Underwater compass navigation is both a challenging and enjoyable activity for divers. Some buying tips are discussed to assist divers in selecting the type of underwater compass that they may need.
Using an underwater compass is challenging and fun. Accurately navigating by compass is a good test of how advanced you are as a diver.
Compasses are deceptively simple instruments. They are relatively inexpensive, sturdy, have few moving parts and do not require batteries. They get thrown in dive bags and ignored for long periods of time, yet will still work when needed. They strap on a wrist or fit in a console – one size fits almost all. They always point north, but will guide you in the direction of your choice.
So, what are the negatives? Simply, if you do not hold a compass properly, the accuracy of your direction of travel is severely affected and you will be far off course over a long distance.
Table of Contents
Primitive sailors knew the winds and named the four primary directions from which they came: north, east, south and west. These four locations were called the cardinal points. As sailors and their ships became more sophisticated, the cardinal points were subdivided into intercardinals and called the half winds: northeast, southeast, southwest and northwest.
Over time the sails and steering improved, so the eight compass points were not accurate enough. The 8 points were divided into 16 and finally 32, thus producing a compass with 32 points. This “wind dial” resembled a 32 petalled flower in the elegant hand-drawn representations of the day. The “rose of the winds” was drawn on charts and eventually became known as the chart (or compass) rose.
Technology improved over the centuries and the compass face was eventually divided into the 360 degrees we know today. This allows for very accurate navigation, if the vessel is able to maintain a steady heading, and allows for variation, deviation, local attractions and the particular compass.
Divers have a hard time maintaining a steady heading, especially with the compass position itself in constant motion. Since a steady heading is the key to accuracy, you will want to select the compass that is easiest to read and maintain in position. Here are some features to look for in an underwater compass:
1.) Large, easy to read markings on the dial.
2.) Corrosion resistant case.
3.) Movable bezel or index mark face (depending on the model of compass).
4.) Liquid filled, non-freezable liquid (usually oil) to slow the swing of the needle’s movement through its arc.
5.) Luminous dial for readability in low light.
6.) A needle that turns freely when the compass is slightly tilted (no needle drag).
7.) Moving parts that are easy to use with the type of gloves you normally wear.
8.) A long enough strap to go around your wrist/forearm and wetsuit or a diameter and depth that will fit in your console.
9.) Fewer than 360 degrees marked on the face (for easier reading).
Most compass brands and models work well technically. Some are better adapted to clumsy use underwater. Since most divers navigate within a range, consider a compass that is easy to read and has fewer degree markings. If a compass has only 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330 and 360 marked on the face, it will be easy to read and easier to use. The only compasses I have not been able to consistently use accurately are the small, ball-style models that look like a key fob.
Parts Of A Compass
An underwater compass has only a few diver-controlled parts. The following definitions are for a needle indirect compass. This is the common diver’s compass that requires you to follow a direction of travel arrow, while keeping the north needle pointed north. The parts are:
HOUSING: The plastic case with the degrees or compass points marked on the outer edge. It may include wrist strap attachment points.
LUBBER LINE: The line or arrow on the compass face. It is sometimes also called the direction of travel arrow. This is the line you want to keep perfectly lined up with an imaginary line that goes the length of your body from your head to your toes.
INDEX MARKS: Short, bold, parallel lines on the outer face of the compass, between which you line up the northseeking needle. These lines are easier to read at night or in low visibility water if they are luminescent.
BEZEL: The rotating dial inscribed with the index marks.
NORTH POINTING NEEDLE: The magnetized needle that rotates freely 360 degrees, always seeking north. This needle is easier to read if it is luminescent.
STRAP: If your compass is not mounted in a console, it needs to be strapped to your wrist. Touch-fastener straps assure a snug fit, so the compass does not rotate on your arm as your wetsuit compresses.
CONSOLE MOUNT: If your compass is to be mounted in a console, be sure its diameter and depth match the console opening and depth. Not all compasses fit in all consoles. To more accurately use the compass, the console hose must be long enough to reach around the body and be held parallel to your lubber line, while still allowing you to read the face. Standard hose lengths may be too short.
THUMB TABS: The bezel will usually have raised tabs or a textured edge. These are for gripping the bezel. If you routinely dive with neoprene gloves, be sure you can get a grip on the bezel and turn it.
Using A Compass
The key to accurately using a compass is to keep it lined up with the imaginary line that bisects your body. In moving water, low visibility and with buoyancy changes, it is a challenge to maintain position.
