Install the Sonar Transducers

Transducer Configurations

Transducer installation; General

Transducer Configurations

Hull mount

Bow mount

Side mount

 

Ship hull mount configuration

This configuration is used where the Bathyswath is to be permanently mounted on a vessel, and where a pole mount is not required

Pole mount configuration

This configuration is used when a permanent hull mount is not required.  The ‘wet end’, consisting of the transducers and the attitude sensor in a watertight pod, is mounted on a special plate at the end of a rigid pole. The pole is deployed over the side, or on the bow, of the vessel. The pole can be hinged or retractable, so that the system and the ship are not at risk in shallow water.  Alternatively, the Bathyswath can be deployed through a moon pool or gate valve.

Use on very small boats

Bathyswath can be used from very small boats, including open ‘Zodiac’ type inflatables and RIBs, and can be configured to use DC power from a battery, rather than mains power. The higher frequency transducers are smaller and lighter than the low frequency ones, and are thus more suitable for this application. A carefully selected system of Bathyswath with laptop and lightweight auxiliary sensors can weigh less than 10kg and consume less than 25 Watts.

The Bathyswath-SPLASH option is best suited for open boats, as its electronics housing is splash-proof.

Use on remote platforms

Bathyswath has been used on a range of remote and underwater platforms, including unmanned surface vessels (USVs) and unmanned/autonomous underwater vehicles (UUVs/AUVs). It is also ideal for use on remotely operated vehicles (ROVs).

Transducer Installation – General

The following points apply to any installation type.

Transducer versions

Bathyswath currently may use three versions of transducer, operating at 117kHz, 234kHz, or 468kHz. The first two transducer types are usually fitted with a 20 metre cable, the 468kHz usually has a 15m cable.

Transducer location

The faces of the transducers must not lie in aerated water. Aeration can occur when fine bubbles are drawn under the hull from the air-to-water interface around the hull, or from the action of propellers or other propulsion mechanisms. The sonar range of the system is severely curtailed when the transducers pass through the wash of a vessel. On no account should the transducers be mounted aft of the propellers. Avoid mounting the transducers aft of obstructions in the hull such as thruster tunnels.

The transducers should be mounted in a location where they remain in the water at all normal roll and pitch angles.

  • Measure the location of the transducers in three dimensions relative to the attitude sensor.
  • Measure the location of the transducers in three dimensions relative to the positioning system aerial.
  • Measure the depth of the transducers relative to the water line.

These measurements should be made with an accuracy of about 10 mm.

See the Bathyswath User Guide for more information on measuring sensor positions and orientation definitions.

Transducers should be usually be mounted so that:

  • The top of the front face is horizontal; i.e., there is no pitch offset
  • The front face points downwards at 30 degrees to the horizontal. That is, the surface of the front face is at 30 degrees to vertical.
  • One transducer points port and the other starboard, both at 90 degrees to the direction of motion of the vessel.

Other configurations are possible, and the software will correct for any transducer orientation. However, the angles described above are optimal for most survey conditions, and the transducers should be set up to within one or two degrees of this. Any slight offsets from these nominal angles can be measured using the post-processing patch test calibration procedure.

In some cases, it can be beneficial to adjust the vertical angle of the transducers. If operating in very deep water, close to the depth limit of the frequency option being used, then increasing the transducer angle to 40° or 45° can improve performance. Similarly, if the application is concentrating on scanning objects close to the water level, then pointing the transducers horizontally or even slightly upwards can give better results.

If the optional third forward-facing transducer is fitted, it may supplied with an installation kit that fits to the transducer V-bracket. See here.

Transducer cabling

The signals from the transducers are at very low voltages and at radio-signal frequencies. They are therefore prone to external interference unless care is taken during installation. The cable length between transducer and electronics rack should be less than 20 metres, and less than 15m for Bathyswath-H systems. The transducer cables supplied are shielded twisted pairs. Similar cables should be used if the installer supplies alternative cables to connect the transducers to the rack. Core sizes must be 16/0.2 (0.5 mm2) or greater. Avoid cable routes in close proximity to equipment or cables operating at radio frequencies, or carrying electrical power.

