Compasses

Requirement 9:

1. Explain the degree system of compass direction.
2. Explain variation and deviation, and show how corrections are applied to correcting and uncorrecting compass headings assigned by your consultant.
3. Name relative bearings expressed in both degrees and points.
4. Be able to report objects in view and wind directions with respect to the boat, and know the duties of a lookout.
5. Name three kinds of devices used aboard ship for measuring speed and/or distance traveled and, if possible, demonstrate their use.
6. Make a dead reckoning table of compass and distances (minimum three legs) between two points, plot these on a chart, and determine the final position.

Compass Degree Systems

A compass is a device used to determine geographic direction, usually consisting of a magnetic needle or needles horizontally mounted or suspended and free to pivot until aligned with the earth's magnetic field.

For centuries, navigators used a system of compass readings, called compass points, to indicate direction. An navigator would use the cardinal points of the compass (north, south, east, and west) and intervening points between each cardinal point to indicate the direction of an object. This system allowed sailors to head in approximately the right direction, but more accuracy is needed today.

Today most navigators use the azimuth system when determining direction with a compass. The azimuth system divides the compass into 360 degrees. We measure direction in degrees, minutes, and seconds clockwise from north in a horizontal plane. The minutes and seconds provide addition accuracy beyond the 360 degrees.

Some marine compasses show both the cardinal point and the azimuth figures on their cards.

Cardinal Points of the Compass

The division of the compass card according to points is an interesting one. The seaman should be familiar with both the degree system and the point system as clearly shown in the above pictures. There are 32 principal points on the compass card (each point is 11.25 degrees) and each point has a specific name. The four cardinal points are North, South, East, and West.. The following table provides the names and directions of the cardinal points and sub-cardinal points of the compass:

Back to Top
Back to Ordinary Rank Requirements

Compass Errors - Variation and Deviation

"True" direction is direction relative to the Geographic North Pole. "Compass error" is caused by anything that causes our compass reading to not point to the Geographic North Pole. We need to understand can cause compass error so that we can correct these errors and derive our actual heading.

Variation

The magnetic poles are close to the actual geographical poles. When doing navigation, especially at sea, close is not good enough. The north magnetic pole is located at approximately 78.9°N latitude and 103.8°W , over 600 miles from the geological north pole. To add to our problems, our compass doesn't point exactly towards the north magnetic pole, it points to a location near it.

Our problem is the fact that a compass will point to a direction other than true north, the difference between the two depending where on Earth the compass is. This error is called Variation.

Look at the picture below to see how variation is different depending on where you are located. The lines are called isogonic lines and are rough boundaries of variation differences.

To determine true heading, you must take into consideration the amount of variation affecting you. When the variation is east, you must subtract the variation from your true heading as determined from your map. When the variation is west, you must add the variation to your true heading.

Example: If true heading is 45 degrees and you have a 5-degree west variation, the magnetic course would be 50 degrees.

Some things to remember:

• Variation is sometimes called declination.
  
  
• An area's local variation can be found within the Compass Rose of your nautical chart.
  
  
• Variation changes a very slight amount every year due to the slow migration of the Earth's magnetic poles so check the year that your chart was printed and note the annual increase or decrease in variation.
  
  
• Some places have no variation. Other areas have extreme magnetic disturbances, to the point where conventional magnetic compasses are useless. These areas will be marked on charts of the area.
  
  
• Variation affects devices that rely on the Earth's magnetic field to work. Therefore, gyroscopic compasses, radio direction finders, and global positioning instruments are not effected by variation.
  
  

Deviation

Variation is not the only error you will experience with your compass. Another force that acts upon your compass to create error is deviation. The environment surrounding your compass causes deviation. Remember that your compass is basically a magnet. Your compass will be attracted to (or repelled by) iron bearing metal and other magnets (including magnetic fields created by flowing electricity).

Unlike variation, deviation is not constant. It is different in every boat. In fact it is even different within the same boat, depending on which direction she's sailing. Deviation is measured by the difference between the magnetic heading and the compass heading.

You should know what the deviation is on your vessel Most good compasses allow you to adjust for the deviation error. What deviation remains can be found and documented on a Deviation Card. This card or graph will list the deviation for various compass courses. It should be referred to when compass courses need to be corrected.

Some more facts about deviation:

• Objects outside your boat can create deviation.
  
  
• Don't forget about everyday objects that you take on and off your boat. Watch out for anchors, electronic devices (i.e. fish finders, radios, etc.)
  
  

Back to Top
Back to Ordinary Rank Requirements

Bearings

A bearing is simply a direction to a target (or object). The principal bearings used are true and relative. Each type serves a useful purpose at one time or another. Other types of bearings are reciprocal and target angle. All of these types of bearings are illustrated here. All bearings are measured clockwise from their reference point.

Line of bearing: The line connecting the positions of two objects. (see view A)

True bearing: The angular measurement between true north and the line of bearing to the object. (see view B)

Relative bearing: The angular measurement between own ship’s head (own course) and the line of bearing to the object. (See view C)

Reciprocal bearing: A bearing that is 180°, plus or minus, from any given bearing. Ship B bears 130° true from own ship. The reciprocal of 130° is 310°. Therefore, own ship bears 310° from ship B. (See view D)

Target angle: The relative bearing of own ship from a target ship. It is the angular measurement from the target’s head clockwise to the relative bearing of own ship. (See view E)

Back to Top
Back to Ordinary Rank Requirements

Shipboard Locations & Directions

• Abaft – A position toward the stern but not behind; between 90 and 180? on the starboard , or 270 and 360 degrees on the port
  
  
• Abeam – At right angle to the centerline of the vessel; at 90 or 270?
  
