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Underwater Navigation

A skill that many of us could improve upon. Getting disorientated or lost underwater can spoil your dive. Use these techniques to improve your underwater navigation.


Introduction

There is no easier place to get lost than in the underwater environment, not only does a diver have to concentrate of factors such as safety, buoyancy, time, depth, air and of course their buddy, they also have to contend with a 3D world, poor visibility, untraceable sounds and currents. If that's not difficult enough, there are no maps to follow and the terrain, with a few exceptions, is random, irregular and unpredictable.

That said, there are a number of techniques that can greatly improve the chances of successfully navigating underwater. Practice is essential so take every opportunity to try out navigational exercises.

A simple wreck dive

Much depends on the intended dive site, diving a small intact wreck in good visibility is relatively simple, identify where the shot line is in relation to the wreck e.g., bow, stern, amidships etc. Investigate the immediate area around the shot line without loosing site of it, again getting orientated. When confident the diver can navigate back, then venture further afield, returning occasionally to the shot line. It is usually helpful to follow a natural line for example; the deck rail, so no time is lost if there needs to be a rapid return to the shot line.

Misplaced shot line

If the shot line has missed the wreck then you may have to do a search. The simplest method is to secure the shot line and tie a line onto it and reel out in different directions (e.g., North, East, South, and West) until you find the wreck. When found, tie off your reel onto the wreck and that will enable the following divers to also locate the wreck. Should your search not succeed you can easily reel in back to the shot line.

Other forms of search that may be useful in this case, are the circular or square search, more on this later.

Whilst descending the shot line get as orientated as possible with the dive site, noting any landmarks. Always note the depth of water at the bottom of the shot line and the bottom terrain. It will help you find it again.

Methods of Navigation

Divers navigate underwater in two ways, dead reckoning and pilotage.

  • Dead Reckoning is following a compass bearing in a specific direction, keeping track of speed and time. Just as in surface dead reckoning, other factors will limit the absolute accuracy of the underwater dead reckoning. Current, variation in a diver's swimming technique, and errors in holding a course will cause the diver's actual track to differ from the diver's dead reckoning. These factors can be compensated for.
  • Pilotage - Navigation by reference to terrain features, both natural and artificial, usually with the aid of an appropriate chart. Typically following bottom contours, gullies, reefs, kelp borders etc.

Planning

The most important part of underwater navigation is the planning. Several things must be considered in advance and may impact on the navigational techniques used:

  • What is the expected scenario, e.g., a flat sandy bottom or rocky or crevasse filled terrain or wreck dive, clearly the latter will have an impact on any compass work. Get as much information about the location as possible.
  • What is the intended dive plan, for example, in a shore dive is the plan to head North for 15 minutes then South or methodical search, or moving from point to point?
  • What resources are available, e.g., compass.
  • What is the navigational skill level of the divers.
  • The average swimming speed of the divers.

Decide which technique will produce the most chance of success while ensuring safety. As a knowledgeable navigator, a diver will have a repertoire of techniques, each suitable for a particular type of mission. Allow for Murphy's Law and consider an alternate plan, just in case it becomes necessary.

Perform as much of the mission above water as possible. Take any bearings you can. Your surface support is usually capable of guiding the diver more accurately than the diver himself can. This can be very important when a task requires methodical coverage of the bottom.

Write any required bearings that are needed onto the divers slate.

Make a map

Before your dive draw a picture a map of the dive site on a dive slate. Sometimes the simple act of making the map helps fix the picture in your head, even if you never look at it again. And you can always refer to the slate under water.

Mark depths to the bottom, if they are known. Be sure to mark the location of the dive boat, or your entry point from shore.

f you're diving close to shore, the shape of land above water often indicates the bottom contour. A steep cliff dropping into the water probably continues to drop steeply under water, for example. A partially submerged rock that's steep on one side and gradually sloping on the other will probably have the same shape below the surface.

Mark down the direction of the current and the position of the sun with arrows. If the visibility is reasonably good, you will be able to see the sun under water. If it is not directly overhead, the direction to the sun and the shadows it casts can be direction indicators.

Plan a Route

Where appropriate plan a route through the dive site and try to stick to it instead of wandering randomly. Draw the route on your map and note which landmarks you'll pass along the way. Where possible follow a chain of landmarks or features.