Transducers shipped after 2008 are usually fitted with a 1-metre ‘tail’, joined to an extension cable with an underwater cable. This allows different cable lengths to be fitted without needing to modify the transducer.

The transducers are activated by a transmission pulse at approximately 450 V rms. This can cause interference with other equipment, and could be a hazard to personnel, unless precautions are taken.

Hull Mounted Configuration

Where the Bathyswath is to be permanently mounted on a dedicated survey vessel, the transducers and attitude sensor will be fixtures on the vessel.

Possible hull configurations

Broadly, there are two possibilities: flush-mounting or chock-mounting:

Flush-mounting

Mounting the transducers so that the active face of the transducer is flush with the surface of the hull will have the minimum effect on the operation of the vessel. However, this requires that the hull include a section at the correct angle (30° from the vertical), and in a suitable, aeration-free position. This option requires that a hole the size of the transducer must be cut in the hull.  Typically, a watertight box section will be fixed into the hull, so that the integrity of the hull does not depend on the presence of the transducer. A watertight tube may be run from the back of the box up to the location of the instruments, which should be above the water level.

Chock-mounting

In this option, the transducer stands out from the hull. It is backed by a mounting frame and surrounded by a fairing to ease the flow. In this case, there is a little more resistance to flow, but this is not likely to affect vessel motion. The shape of the chock may cause the flow over the transducer face to be a little more turbulent, but on the other hand, the transducer face will be lifted away from surface effects on the vessel’s hull. This option requires only a small hole to be drilled in the vessel’s hull, to allow the transducer cable to pass through.

Transducers

  • These points are in addition to those listed in ‘Transducer Installation -General’, above.
  • Mount the transducers with their faces as flush to the hull as possible, so that turbulence across them is kept to a minimum. If flush mounting is impossible then a fairing must be constructed to smooth water flow across the transducer face. This helps to reduce vibration and entrained air, and helps to protect against damage to the transducers caused by striking submerged objects.
  • The horizontal axes must be horizontal with respect to gravity, when the vessel is at survey speed.
  • The vertical axes of the transducers must be at as near to 30 degrees, looking down from the vertical, as is possible. In other words, the face of each transducer must make an angle of 30° with vertical.
  • Design the location of the transducers to avoid multi-path reflections from the hull or other structures.
  • The location of the transducers should be as close to the apex of the keel as is practical.
  • The transducer pairs should be ideally laterally opposite each other. However, this is not essential, if the location of each transducer is accurately measured.
  • Pass the transducer cables through watertight glands in the hull, or a sealed tube running to above the waterline, in order that they may be connected to the electronics.
  • The location of each transducer must be accurately recorded, relative to some fixed point on the vessel.
  • It is often convenient to use land-survey techniques to measure these locations.
  • The locations must be measured in three dimensions, to an accuracy of about 10mm.
  • Measure the angles that a normal to the transducers makes with vertical (elevation) and the fore-and-aft line (azimuth). An accuracy of about 1 degree is sufficient at this stage. The actual elevation angle of the sonar ‘boresight’ will be measured using the acquired depth data. See the Online User Guide topics on Calibration.
  • When the vessel is afloat, record the location of the water line relative to the fixed point. The height of the vessel in the water may change as the vessel moves; this effect is called ‘squat’. This effect is difficult to assess on a particular vessel, but could be significant if ultimate depth accuracy is required. Consider consulting the manufacturer of the vessel, or using a GPS system that provides accurate height information instead.
  • Make sure that the transducer installation will not adversely effect the operation of the vessel, or cause the transducers to be damaged. If the vessel is to lie on the bottom at low tide, ensure that the transducers are not placed in a position where they will be under mechanical stress or be abraded. Similarly, for small vessels that are routinely lifted by strops, or placed in transport or storage cradles, ensure that the transducers will be safe during such operations.
  • Consider the effect of the transducers on vessel handling.
  • The transducers are extremely robust, and contain no active electronic parts other than the piezo-electric ceramic elements themselves. However, a ship’s hull is a very harsh environment, so consideration should be given to ease of repair in the case of damage.

