Sailing charts--scales 1:600,000 and smaller are for use in fixing the mariner's position as he approaches the coast from the open ocean, or for sailing between distant coastwise ports.

General charts--scales 1:150,000 to 1:600,00 are for coastwise navigation outside outlying reefs and shoals.

Coast charts--scales 1:50,000 to 1:150,000 are for inshore navigation leading to bays and harbors of considerable width and for navigating large inland waterways.

Harbor charts--scales larger than 1:50,000 are for harbors, anchorage areas, and the smaller waterways.

Special charts--various scales, cover the intercostals and miscellaneous small-craft areas.

True Virgins Make Dull Companions

T     +     V     =     M     +     D    =     C

True    +   Variation   =   Magnetic   +   Deviation   =   Compass

When dealing with variation and deviation remember "West is Best" and "East is Least," meaning you add when it is West and subtract when it is East.

Variation is recorded on the compass rose on the chart.  

Deviation must be manually calculated for each compass onboard.

Example:

The bearings are 240 degrees True with 7 degrees West variation and 4 degrees East deviation.

T + V = M + D = C

240 T + 7 V (West is Best) = 247 M - 4 D (East is Least) = 243 C

You would then steer 243 degrees using your boat compass in order to proceed 240 true degrees.

Deviation Card

1.  Create a card like the one displayed.

2.  Sail the boat at each heading listed on the card using the binnacle compass to determine the correct heading.

3.  While on each heading, also record what a hand compass determines the heading to be.  Have the person using the hand compass stand as far away as possible from the engine and any other metal object.  The best place is at the bow of the boat.

4.  After sailing through all the headings (i.e. swinging the ship), subtract the compass reading from the binnacle reading for each heading.  If the number is positive, it is West (west is best).  If the difference is negative, it is East (east is least).  For example:  90 (binnacle) - 87 (hand) = + 3  (West); 240 (binnacle) - 244 (hand) = - 4 (East).

5.  Plot the differences for each point of sail on the deviation card and draw a line to connection the points.

Using the Deviation Card

1.  Calculate the Magnetic course (True +Variation = Magnetic).

2.  Look at the card for the deviation for that particular heading.

3.  Add the deviation for a West correction, and subtract the deviation for an East correction.

4.  Use the binnacle compass to steer a course based on the deviation calculation (Magnetic + Deviation = Compass).  For example, if the course is 90 degrees and the deviation is 3 degrees West, steer 93 degrees.

At some predetermined point (or when circumstances present themselves), you take a "fix" to determine your actual position and compare it to your dead reckoned position.  From there you chart a new course from the actual "fixed" position, making allowances for conditions that may have caused you to veer from your original charted course (e.g. current, leeway, etc.)

D = ST

Distance (in nautical miles)  = Speed (in knots) X Time (in hours)

For example:  Traveling at 6 knots for 2 hours = 6 X 2 = 12 nautical miles

If you wish to record time in minutes instead of hours, use the formula 60D = ST 

Remember the formula as "60 D Street" (i.e. 60 D = ST)

Minutes per Nautical Mile

To calculate the number of minutes it will take to travel 1 nautical mile, divide 60 by the speed in knots:

Minutes to travel 1 nautical mile = 60 ÷ speed in knots

For example:

60 ÷ 2 knots = 30 minute nautical mile

60 ÷ 3 knots = 20 minute nautical mile

60 ÷ 4 knots = 15 minute nautical mile

60 ÷ 6 knots = 10 minute nautical mile

Lines of Position from Charted Objects

Fixes  are taken using a hand bearing compass.

Fixes are better with nearby objects.

Tangent lines from the sides of islands also serve as good lines of position.

Take fore and aft bearings first.

Take the side bearings last.

Use three lines of position, ideally 60 degrees apart.

The center triangle where they intersect (called a "cocked hat") is the position of the boat.

The average error for a line of position is 5 - 6 degrees, and the error increases with the distance from the objects.

By computing a dead reckoned position and taking a fix, you can calculate the set and drift of a current.

1.  Plot a course and travel along it for 1 hour.

2.  Compute a dead reckoned position along that course.

3.  Take a fix and mark your actual position.

4.  Join a track line to your actual position.

5.  Join a current vector from your dead reckoned position to your fixed position.  This is the set (direction) of the current.

6.  Use your dividers to measure the current vector, and use the  latitude markings to determine distance in minutes.   Since you've traveled 1 hour, the length of the current vector is the drift (speed) of the current in knots.

Example

You travel for one hour at 6 knots.

Your dead reckoned position is 6 nautical miles down the course line.

