US Coastal Chartwork: Currents, DR and Fixes

Prerequisites

• Latitude, longitude and measuring distance on the latitude scale. The basics are in Position Fixing.
• Converting between true, magnetic and compass (TVMDC) and using NOAA charts and Chart No. 1. See NOAA Charts, Tides & Coast Pilot.
• The UK-style version with tidal diamonds is in Course Plotting and Tidal Navigation. This lesson does the same jobs the American way.

Learning Objectives

This lesson covers the chartwork part of the ASA 105 Coastal Navigation standard. By the end of it you will be able to:

• Get a current prediction from NOAA Tides & Currents and estimate the current between slack and maximum.
• Draw a current triangle to find the course to steer and the speed made good.
• Plot and label a DR and EP using US conventions, and find set and drift from a fix.
• Plot a running fix from two bearings of one light.
• Check your compass deviation by steering down a range.
• Read a light's characteristic on the chart and in the USCG Light List.

NOAA Current Predictions

In UK waters you find the tidal stream from diamonds printed on the chart. US charts have no diamonds. Instead, NOAA publishes tidal current predictions for hundreds of current stations on its Tides & Currents website.

For each station you get, by default, a table of the times of slack water and the times and speeds of maximum flood and maximum ebb. Some useful details from NOAA's own help page:

• Speeds are in knots by default.
• Times are in local time, LST/LDT, so daylight saving is already applied unless you choose otherwise.
• Speeds are measured along the station's mean flood direction. A positive number is flood; a negative number is ebb. The flood and ebb directions are given in degrees true at the top of the table.
• Harmonic stations have predictions worked out directly for that spot, and can show speeds every 6, 30 or 60 minutes. Subordinate stations take a nearby reference station and apply time and speed corrections.
• Some stations list a depth after the name. If no depth is given, the prediction stands for all depths there.

Estimating the current between slack and maximum

The table gives only the turning points. To find the current at, say, 1000, you have three options:

1. Use a harmonic station's 30-minute output. This is the easiest and most accurate.
2. Use the "speed of current at any time" table. The printed tidal current tables included one (Table 3), and Bowditch explains its use. It assumes the current follows a smooth curve between slack and maximum.
3. Use the 50-90-100 rule of thumb. Split the time from slack to maximum into thirds. At the end of the first third the current is running at about 50 percent of its maximum; at the end of the second third about 90 percent; at the end of the last third, 100 percent. It then dies away in the same way towards the next slack.

The rule works because the speed curve is close to a sine wave: the sine of 30 degrees is 0.5 and of 60 degrees is 0.87. It is only an estimate: round to a tenth of a knot and be wary of it near headlands.

Example. Slack at 0800, maximum flood of 2.0 knots at 1100. At 0900 the flood is about 1.0 knot; at 1000 about 1.8 knots.

Set, Drift and the Current Triangle

Two words do all the work:

• Set is the direction the current flows towards, in degrees true.
• Drift is its speed, in knots.

So a current "setting 105 at 1.0 knot" pushes you towards the east-south-east at one knot. Notice this is the opposite of how wind is named.

Course to steer

To make good a track across a current, draw a triangle on the chart for one hour (or any convenient time):

1. Draw the track you want to make good from your start point, and extend it well past your expected position.
2. From the start point, draw the current vector: in the direction of the set, as long as one hour of drift.
3. Open your dividers to your speed through the water. Put one point on the end of the current vector and swing an arc to cut the track line.
4. The line from the end of the current vector to where the arc cuts the track is the course to steer (true). The distance along the track from the start to that cut is your speed made good.

If you also expect leeway, steer a few degrees further into the wind as well.

Track and speed made good

The other way round: draw your course and speed for one hour, then add the current vector to its end. The line from the start to the end of the current vector is your estimated track and speed made good.

Dead Reckoning and the Estimated Position

US practice, as set out in Bowditch, uses a few simple conventions:

Bowditch gives four rules for when to plot a DR:

1. At least every hour, on the hour.
2. After every change of course or speed.
3. After every fix or running fix.
4. After plotting a single line of position.

The DR uses only the course steered and the speed through the water. It ignores current and leeway on purpose, so that the difference between the DR and a fix tells you what the water has done to you.

The estimated position (EP) is the DR plus your best guess of current and leeway.

Finding set and drift from a fix

When you get a fix, compare it with the DR for the same time:

• The set is the direction from the DR to the fix.
• The drift is the distance from the DR to the fix, divided by the hours since you last reset the DR.

Then restart the DR from the fix, not the old DR, and use the set and drift for your EPs until the next fix. Strictly, the offset includes leeway and steering error too, but it is the best number you have.

The Running Fix

With only one charted object in sight, such as a lighthouse on a featureless coast, use two bearings of it taken at different times:

1. Take a bearing and plot it as a line of position (LOP). Note the time and log.
2. Keep a steady course and speed. Later, when the bearing has changed by at least 30 degrees or so, take a second bearing and plot it.
3. Work out how far you have run between the two times along your course.
4. Move (advance) the first LOP that distance in the direction of your course, keeping it parallel to the original.
5. Where the advanced LOP crosses the second LOP is your running fix. Label it with the second time.

A running fix is only as good as your estimate of distance run. Bowditch's advice for a light on shore: assume the current is slowing you down. A head current makes the true position closer to the shore than the plotted fix, so allowing for less distance run errs on the safe side.

Checking Deviation on a Range

A range (what the British call a transit) is two marks that line up on a known bearing. Many US channels have lit range lights. The Light List gives range bearings in degrees true, to a tenth of a degree, and the bearing is printed on the chart.