Bow-mount Configuration

The bow-mount is one of two deployment configurations available for vessels of opportunity.

For bow mounting, the pole is fitted with V-shaped brackets that fit around the bow of the vessel. The pole is then pulled back against the bow using ratchet straps. This method has the following advantages:

  • It is very quick to fit, often requiring only a few minutes.
  • It can be fitted to most vessels with no modification to the vessel at all.
  • The transducers have a clear view of both sides of the vessel without needing to be below keel level.
  • The sides of the vessel are left clear for docking.
  • As the transducers are on the centre-line of the vessel, the effect of vessel roll is minimised.

It has the following disadvantages:

  • The straps must be kept tight; otherwise, the pole will fall off.
  • The pole cannot be fitted to vessels whose bow angle is too shallow. In general, the bow must make an angle of less than 45° with the vertical.
  • The water at the bow of the vessel can sometimes be aerated by the bow wave.
  • The transducers may be lifted out of the water by vessel pitch.
  • On large vessels, the bow can be more than the length of the transducer leads
  • (20 metres) from a suitable location for the electronics.
  • It is difficult to make the pole mount rigid relative to the vessel. This is a problem if the vessel’s gyrocompass is used as a heading reference.

 

Bow-mount components

The installation kit consists of pole, transducers, transducer frame, attitude sensor mount, adjustable angle bracket, bow-mount brackets and adjustable ratchet straps.

The two bow-mount brackets are spaced on the pole so that they both make contact with the straight section of the rake of the bow. The brackets are fitted with padding on the surfaces that are in contact with the bow, in order to protect the ship. Make sure that this padding is intact before fitting the bow-mount.