You take a GPS fix and mark your actual position. Your track line shows that you've traveled 5 nautical miles down the track, less than the 6 nautical mile shown on the course line due to the effect of the current.

You connect the current vector and measure it with your dividers.  It measures 1 minute of latitude; therefore, its drift is 1 knot.

When the current is 90 degrees to the boat, you can do a quick calculation of the lead angle.

Lead angle = (60 X current in knots) ÷ speed in knots

Since a boat covers a nautical distance in one hour equal to its speed in knots, you would allow a 10% margin of error for each hour.

For example:  

At 6 knots, you would have a .6 nautical mile error potential in the first hour.

At 6 knots, you would have a 1.2 nautical mile error potential in the second hour.

At 6 knots, you would have a 1.8 nautical mile error potential in the third hour.

That is why you need to take regular fixes because the error potential of dead reckoning steadily increases with time.

Since dead reckoning is accurate only to 10% of the distance, it is possible that you could miss an mark to which you are sailing.

You therefore need to keep track of your estimated time in route to know when you should be encountering the mark.  Before leaving, you should calculate your estimated time to the mark. 

When 5 minutes have past and you have not encountered the mark, you need to begin a search pattern to find it (if finding it is crucial to your safe passage).

Searching for a mark (or even for a man overboard) involves an expanding square.  You can use the shoreline and water depth contours for reference.  With such a pattern, you are never more than one mile from a prior pass.

A.  Take the first line of position (LOP) from a charted object, and record the time.

B.  Travel a distance and  take a second LOP.  Record the time.

C.  Pick a point anywhere along the first LOP.  

D.  Based on speed, direction and time, do a dead reckoning (DR) from the time of the first LOP to the time of the second LOP.  Draw a course line from the first LOP to the DR point.

E.  Draw a transferred LOP through the DR point parallel to the first LOP.

F.  Where the transferred LOP intersects the second LOP is the fix.

1.  Identify a charted object on shore.

2.  Draw a line of position from the object on shore through the mark.

3.  Move to the bearing of the line of position and steer toward the object until the mark is located.

Instead, you can deliberately miss the mark and then know in which direction to proceed.

1.  Set a course directly to a destination.

2.  Veer off shortly before arriving.

3.  Follow the shoreline to the destination.

1.  Perform a deliberate miss of the destination, but know which side of the mark you are on.

2.  Use the depth sounder and the depth contour lines of the chart  to proceed along the shore to the destination.

1.  Determine the distance of visibility for an object subject to the current conditions.

2.  Draw a circle around the mark equal to the area of visibility.

3.  Plot a course to the mark.

4.  A boat normally sails 7 - 8 degrees to either side of a track line creating a cone of 15 degrees.

5.  Plot a 15 degree arch from the boat on either side of the track line.

6.  The cone must pass within the circle of visibility of the mark if you expect to see the mark.

By knowing these bearings, you can sail to the mark and only be concerned that the mark stay below the unsafe bearing you calculated.

In the example to the right, as long as the mark does not go higher than 15 degrees, you are okay.  It must remain between 15 & 5 degrees for you to reach the mark safely.  If it goes to 25 degrees, you're on the reef.

You can use the lifelines and stanchions (or any other stationary object onboard) to align with the boat in order to gauge its bearing change.  If it remains aligned with the object and continues to get closer, you should consider altering course.

Frozen Range

Leading Mark

Bow & Beam Bearings

1.  Record the time when a charted object onshore is 45 degrees from the boat.

2.  Record the time when the object is abeam of the boat (i.e. 90 degrees).

3.  The distance traveled (A) computed from time and speed is the distance from the object (B).

Angle = Speed

1.  Take a bearing with a hand bearing compass as you pass a charted object onshore.

2.  Record the time in minutes that it takes the bearing change to equal the boat speed in knots.

3.  The time in minutes will equal the distance from the object in nautical miles.

4.  If you combine the calculation with a line of position from the object to the boat, it provides you with a good estimated position.

1.  Use the GPS to determine the direction and bearing of two or three prior waypoints.

2.  Draw lines of position from these waypoints.

3.  Where they intersect is your estimated position.

1.

Draw a circle of error around a dead reckoned position.

Assume you are in the circle in the position closest to danger.

When the weather is too rough to site a hand bearing compass, aim the boat at the mark and record the bearing using the binnacle compass.

Likewise, steer abeam of the bearing and record the bearing using the boat's binnacle compass.

An average 35 foot sailboat sailing close hauled will experience leeway up to 7 degrees in 20 knot winds.

It will experience leeway of not less than 10 degrees in 30 knots of wind.