Navigation

As a minimum, the diver must carry an underwater compass. Choose a well know brand with a proven track record. Remember that an underwater magnetic compass is still affected by variation and deviation caused by any local magnetic influences, such as a wreck, air cylinders etc. Variation for an area is the same underwater as it is on the surface, but unless the dive plan contains a really long swim, there is no real need to consider variation. Most underwater navigation is planned and executed using simple magnetic headings.

To navigate underwater with a compass, there are 7 techniques divers need to learn:

1. Understanding the compass and how to position and read it.
2. How to take "get me home" bearings
3. How to take a bearing to an object
4. How to swim accurately on a bearing
5. How to measure the distance swam
6. Reciprocals bearings
7. Squares and Triangles

1. Understanding the compass

Most underwater compasses differ from conventional ones in that the north seeking needle is not mounted to a compass card. Rather the degree scale and the lubber line are fixed on the instrument with the lubber line and 000° always facing in front of the diver. Instead, a rotating bezel contains needle index marks.

There are generally two type of compass, both can be either wrist mounted, mounted on a slate, instrument console, or attached to your kit. In any event it must be held centrally to your eyes, i.e., centrally and directly in front of you, if not your directions will be out. This typically happens with a wrist mounted compass.

Also critical is that the compass is held perfectly level so that the north seeking card can freely move. If the compass is not level and the card cannot revolve your directions will be widely out.

Types of compass

There are really two type of non electronic compass, the Suunto type and the Uwatec type.

There is a significant difference between the two types of compass; the degree marking on the Suunto is fixed and the inner bezel rotates also the degree markings run anticlockwise, where as on the Uwatec type the whole bezel can rotate and the degree markings run clockwise.

2. "Get me home" Bearings

As the name suggests, this bearing aims to get the diver home again after the dive or at any time during the dive, it if the diver becomes lost or needs to get back. They are simple to set up and use.

Example 1 - A diver is shore diving and the sea is to his North (0°), he intends to swim North for 15 minutes and then South for 15 Minutes. To set the bearing he simply turns his back to the sea (he will then be facing the same way to get to the shore) and rotates the bezel so that the North seeking needle is between the two needle index marks.

During the dive the diver simply rotates himself until the north seeking needle falls again within the two needle index marks (i.e., he will be facing South or 180°) and he swims in that direction towards the shore. A key factor in this is that once the bearing is set the bezel must not be changed.

3. How to take a bearing to an object

To take a bearing to something you simple point the direction of swim arrow to the object, holding the compass at eye level, ensuring that the swim arrow remains pointed at the object, look into the viewing window (right) and take note of the bearing.

4. How to swim accurately on a bearing

Swimming on a bearing can be achieved by two methods. Note that all compass work depends on two skills. Firstly, holding the compass absolutely central to the diver. In practice this can be difficult to achieve with a wrist mounted compass unless you adopt the positioning on the right (assuming the compass is on the divers right wrist). Where possible hold the compass with both hands or mount it on a navigation board (see picture below).

Secondly, The compass must also be held perfectly horizontal, so the North seeking needle can rotate freely. Not achieving these two factors accounts for most navigation errors.

  1. The diver rotates underwater whilst holding the compass directly in front of them with the direction of swim arrow pointing outwards. Looking into the viewing window stop when the correct bearing is displayed. The direction of swim arrow points the way.
  2. Navigation Board The second method allows the diver to look down at the compass. This is achieved by setting the compass by rotating the bezel to the bearing required. Note that this is slightly different with the two compasses shown.

    In the Suunto example, if a diver is required to swim on a bearing of 100° Then rotate the inner bezel so that it reads 100° on the outer scale (there is a white mark adjacent to the two index marks), then rotate the compass so that the north seeking needle aligns to the two index marks. The direction of swim arrow points the way.

    In the Uwatec example, turn the bezel do that it reads the correct bearing on the front edge (furthest away), then rotate the compass so that the north seeking needle aligns to the two index marks. The forward edge of the compass or lubber line points the way.

In practice it is not a good idea to constantly watch the compass, it is better to check bearings frequently. It should be noted that a 2 degree error on a 300 meter swim could mean an error of 10 or so meters.