Installation sequence

  • Bolt the transducers to the cradle with the stamped ‘TOP’ labels uppermost.
  • Attach the attitude sensor to the transducer frame. Each model of attitude sensor has a different mounting assembly to fix it to the Bathyswath frame. Some attitude sensors have locating holes that need to be correctly aligned. Ensure that the attitude sensor is located the correct way around.
  • Attach the attitude sensor and the transducer assembly to the pole.
  • The angle that the transducer frame makes with the pole should be adjusted at the adjustable bracket so that when the pole is secured to the bow the transducers are horizontal. Adjustment is accomplished by loosening the four bolts that run through the radiused slots.
  • The bow angle may be estimated as follows. With the vessel at rest alongside a quay, hold an adjustable set square or mathematician’s protractor at arm’s length and use your eye to align the bottom of the square with the edge of the dock or surface of the water, and adjust the square so that the moving edge aligns with the rake of the bow. Fix the square at this angle and then transfer the angle to the bow mount (or 90° minus the measured angle, depending on the angle taken).
  • It is necessary to know the depth of the transducers below the water line when the vessel is under-way. This depth can change with loading and speed. You may therefore find it helpful to fix markers to the pole at intervals. Electrician’s tape wrapped around the pole is good for this. Carefully record the location of each mark relative to the centre of the transducers.
  • Using cable wraps, tidy the cables from the transducers and the attitude sensor, securing them along the pole.
  • Shackle the ends of the long sections of the ratchet straps to the holes provided on the V-brackets.
  • Attach a safety line to the top of the pole and secure its free end to the vessel’s bow.
  • Pass the connectors (take care not to drop them in the water) to the vessel, along with the free ends of the straps.
  • Attach four ratchet blocks to secure points. Two blocks are attached on the port side, and two on the starboard side. Fix two of them 2-3 metres aft of the bow, and the other two 5-8 metres aft of the bow.
  • Two people are now required to lift the entire assembly over the quayside into the water.
  • A third person, on the bow of the vessel should maintain tension on the safety line attached to the top of the assembly.
  • Once the assembly is in the water its weight will decrease and the bow person should be able to support it whilst the two persons on the quay come aboard.
  • These two should then take the free ends of the straps, attached to the top V-bracket, and pass them along the appropriate port and starboard sides.
  • Attach the ends of the straps to the forward pair of ratchet blocks.
  • Use the ratchets to tension the straps so that the top V-bracket is firmly in contact with the bow.
  • At this point, it is useful to check the alignment of the pole with the bow, and to give correcting instructions to the people tightening the straps.
  • Once the top mount is secure, take the bottom straps to the aft ratchet blocks.
  • Again, during tensioning the quayside person is required to assist in alignment.
  • On completion of strap tensioning, tie off the safety line to a bow strong point.
  • Inspect the straps to ensure that they are not twisted, and that they do not cross any sharp edges in the hull. The straps can become abraded if they rub against such sharp edges, or if they are caught between the hull and the dockside when the vessel is moored. If this is the case, then re-route the straps, or protect the straps with robust covering.
  • An alternative to using straps is to use steel rope between the pole and the ratchet blocks. Fix the steel rope to a length of strap a metre or two on front of the ratchet block. This allows the rope to be tightened with the ratchet. Where the rope touches the hull, it could damage the hull. Putting a protective tube over the wire rope can prevent this. Garden hose is ideal for this purpose.
  • Route the cables through the vessel and connect them to the Bathyswath electronics, as specified above.
  • Apply power to the Bathyswath system, and start the Swath software. If the attitude system is fitted in an underwater bottle on the transducer frame, note the roll and pitch angles. Use the ‘Text' window in Swath; make sure that the angles are displayed as numerical text. The sign convention of pitch is such that it is positive when the vessel is bow-up. Roll is positive port-side up.
  • If the pitch angle is more than two degrees from zero, consider lifting the pole and adjusting the angle bracket to bring the transducer and attitude sensor assembly level.
  • If the attitude system is not fitted to the transducer frame, examine the angle of the seabed by eye. However, remember that the seabed under the ship in dock may not be flat.
  • Measure the depth of the centre of the transducers, relative to the water line. Note that this can change once the vessel is moving at survey speed. It will also change with loading and the location of personnel. Use the fixed marks on the pole, if you fitted them. Record the pitch angle whilst this depth measurement is made. Also, measure the fore and aft distance from the transducers to the centre of pitch of the vessel. These measurements will allow you to estimate the change in water depth of the transducers, as the pitch of the vessel changes with speed.

Side-mount Configuration

The side-mount uses the same pole, attitude sensor and transducer frame as the bow-mount, but without the angle bracket and bow brackets. Alternatively, a pole may be custom-built for a particular vessel. This may be required if the pole needs to be longer than the Bathyswath pole supplied or if a swinging pole is required to allow quick recovery at sea.

The advantages of a side-mount are:

  • It is usually more rigid than a bow-mount.
  • It can be deployed at sea, provided that suitable fixings are designed.
  • It is stable in pitch.

The disadvantages are:

  • A small modification to the vessel is often needed.
  • It is less stable in roll than a bow-mount.
  • It needs to be lower than the bottom of the hull, and is thus liable to damage from striking the seabed. This also brings it closer to the seabed, thus reducing range in shallow water.

The pole may be fixed to the side of the vessel using a variety of techniques. Some vessels may have brackets fixed to their side that can be adapted for use with the Bathyswath pole. Others may need a small modification. The Bathyswath pole assembly includes a special bracket that enables the pole to be fixed to many vessels with a minimum of alterations. The simplest approach is to fix the bracket to the side of the vessel, using bolts, clamps, or similar fixings. The pole is fixed to the bracket using U-clamps. The end of the pole is pulled tight using guys running fore, aft, and sideways using a belly-strap running under the vessel to the rail on the other side. Steel rope is better for this application than rope, as it is less elastic. Screw-up tighteners (bottle screws) are useful to make the whole assembly rigid. The problem with this approach is that it can be difficult to recover and deploy at sea, so it may not be suitable if long transits to the survey site are needed. If the pole is mounted alongside an open deck area, recovery at sea can be possible. A drill for this must be thoroughly practised whilst alongside. Review this drill for safety.

