Aids to navigation can pretty much be summed up as any object that might be used to aid in the process of safe navigation. Here, we will just be talking about some of the major aids found in the US Coast Guard U.S. AIDS TO NAVIGATION SYSTEM card.

Preferred Channel Marks

In some cases, multiple channels or routes may be available with one considered to be better than the other. The preferred channel is indicated using a preferred channel mark. These are either Red over Green, or Green over Red, with the top color being considered the color of the mark.

To remember what to do with a preferred channel mark, just look at the top color. If it is red, leave it to the right if you are returning. if green, leave it to the left while returning.

Safe Water

Advises of “safe water” all around. A common location is at the entrance to a port, placed in a location to line vessel traffic up with the entrance.

These are often mistakenly referred to as “mid-channel” marks because they may be found marking the approximate middle of a wide channel.

Safe water marks flash Morse Code letter A – Mo(A) – {dot dash}.

Intra-coastal Waterway

Aids to Navigation marking the Intracoastal Waterway (ICW) display unique yellow symbols to distinguish them from aids marking other waters.

Yellow triangles indicate aids should be passed by keeping them on the starboard (right) hand of the vessel.

Yellow squares indicate aids should be passed by keeping them on the port (left) hand of the vessel.

A yellow horizontal band provides no lateral information, but simply identifies aids as marking the ICW.

Common ATON terminology

• beacon – A fixed artificial navigation mark. Daybeacon, Daymark, Lighted Beacon, and Radio Beacon are all considered types of beacons.
• daybeacon – An unlighted beacon. A daybeacon is identified by its color and the color, shape and number of its “daymark.” The simplest form of daybeacon consists of a single pile with a daymark affixed at or near its top.
• daymark – The daytime identifying characteristics of an aid to navigation.
• buoy – A floating device moored or anchored to the bottom as an aid to navigation.
• can buoy – An unlighted buoy of which the upper part of the body (above the waterline), or the larger part of the superstructure has the shape of a cylinder or nearly so.
• nun buoy – An unlighted buoy of which the upper part of the body (above the waterline), or the larger part of the superstructure, has a cone shape with vertex upwards.
• lighted beacon – A beacon exhibiting a light.
• lighted buoy – A buoy exhibiting a light.

ALA Maritime Buoyage System

A uniform system of maritime buoyage which is now implemented by most maritime nations.

Within the single system there are two regions, designated as Region A and Region B, where lateral marks differ only in the colors of port and starboard hand marks.

In Region A, red is to port on entering.

in Region B (used in the United States), red is to starboard on entering.

TVMDC

Unfortunately, this isn’t true when using a compass. As we will be exploring in more detail below, a compass reads “magnetic north,” a direction based on the location of the magnetic north pole in relationship to the compass location on the earth.

Shown below is an example of a “compass rose” printed as part of chart no. 18649 (Entrance to San Francisco Bay). The compass rose has two superimposed compass circles, the outer circle aligned with true north, and the inner circle aligned with magnetic north.

The center of the rose indicated variation (VAR). In this case, as of 2013 (the latest NOAA published chart) VAR is 14°00’ to the East of true north. Also included here is an annual change in variation, decreasing by 7’ (minutes) each year. 

At the time of this writing, seven years after publication, variation would have decreased by some 49 minutes.  There are 60 minutes to a degree, which in theory would make variation approximately 13°11’E in 2020. However, the amount of change is not constant from year to year, making this calculation less than exact. In fact, as of February of 2021, variation at this location is 13°19’E, approximately 8 minutes different than the calculated value.

The net result? If using the chart listed variation, don’t worry too much about adjusting it. Just use 14°E. If you have actual knowledge of what what the updated value is, then use it, rounded to the nearest whole degree. In this case, 13°E

For simplicity sake, always round to the nearest whole degree of variation.

How it Works

In the example above, a course plotted from the San Francisco approach buoy through the San Francisco Main Ship Channel and under the Golden Gate Bridge requires a course of 070T until the Point Bonita Lighthouse is abeam on the port side, then a course change to 054T passing under the bridge.