5. How to measure the distance swam

Accurately measuring the distance travelled is probably the most difficult aspect of underwater navigation. Using time (at a known speed) or kick cycles as a measure for distance is somewhat inaccurate and varies considerable with each diver. This is because every diver is different, the length of their legs, strength, amount of drag created by equipment, and the bottom's condition, will all affect the distance covered. However since underwater dead reckoning will be relative to the diver, this doesn't matter. Diver "A" may require 4 minutes or 100 kick cycles to get to a target while diver "B" may need 6 minutes or 140. However long it takes becomes the baseline for that particular distance. What does matter though is that each diver know their average swim speed and the average number of fin cycles to cover say, 100 meters.

When the objective of the navigation is to arrive at a particular destination, the diver may not be accurate enough to get there and being off course by a meter, in bad visibility, may cause you to miss the target completely. If the divers’ measurement of distance is poor, he could end up a considerable distance from the target.

Swimming the right distance improves greatly with experience and remember, of course, that all of these methods are influenced by currents:

  • Lapsed time – An average divers speed is say, 0.75 of a meter per second, so if the target is 50 meters ahead, then simply swim for 66 seconds. (using the formula Time = Distance / Speed).

    For a square search a diver could swim two minutes in one direction, turns right and does the same, repeats this another two times and theoretically the diver should return to his starting point.

    An alternative approach may be to simple count 1,2,3,4 etc.

  • Fin strokes or cycles – The diver uses the number of kick cycles (2 kicks per cycle) to determine relative distance covered. For instance, if a diver followed a magnetic course of 090° for a total of 100 kick cycles, turned around and followed the reciprocal course of 270°, he could expect to arrive back at his starting point by about the 100th kick cycle on his return.

    With this method it is essential to maintain consistent kicking cycles. The diver must resist the urge to kick too energetically at the beginning of the dive, only to have his efficiency suffer toward the end. He must try not to favour his dominant leg, which prevents him from swimming a straight course. It’s a good idea to get your buddy to count kick cycles for the duration of the dive.

    An advantage of using kick cycles to measure distance is that you can stop on route and pick up the counting again when you move off.

  • Air consumption – Note the air pressure and swim until it reads 20 or 30 bar less. This method is not very accurate and requires the divers to maintain breathing rates. It also doesn't allow divers to stop mid journey.

  • Arm widths – Probably more accurate for short distances, but it takes a bit of effort.

  • A reel or Tape Measure – Fix the end of a line and reel out to the required distance. The lines can be marked (or have a knot or loop) at the required distance. Reels can be tied together for longer distances. It may be useful to use a carefully tied “waster” so that when the target is reached a strong pull will enable the line to be reeled in. This method works best in a snag free environment e.g., sandy bottom.

Whilst traveling on a bearing, and if the visibility permits, it may be feasible for divers to separate a little, perhaps with a buddy line, so the chances of sighting the location are increased.

6. Reciprocals bearings

Swimming and a bearing and returning on its reciprocal bearing are a basic technique that all divers need. To calculate the reciprocal you simple add 180° or take it away. Examples:

The reciprocal of 125° is 305° (added 180°)
The reciprocal of 235° is 55° (minus 180°)

7. Squares and Triangles

To swim in a square clockwise simple:

  1. Swim for a specific distance (e.g., 100 fin cycles) on a specific bearing (e,g, 90°).
  2. Stop, add 90° to that bearing (now 180°) and swim on that bearing for the same distance.
  3. Stop, add 90° to that bearing (now 270°) and swim on that bearing for the same distance
  4. Stop, add 90° to that bearing (now 360°) and swim on that bearing for the same distance

If the distance and bearing were accurate the diver should return to the start. To do this anticlockwise merely deduct 90° each time.

Triangles are the same but add 120° and of course there are only three legs.

Don't forget the buddy!

It is very easy to become over preoccupied with the bearing and navigation and complete forget the buddy. All Divers have a duty of care to their buddy.

Planning Examples

A pair of divers wish to dive the three wrecks in the order of A, B and C. The start and finish for the dive is the Jetty. Assuming all the wreck are buoyed, it would be possible to identify all of the bearings required.

  • Jetty to Wreck A - Stand at the end of the jetty and take the bearing (about 335°)
  • Wreck A to Wreck B - Move to point D, line up A and B and take another reading (about 53°).
  • Wreck B to C - Move to point E, line up C and B, and take a bearing (about 332 degrees). Get the reciprocal of this by removing 180° (about 152°)
  • Wreck C to the Jetty - Stand on the end of the jetty and take a bearing to wreck C (about 79°). Get the reciprocal of this by adding 180° (about 259°).