An alternative approach is to weld U-shaped brackets to the side of the vessel, and fit the pole with mating studs. The pole can then be lowered into position when at sea. This procedure usually needs a hydraulic crane or winch, and can be hazardous in rough sea conditions. For this reason, another popular approach is to fit the pole with a swivelling mount at the top, so that it can be rotated up out of the water when not in use. The pole can be raised and lowered using a winch, and tightened fore and aft with ropes.

If a pole is fabricated specially, then it needs a horizontal square plate to be firmly welded to the end. This plate must have four 12 mm diameter holes, drilled in a square pattern on 120 mm centres.

Side-mount components

The installation kit consists of pole, transducers, transducer frame, attitude sensor mount and pole bracket. Wire rope, shackles, bottle screws, and fixings allowing eyes to be made in the wire rope are also required. These latter components are available from most ship’s chandlers, and need to be selected to suit the particular vessel used.

Installation sequence

  • The precise installation procedure will vary from installation to installation, depending on the various factors discussed above.
  • Shackle three guy lines to the pad-eyes on the base of the pole. Three lines are required; one forward, one aft, and the third as a belly-strap.
  • Locate hard-points for the three lines, and fix shackles to them. Two of the hard points should be as far forward and aft as practical. The third should be on the other side of the vessel, directly across from the location of the pole.
  • Assemble the attitude sensor and transducer frame assembly.
  • Fix the assembly to the pole.
  • Carefully measure and record the distance from the top of the pole to:
    • the bottom of the transducers, or the lowest point of the pole;
    • the centre of the transducers; and
    • the centre of the attitude sensor.
  • It is also useful to mark the pole at intervals with tape, so that any changes in the depth of the transducers can be estimated when the vessel is underway.
  • Estimate the depth of transducers that will be needed to clear the bottom of the hull.
  • It is usually easiest to fix the side-bracket to the pole whilst the pole is out of the water. Estimate the correct location of the bracket that gives the required transducer depth.
  • Fix the transducer and attitude sensor cables to the pole.
  • Loosely fix the fore and aft guy lines to their hard-points.
  • Run the belly-strap under the vessel, and loosely fix it to its hard-point. A shackle or similar weight loosely passed over the line will help to sink it under the vessel. Be sure that the belly-strap does not foul the propellers or other hull fixtures.
  • Lift the pole over the side and clamp it in place. Three or more people are required for this task.
  • Secure the three guy lines, with adequate tension to steady the assembly when underway. Care should be taken not to bend the pole too much when doing this. However, a small amount of bend in the direction of the belly-strap is inevitable. The trick here is to fix the side-clamp so that the pole is angled outwards slightly, and then to pull it in with the belly-strap.
  • Route the cables through the vessel and connect them to the Bathyswath electronics, as specified above.
  • Apply power to the Bathyswath system, and start the Swath software. If the attitude system is fitted in an underwater bottle on the transducer frame, note the roll and pitch angles. Use the ‘Text' window in Swath; make sure that the angles are displayed as numerical text. The sign convention of pitch is such that it is positive when the vessel is bow-up. Roll is positive port-side up.
  • The pitch angle can usually be brought to zero by tightening and loosening the fore and aft guy lines as appropriate. The roll angle can be altered by tightening and loosening the belly-strap. If this does not provide sufficient control, place a chock on the appropriate side of the pole clamp.
  • If the attitude system is not fitted to the transducer frame, examine the angle of the seabed by eye. However, remember that the seabed under the ship in dock may not be flat.
  • Measure the depth of the centre of the transducers, relative to the water line. Note that this can change once the vessel is moving at survey speed. It will also change with loading and the location of personnel. Use the fixed marks on the pole, if you fitted them. Record the pitch angle whilst this depth measurement is made. Also, measure the fore and aft distance from the transducers to the centre of pitch of the vessel. These measurements will allow you to estimate the change in water depth of the transducers as the pitch of the vessel changes with speed.