Subtracting the San Francisco area variation of 13E, the magnetic courses would be 057M and 041M

The deviation card above indicates a bit less than 4W for each of those magnetic courses. Rounding to the nearest whole degree of deviation results in adding 4W to each. Course to steer would be 061 PSC and 045 PSC.

There are a number of plotting tools available to assist in piloting. Every navigator develops his or her own favorites over time, and most find that a very few are used on a regular basis.

In its simplest form, you will need something to measure distance and something to draw straight, parallel lines.

Compass

Used to draw arcs and circles. Put a pencil lead in one leg of a divider and it becomes a compass. Many marine compasses are similar to ones used in drafting, with one notable difference. The screw mechanism used to adjust the arc radius can be bypassed and the legs closed by simply squeezing.

Rolling Plotter

The roller plotter is a great tool for transferring a parallel line to another location on the chart. It moved “up and down” simply by rolling it. To move side to side requires an additional straight edge such as a ruler, parallel ruler, or triangle.

There is a bit more of a learning curve to use a rolling plotter well, however, it is more accurate than parallel rules. There is however a tendency to slip while rolling across the chart.

Miscellaneous Plotting Tools

There are a number of miscellaneous plotting tools designed to draw and transfer lines. The three discussed above are the most common, however many variations exist. When you have any opportunity to try out something new, do it. You may find a different tool you like better.

Coast Pilot

The United States Coast Pilot consists of 10 volumes covering a wide variety of information important to navigation of U.S. water. Typically, this is information that is not able to be shown in graphic form on a chart. 

Information contained in each Coast Pilot includes area specific navigation regulations, visual landmark clues, channel and anchorage data, weather, currents, and navigational hazards.

Each Coast Pilot can be purchased in printed book form or downloaded for free as a PDF file. PDF downloads can be completed through NOAA’s Office of Coast Survey Publications page. https://nauticalcharts.noaa.gov/publications/coast-pilot/index.html

Planning and approach to an unknown harbor should never be attempted without a thorough review of the appropriate Coast Pilot.

American Practical Navigator

Also known as Bowditch, The American Practical Navigator was first published in 1802 by Nathaniel Bowditch who referred to it as the “epitome of navigation.” In my opinion, it remains that to this day.

Bowditch has been continuously updated to include advances in navigation theory and technology since its original publication. It is among the foremost guides to navigation and is a must have in any navigator’s library. As of this writing, the most recent update is the 2019 edition.

The 2019 edition of Bowditch is presented in two volumes.  Volume I is “the book” itself, while Volume II includes useful tables, calculations, and a marine navigation glossary. 

Both volumes, as well as 2002 and 2017 past editions are available for free download in PDF format from NGA publications.

Note: There is a “Wikisource” version of the American Practical Navigator available for viewing in HTML form. However, it is the 2002 edition, and does not include the most up to date information available.

There is a warning on the site that “The source document of this text is not known.” Downloading the PDF directly from the NGA is the better way to go.

Sailing Directions Enroute

Containing information similar to the United States Coast Pilot, Sailing Directions Enroute are a series of publications with navigation information on non-U.S. waters.

As with the Coast Pilot, any voyage to a foreign location must include a review of Sailing Directions Enroute for that area. 

For convenience of review, they may be downloaded in PDF format. I personally like to print and highlight sections which may impact my route.

Notice to Mariners

The US Notice to Mariners corrects NGA and NOS (National Ocean Service) charts using information collected from many sources, among them the Local Notice to Mariners. The US Notice to Mariners will contain only those chart corrections of interest to ocean-going vessels.

A vessel entering U.S. waters would benefit from downloading the current US Notice to Mariners.

Current and previous, Notice to Mariners may be downloaded as PDF files via https://msi.nga.mil/NTM.

International Notices

While navigating outside of U.S. waters, it continues to be important to keep abreast of changes and updates to the maritime environment. Most countries with a maritime tradition have publications similar to Notice to Mariners and/or Local Notices to Mariners. Two examples follow:

• NOTMAR – The Canadian Coast Guard Notices to Mariners
• ADMIRALTY Notices to Mariners – The British Notices to Mariners

Before operating in foreign waters, every effort should be made by a diligent navigator to obtain up to date information about the intended sailing area(s).

