one place while the ship moved forward,
with the log line running out as the ship moved.
3. The log line had bits of cloth knotted into it at a set distance of 47 feet 3 inches.
4. The chip log was heaved into the water and as the line ran out, a particular
mark on the line was noted.
5. At that time a 28 second time glass was turned.
Answer: 6,080 feet.
5. When the time ran out, the number of knots in the log line that ran out during
the 28 seconds was counted, and this gave the speed the ship was travelling in
knots, or nautical miles per hour.
D. A second method called the Dutchman’s log could also be used.
1. The Dutchman’s log was simply a piece of wood thrown into the water.
2. When it passed some point near the bow, the time was noted.
3. When it passed some point near the stern, the time was again noted and elapsed
4. By plugging these numbers into the proportion distance in feet (bow to stern
marks) is to feet per hour; as the time elapsed (in seconds) is to the entire hour
5. The answer would give feet per hour. Simply divide by 6,080 feet to get the
ship’s speed in knots.
A. A ship’s course is measured in points.
1. There are 32 points on a mariner’s compass.
Question: If there are thirty-two points on the compass, and 360 degrees in a
circle, how many degrees are there per point?
Answer: 11.25 degrees per point.
2. Points are referred to by their relationship to the cardinal directions (north,
south, east, and west). I.e., north; north by east; north, north, east; northeast by
north; north east; and so on around the circle.
Answer: See Plate 7, The Mariner’s Compass.
3. To be more accurate, points could be subdivided by quarter points. I.e.,
southeast by east ¾ east.
B. Magnetic variation could alter the compass’s reading substantially.
1. The earth’s magnetic field is not uniform over the surface of the earth.
2. The north magnetic pole is several hundred miles south of the north geographic
pole. I.e., the compass “thinks” that the north pole is over Baffin Island in
3. Because of this, magnetic variation is most pronounced in northern latitudes.
4. Eighteenth century sailors knew of this phenomenon and knew how to adjust
for it to some extent.
C. Wind direction was also recorded in points.
IV. Depth of bottom
A. Depth of bottom is recorded in fathoms.
1. A fathom is equal to 6 feet.
B. Measuring the depth of bottom helps sailors in three ways.
1. It tells them if they have enough water under them to allow passage.
2. By comparing the sounding (depth measurement) and calculated position to a
chart, or nautical map, it could help them pinpoint their location more exactly.
3. When the depth was measured, a device was used that brought up a sample of
the bottom so that sailors knew if it was sandy, muddy, rocky, etc.
a. This would tell them about the nature of the water they were in and if their
anchors would hold.
b. Today’s sailors use an electronic depth sounder to determine the depth
but it does not tell them anything about the bottom.
V. Currents and tides
A. Because water is always moving, it will alter a ship’s actual position compared to
its calculated position.
B. Today’s sailors have this information printed in tables and sometimes directly on
C. Eighteenth century sailors had tables showing tides, but currents were often
D. A master or pilot, after calculating his position, would make a guess as to how
much the current affected their position.
E. A good master or pilot could place his ship within one nautical mile of his true
position, and many could place it within three nautical miles.
F. Today’s global positioning systems can place a ship within ten feet of its true
VI. Practice in dead reckoning
A. Table 7 shows part of H.M.S. London’s log beginning at 6 a.m. on Sunday,
September 9, 1781, as the London and a British fleet were manoeuvring to do
battle with a French fleet.
Note: Instruct students to use a convenient scale such as ¼ or ½ inch per knot when plotting this manoeuvre. This exercise can be made more challenging by introducing vectors and giving students current and wind information and instructing them to factor those into their plot either on an hourly basis (easiest way to do it - use scale of 1 inch per knot) or in their final position fix.
Table 7 and Plate 7 in either handout form for each student or on a transparency.
Student assessment will take place through quality of work on
assignments, interaction with students in discussion, and by their responses in