To check your steering compass:

1. Look up the true bearing of the range on the chart or in the Light List.
2. Apply variation from the chart's compass rose to get the magnetic bearing.
3. Steer straight down the range, with the front and rear lights lined up ahead or astern, and read the steering compass.
4. The difference between the magnetic bearing and the compass reading is the deviation on that heading.

If the compass reads less than the magnetic bearing, the deviation is east. If it reads more, the deviation is west. Deviation changes with heading, so a range only checks one heading (and its reciprocal, roughly). Record each check in your deviation table.

Reading Light Characteristics

On a NOAA chart a lighted aid has a short label next to its magenta flare. A typical one reads:

Fl G 4s 15ft 4M "3"

Other common rhythms are Q (quick), Iso (isophase, equal light and dark), Oc (occulting, more light than dark), Fl (2) (group flashing) and Mo (A) (Morse A, used on safe-water marks). US Chart No. 1, Section P, shows every abbreviation, and the colour and shape rules are in US Aids to Navigation.

The USCG Light List describes each aid in eight columns: number, name, position, characteristic, height, nominal range, structure and remarks. Points worth knowing:

• Heights are in feet above mean high water (on the Great Lakes, above low water datum).
• Nominal range is the distance a light can be seen in a meteorological visibility of 10 nautical miles. In poor visibility you will see it at less than that; the Light List has a diagram to estimate this luminous range.
• In Volume 7 (Great Lakes), ranges are in statute miles.
• There are seven volumes, each covering one or more Coast Guard districts. Volume I covers the First District, in New England.

Worked Example

A summer passage of 20 nautical miles in three legs, for a boat that sails at 5.0 knots through the water. Variation on the chart is 15 degrees W. You leave the harbour entrance buoy at 0900.

Leg 1. 6.0 M at 5.0 kn takes 1.2 hours, which is 72 minutes. Turn at 1012.

Leg 2. The current sets at exactly 90 degrees to the right of your track. In the triangle, the current vector is 1.0 M long and the boat's speed is 5.0. The angle you must steer into the current is the angle whose sine is 1.0 divided by 5.0, which is 0.2: about 11.5 degrees. So steer 015 minus 11.5, about 004 T. Your speed made good along the track is the square root of (25 minus 1), which is about 4.9 knots. 8.0 M at 4.9 kn takes about 98 minutes. Turn at 1150.

Leg 3. 72 minutes again. Arrive at 1302. Total time 4 hours 2 minutes for 20.0 M.

Compass courses. Add westerly variation to go from true to magnetic. Your deviation card says 1 degree W on leg 1, 2 degrees E on leg 2 and nothing on leg 3.

A fix on leg 2. At 1012 you start a new DR from the turning mark, steering 004 T. At 1112, one hour later, a three-bearing fix puts you 0.8 M from your DR, in the direction 120. Set is 120, drift 0.8 knots: a little south of what you expected, so the current is pushing you back slightly as well as sideways. Restart the DR from the fix and adjust your course.

Checking the compass. On the way in, you steer down a range listed at 352.4 T. With 15 W variation, that is 007.4 M. The compass reads 005. It reads less than magnetic, so deviation on that heading is about 2.4 E, which matches your card.

Common Mistakes

• Using tide times for current. High water and slack water can be hours apart in US bays and rivers.
• Naming set the wrong way. Set is where the current goes to, not where it comes from.
• Putting current into the DR. The DR is course and speed only. Current goes into the EP.
• Advancing an LOP the wrong distance. Use distance run through the water between the two bearing times, along the course steered.
• Mixing up feet and metres. NOAA charts and the Light List give light heights in feet.

Summary

• US charts have no tidal diamonds. Get current from NOAA's current predictions: times of slack and maximum flood and ebb, in knots and local time.
• Set is direction towards, drift is speed. A one-hour current triangle gives course to steer and speed made good.
• Label C above and S below the course line, circles for fixes, semicircles for DRs, squares for EPs, four-figure times.
• Set and drift come from the DR-to-fix offset. Always restart the DR from a fix.
• Running fix: advance the first bearing by distance run, and allow for a possible head current.
• Steer down a range to check deviation on that heading.

Check Your Understanding

1. A NOAA current table shows slack at 1300 and maximum ebb of 3.0 knots at 1600. Roughly how fast is the ebb at 1500? About 2.7 knots, 90 percent of maximum, by the 50-90-100 rule.
2. What is the difference between set and drift? Set is the direction the current flows towards, in degrees true. Drift is its speed in knots.
3. How do you mark a DR, a fix and an estimated position? A semicircle with the time at an angle for a DR, a circle with the time horizontal for a fix, and a square with the time horizontal for an EP.
4. Your DR is 1.5 M from your fix, two hours after the last reset. What is the drift? 0.75 knots, in the direction from the DR to the fix.
5. A range bears 180.0 T. Variation is 10 W. Steering down it, the compass reads 192. What is the deviation? Magnetic is 190. The compass reads 2 degrees more, so deviation is 2 W.

Sources

• https://tidesandcurrents.noaa.gov/noaacurrents/help
• https://tidesandcurrents.noaa.gov/currents_info.html
• https://msi.nga.mil/Publications/APN
• https://thenauticalalmanac.com/2024_Bowditch-_American_Practical_Navigator.html
• https://www.navcen.uscg.gov/light-lists
• https://nauticalcharts.noaa.gov/publications/us-chart-1.html