Tide and Current Tables

The effects of moon and sun on the level of the water in our oceans, especially in the coastal areas, is very noticeable. Most coastal areas of the world experience two periods of high water, and two periods of low water each day. the volume of change depends on too many factors to easily discuss here, however, knowing the amount of change in your sailing area is critical to safe navigation! This is where tide and current table come in.

In the United States, the meaning of the phrase tide tables was turned upside down January 1, 2021. On that day, due to the availability of electronic predictions products, NOAA ended production of printed tide table publications. The final printed edition provided tide and tidal current predictions for the calendar year 2020.

Tide and Tidal Current predictions are now available through NOAA online services. While others may disagree, in my opinion, this is a good change, simplifying the process of getting tidal information for a specific area.

Today, the two primary sources of tidal information are:

• https://tidesandcurrents.noaa.gov/tide_predictions.html
• https://tidesandcurrents.noaa.gov/noaacurrents/Regions
• Both of these online pages my be reached from https://tidesandcurrents.noaa.gov/, which also includes an interactive map designed to help located site specific data.

Note: Private printing companies will still use NOAA data to print tide and current booklets specific to a given area. These “Tide Books” will have very similar appearance and included information as included in past booklets.

Other Publications

As you explore the world of marine navigation, you will find a large number of other publications. Publications like Nautical Almanacs, Sight Reduction Tables, and Radar Navigation Manuals. Indispensable in their place, they are not required for coastal navigation and piloting.

Magnetic Bearing

A compass does not point in the direction of true north. Instead, it points towards the magnetic north pole. In marine navigation, the difference between true north and magnetic north is known as variation.

Note: You may also hear the term declination used to describe the same thing, however, in marine navigation the term declination is used only in celestial navigation.

Variation is not the same in different parts of the world. For example, in San Francisco Bay, variation is 13° 21′ E, meaning the compass points a bit over 13 degrees to the east of true north. In the waters near Martha’s Vineyard, variation is 14° 17′ W, and near Hawaii, it is 9° 30′ E.

To make things a bit more confusing, the magnetic north pole is constantly moving, resulting in magnetic north changing from year to year. For example, in San Francisco Bay, variation decreased from 14° 00′ E to 13° 21′ E between 2013 and 2020. In Martha’s Vineyard variations increases each year, and in Hawaii, it currently remains fairly constant.

The center of the compass rose includes a written description of variation, an “as of” date, and the annual increase or decrease rate [14° 00′ E (2013) Annual Decrease 7′]

R + S = T

I hope you will always know the direction your ship is heading. If you do, and you know the relative bearing to a target you can always calculate its true bearing. Likewise, with your heading and the target’s true bearing, you can easily calculate its relative bearing.

R+S=T is easy to remember. It is in alphabetical order. It stands for:
Relative Bearing + Ship’s Heading = True Bearing.

If you have a relative bearing of 020° and a ship’s heading of 090° then the true bearing to the target is 110° (20 + 90 = 110).

If R+S=T is true, then simple math tells you that T-S=R must also be true. Therefore, a true bearing of 110° minus a ship’s heading of 090° yields a relative bearing of 020°. If you get confused about whether to add or subtract, if you are working the alphabet from A to Z, then add. If you are going backwards, from Z to A, then subtract.

What is an estimated position

Dead reckoning uses only course, speed, time, and distance to establish an approximate position.

Two or more lines of position are required for a fix of a vessel’s actual location.

An estimated position is anything in between. As a result, it may be a DR position with set and drift taken into account. It may also be a single line of position in plotted into a DR location.

While estimated positions are approximate, they are more accurate than a simple DR position.

Set and drift and the estimated position

Expanding on the illustration above, set and drift have been added in this example.

A complete discussion of set and drift, how to calculate it, and how to compensate for it may be found in the Set and Drift lesson.

Set and drift and a single line of position

Establish an estimated position (EP) using a bearing to an object as a single line of position (LOP). The closest point along the LOP to the DR position is the EP. A 90° angle from the LOP to the DR position is always the closest point.