The divers slate should read:

Leg From / To Bearing Distance Kick
Cycles
1 Jetty to Wreck A 335 100m 125
2 Wrecks A to B 053 300m 375
3 Wrecks B to C 152 220m 275
4 Wreck C to Jetty 259 320m 400

The "Get me home" bearing would be 180 degrees.

The kick cycles assume 1 complete cycle = 0.75m. This will vary with each diver.


Directional Indicators

  • Currents - Divers can benefit from a running current is that it can give the diver a directional reference. For example, if the diver is swimming a course of 000° and the current has a SET of 180°, the diver can keep himself oriented by always swimming into the current. Experience suggests that in open water this has some value, but in some areas caution is advised as:
    • Currents change as they move around wrecks and underwater reefs.
    • Current underwater may be different form the current on the surface.
    • If the dive is close to shore there many be many rip currents.
    • Current naturally change direction as tides ebb and flow.

    Dive planning should include an evaluation of the SET and DRIFT of the site's current. Remember that we measure current by the direction to which it flows, so a SET of 0125° will have a current flowing to the SE. Since the diver should know the direction and force of the current, he can anticipate it's effect on his course and compensate for it using conventional techniques.


  • Surge - Surge is a fairly reliable indicator of direction as the back and forth motion is to and from the shore.

  • Sand ripples - Sand ripples are cussed by the currents and when inshore they are a good indicator, as they always run across the direction of the tidal flow. If a diver swims across these, they will be swimming to or from the shore, measuring depth will identify which.

  • Sunlight - If the water is clear enough and shallow enough you can use the sun as a directional reference. For example in a morning dive the sun will be in the east.

Course Holding Errors

Swimming underwater is similar to walking in the desert, in the absence of visual cues, the diver will assume he was swimming a straight course, but in reality his course will degrade very quickly. To keep on a desired course, the underwater navigator can use several techniques:

  • Compass Orientation - The compass must be held in-line with the centerline of the diver's body. This is almost impossible if the compass is worn on the wrist. Some divers attach the compass to a neck lanyard and hold it out in front of them with both hands, whilst they're swimming, while others have it mounted on their gauge console or mounted it onto a navigating board. This device is merely a white, non-magnetic slate with a compass rose printed on it for quickly calculating course vectors, and lots of room for writing down a rough DR plot. It also has a very visible centerline which makes orientating the compass a cinch. Held level, in front of the diver and below his horizontal plane, it is easily oriented and observed. These devices are available from dive shops or can be easily made.

  • Bottom Topography - At many dive sites the observant diver will note that there are visual cues available to help keep them oriented. Many dives involve structures such as wrecks, pipelines, bulkheads, or even debris fields. If the diver understands the layout of the structure, it can be used as an adjunct to the compass in holding a desired course. In fact sometimes the nature of the structure prevents the accurate use of the compass, such as when it's made of magnetic or ferrous material.

    Also consider rock lines, depth contours, gullies, walls, kelp and sand borders, all of which may be useful. Finally, if visibility permits it, after using your compass to determine your course, find a landmark on the bottom in line with that route and swim to it. Upon reaching the landmark, repeat the process. This allows you to reduce the total number of times that you have to take compass readings.

Another technique is to leave markers, in similar manner to “Hansel and Gretel”. For example, when swimming out from the shore in a known direction the diver hits the kelp sand border. A marker of a few piled up stones is placed at that point. The diver can then follow the border and return back to the stones whereupon he can take the reciprocal bearing back to the start point on shore.


Summary

In summary follow these tips to improve underwater navigational skills.

  1. Find out as much as possible about the dive site.
  2. Estimate current and direction from charts.
  3. Always write bearing down on the dive slate.
  4. When descending the shot line, look around and get orientated.
  5. At the bottom of the shot line note the depth and the composition of the sea bed.
  6. Where diving close to shore set a panic bearing to the nearest shore.
  7. Practice as often as possible, compass work, searches and measuring distance.
  8. Remember, at best, the accuracy of compass navigation underwater is not good.
  9. Never forget to monitor your buddy.

 

 

 

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