If the compass is wrist mounted, hold your right arm straight out and grasp the extended right arm at the elbow with the left hand. Lock your arms in this braced position. This puts a 90 degree corner on the left elbow and should put the compass in front of your face. This rigid position doesn’t feel natural and a tight wetsuit tries to snap your arms back down to your sides. If your guiding straight arm strays inward (which it wants to do constantly), it cocks your left arm and shoulder and the 90 degree corners are lost. This makes the compass cockeyed to your lubber line and you will be off course. The farther your arms move away from the 90 degree square, the farther off course you will travel.
When buddies are navigating by compass, one reads the compass and sets the direction of travel. The other diver stays one-half body length ahead of the compass-wearer. This keeps the navigator’s eyes on the compass, not trying to find an out of sight buddy. The non-navigating buddy is the pair’s eyes for the big picture. Are they about to hit something? Has their depth changed? Is the intended target in sight? When concentrating on a midwater compass swim it is very easy to gradually swim deeper since it is hard to read a compass and a depth gauge at the same time.
To use a console mounted compass, have the hose come over your shoulder and hold the compass in outstretched arms in front of your face. Your arms will feel as if they are up in the air since you want to sight over the compass, not look down on it.
If you can keep your head up and looking in the distance, versus looking down, your direction of travel will be more accurate. The best (most accurate travel) is achieved when you sight looking over the compass face and toward your intended destination. One trick I’ve used to force myself to look over the compass is to raise my arms until their upper parts almost touch my ears.
A third option that recreational divers seldom use is a compass board. This is about the size of an extra-large clipboard with the compass mounted on the face. Hand holes on each side and a vertical lubber line painted on the face complete a simple compass board. More sophisticated models have a watch, slate, pen and depth gauge, as well as a lanyard for the diver’s wrist. A compass board is held like a console. The advantage is that it is a bigger surface, easy to hold and, when it gets angled off course, the lubber line is more obvious.
The magnetic field around the earth is not perfect. Thus, a compass will not read true north at all times. The readings on a compass are affected by variation, deviation and local attractions. Ships’ navigators must quite accurately account for these adjustments or they will be miles off course. Divers travel short distances and use a compass so poorly that variation and deviation are rarely practical considerations.
Local conditions can have a far greater impact on divers. For example, if you dive in Lake Superior or anywhere on an iron ore carrying wreck, the amount of iron in the area can cause a compass needle to spin in circles!
If you have an interest in the scientific details of variation and deviation, read the compass chapter in Piloting, Seamanship and Small Boat Handling, by Charles F. Chapman. This is a very useful reference for any diver’s library.
Simple Compass Exercises
If you’ve never used a compass, start out on land. Walk on a smooth, sandy beach and your footprints will be an easy evaluation of your route. If a beach isn’t handy, fill a dish pan with water, get your bare feet wet and then walk the route on a paved surface. Hopefully, you will walk fast enough that your tracks won’t dry before you finish the course. Wear your mask to restrict your vision, hold your arms and compass in a diving position and don’t cheat by watching your feet!
From your starting position, hold the compass parallel to the ground and point the lubber line toward your destination – say a small rock 50 paces away. Watch the needle swing toward the north. Once the needle stops moving, rotate the index marks so they straddle the north needle. Hold the compass with both hands, arms stretched out in front of you, elbows locked, looking over the top of the compass. Keep the direction of travel arrow and your destination in sight. Start walking, always keeping the north needle between the index marks. At the end of 50 paces, evaluate your progress. Are your footprints in a straight line? How close are you to the intended destination?
To return to the starting point you want to take the reciprocal heading. This is the exact opposite direction on the compass. If you had originally walked with the index marks at 30 degrees, you add 180 degrees to that heading and now walk with the index marks at 210 degrees. Add 180 degrees to get a return heading for a straight course.
After you master beach navigation, it is time to go diving. The exact same principles apply: Point the compass where you want to go, rotate the index marks over the needle, keep the north arrow pointing north, follow the direction of travel arrow.
The bezel needs to turn, so keep sand and muck from getting between the bezel and the compass body. Keep compasses away from magnets or your compass will catch northward confusion, which is fatal. Most compasses are filled with oil so the needle swings slowly. Heat is hard on the oil as it changes the viscosity and may cause expansion, which can result in leaks. Shocks or violent shaking can cause a compass needle to fall off its mount.
Following a dive, rinse your compass in freshwater, rotate moving parts to be sure they are grit-free. Buckle the strap so the compass can’t slide off accidentally and dry it in a cool place. Store a compass with your other fragile dive gear.