Continuing the DR plot

In the Dead Reckoning and Fixes lessons you were instructed to “reset” the DR plot anytime a fix is established. An estimated position is not a fix. It is still considered an approximate position. Consequently, the plot should not be reset. Instead, continue with the existing DR plot.

Learning Objectives

• Recognize factors contributing to DR vessel location “inaccuracy.”
• Comprehend what a line of position (LOP) is.
• Understand steering error
• Define Leeway
• Describe Set and Drift.
• Explain Course Made Good and Speed Made Good.
• Establish an estimated position using DR and a single line of position.
• Resolve set and drift by comparing a DR position with a Fix.
• Determine course to steer to compensate for known set and drift.

Set and Drift

Changes in movement of a vessel’s intended course due to current are known as set and drift.  Again, set is the direction of movement.  This direction is marked on the chart using a 3 digit compass direction the current is moving toward. Always plot set using degrees true. Drift is the speed of movement. Another way to say it is; the distance traveled in nautical miles in one hour. When set and drift are known, an estimated position must be plotted with each DR position.

Unknown set and drift

It goes without saying, if set and drift are unknown simply plot a normal DR position.  When at some point in the future you establish a position fix, set and drift can be calculated. Assuming your fix is different than the DR position, simply draw a vector from the DR position to the Fix. The direction of the vector is your set. The length represents drift over “total time”. Make sure to convert drift to a one hour time span.

Correcting for set and drift – determining course to steer

Back to our first example. The course made good will be substantially North of desired if set and drift are not compensated. Obviously, steering somewhat South of your desired course will put you on the proper course. The question is how much.

Course to steer example

Assume the following

• Desired Course 090T
• Estimated Vessel Speed 5 knots
• Set 323T
• Drift .7 knots

Steps – refer to the illustration below

1. Beginning at the vessel’s starting position (A) draw a vector representing the desired course, extending the line to point B, well past the distance to be travelled in one hour. Line AB represents the desired Track Made Good.
2. From point A draw a vector the direction and length of the set and drift ending at point C
3. Using point C as the center, strike an arc the distance traveled in one hour.  The point the arc crosses line AB becomes point D
4. The course to steer is the direction represented by line CD (096)
5. The estimated Speed Made Good is the length of line AD (4.5 kts)

Where am I? Where am I going?

Dead Reckoning is the systematic determination of present position, based upon course and distance travelled from a known location. It is a way to find an estimated future vessel position. In fact, possibly its most important use is projecting into the near future to see hazards. As a result, avoiding dangers along the way to a destination. In other words, dead reckoning shows the best route to follow to arrive at your destination safely.

Things to consider

Four factors become part of a dead reckoning plot.

• Course
• Speed
• Time
• Distance

First, a couple of definitions. Course is the intended direction of movement through the water. Heading is the direction the bow, or head, of the boat is pointed. In reality, heading and course may or may not be the same at any given point in time, however, to start we will treat them the same. Later, we will discuss variations on the idea of course such as course made good and course to steer.

Constructing and Labeling the Plot

Taking Departure

A DR plot requires a known starting point referred to as “taking departure.” This is your last known accurate position and is marked on the chart with an ⊗. The time of departure is noted horizontally on the chart. The size of the circle represents the degree of accuracy of the fix.

Items included in a DR plot

• Time of each DR and/or fix position using 4-digit military time with no punctuation (e.g. 0800 or 1500)
• Time – written horizontally for fixes, and diagonally for DR positions
• Course line (carried out for two estimated future DR positions)
• Above the course line, the letter C followed by the 3-digit course in degrees true for the ordered course (e.g. C 060).
• Vessel speed in knots (e.g. S 5)

Fix Expansion

By its very nature, dead reckoning contains errors.  The effects of compass error, helmsperson error, wind, and current alter your actual direction of movement through the water so that actual course achieved (course over ground – COG) differs from your intended course or heading. 

Errors in course and speed are a function of time

Consequently, as time passes, the error grows. To offset, the navigator incorporates an estimate of the maximum cumulative error into his plot. The he draws a circle of the appropriate size around his DR position.  The area covered by each circle represents every possible position of the boat.