  
• Aft – Toward the stern
  
  
• Ahead – Toward the bow
  
  
• Aloft – Up above the deck
  
  
• Astern – Behind the vessel
  
  
• Athwart – Perpendicular to the keel
  
  
• Below – Beneath the deck
  
  
• Bow – The front or forward-most part of the vessel
  
  
• Fore and Aft – Parallel with the keel
  
  
• Forward – Toward the front of the vessel
  
  
• Lee Side – The side of the vessel that does not get the first blast of the wind
  
  
• Port – The left side of the vessel when observer faced forward
  
  
• Starboard – The right side of the vessel when observer faced forward
  
  
• Weather Side – The side of the vessel where the wind is hitting first (is coming from)
  
  

Back to Top
Back to Ordinary Rank Requirements

Duties of a Lookout

The lookout's responsibility is to immediately report any dangers on the water and in the boat.

Back to Top
Back to Ordinary Rank Requirements

Measuring Speed

Today, mariners measure their ship's speed using a Global Positioning System receiver. Global Positioning System or GPS is a network of man-made Earth satellites that constantly send radio signals to Earth. These signals contain the satellite position and exact time. These signals, each travelling at the speed of light, arrive at a GPS receiver at slightly different times due to the differences in the distance to each satellite. After receiving signals from at least four different GPS satellites, the receiver can calculate its position in all three dimensions. Once the receiver knows exact position at any given time, the internal electronics tracks that position in time and calculates ship's speed.

In the ancient times, the only way to measure ship speed was to throw a wood log into the water and observe how fast it moves away from the ship. This approximate method of ship speed measurement was called 'Heaving the Log' and was used until 1500-1600s when the Chip Log method was invented. The term Log comes from the history of 'Heaving the Log'.

The Chip Log apparatus consisted of a small weighted wood panel that was attached to the reel of rope, and a time measuring device. The rope had knots tied at equal distances along the reel. Sailors would throw the wood panel into the sea, behind the ship, and the rope would start unwinding from the reel. The faster the ship was moving forward the faster the rope would unwind. By counting the number of knots that went overboard in a given time interval, measured by the sand glass, they could tell the ship's speed. In fact that is the origin of the nautical speed unit: the knot.

The Taffrail Log consists of a stiff braided cotton line, a rotator (or four fan-line screws) trailing at the end of the line, and a recording mechanism with a dial. It shows the distance traveled through the water. It can be effected by currents. It allows you to calibrate the recording mechanism to allow for current. The Traffrail Log is placed outside of ship interference, usually on an outrigger and 150 feet behind the ship.

Back to Top
Back to Ordinary Rank Requirements

Dead Reckoning

What do you do if you can not see the celestial bodies to determine you position for navigation? If a storm or overcast skies are preventing your navigation with a sextant or other means, you will need to use dead reckoning.

Dead Reckoning is a method of deducing the ship's position by keeping account of the course and distances sailed from a known point of departure. Not only must the log be kept streaming and constantly read, but consideration must also be given to known currents and leeway. (Leeway is the drift of a vessel due to being driven leeward into the wind.) Dead Reckoning is an inexact science, but skippers of the old days of sail made world passages and perfect landfalls by dead reckoning alone.

Dead Reckoning Exercise

Our ship managed to get a sun shot at position A, placing her at a known spot. Immediately after, fog shut drifted in and stayed for four days. We kept the ship on course A-B, a port tack against the easterly wind, running dues SE. Just before she was tacked, the log was read and indicated that the ship had made 65 miles. It was simple enough to make a check mark on the chart, exactly 65 miles SE of the last position.

The ship was lightly laden and the skipper, remembering some of his past runs and dead-reckoning results decided that he had made leeway of eight miles. It was only a guess; all it could be. Therefore, the check was moved leeward, or westward, eight miles. So far it was easy and the ship spotted within yards of her true position.

The course from B to C was easy, too, only the skipper felt that there was a little current setting him to the west. The current tables didn't say anything about it, but he had a hunch. So at the end of that course he not only allowed for leeway, but added five miles fro a westerly current. He wasn't worried for some time the sun would shine and he could get an accurate fix. Land was still a thousand miles away.

On the course C to D, he knew from the tables that he would be a southwesterly current of two miles and hours and be in it, with this wind, four about 4 hours. This had to be allowed for, so the course steered and the course made good were really quite far apart. Current and leeway had to be estimated. From D to E he had the same things to contend with, only the current here was faster and set to the northwest.

To all this must be added loss from slippage of the log or perhaps a change in wind direction or a few hours of flat clam or a day of sailing with sheets and braces eased, off the wind, and with less leeway to subtract.

The whole reckoning, as is easily seen, becomes a series of guesses and estimates, accurate only insofar as the master's knowledge of the sea and his ship allow. An error of yards on the first course might result in an error of miles on the last. Fortunate is the skipper, wen the sun finally smiles again, or a star or lunar sight can be taken, to find himself within thirty miles of his estimated position.

In gales and hurricanes, a ship will drive before it with bare poles for days, unable to even dead reckon her position and must wait for a sight or the comfort of meeting a steamer from which she can always request her position.

Back to Top
Back to Ordinary Rank Requirements