This publicatIon °IS number 89 in the Technical Report Ser"tes of the Fishermen's Services Branch (formerly Industrial Development Branch)

Ottawa 1976

DESIGN for a VARIABLE DEPTH

o SONAR SYSTEM

Project Officer

L. W. Proctor

for

Engineering Development and

Assistance Programs Division

Fishermen's Services Branch

Fisheries and Marine Service

Environmellt Canada

Division Chief

H. A. Shenker

Opinions expressed and conclusions reached by the author are no~ necessarily

endorsed by the sponsors of this project

VARIABLE DEPTH SONAR

Introduction

As traditional fish stocks decrease, commercial fishina is being conducted in deeper water than has been fished in the past~for both traditional and previously unexploited species.

With trawling to depths up to 800M now being tried, it became evident that ne~'1 means of fish detection would be necessa.ry for maximum efficiency. To provide this facility, the Vessels and Engineering Division of the Industrial Development Branch, Fisheries and Marine Service, initiated a project for design, construction and trials of a fish detection ~y5tem to fill this need.

Design Considerations ·

A number of problems exist in the use of sonar which may be broadly described as noise and refraction. The former raises the detection threshold of the set and the latter deflects the beam to either miss the target or, at best, give a false impression of its location. The obvious

' solution to these problems is to remove the sensor from the sources of noise and beam the sonar through interfaces at an angle other than close to 0° or 90°.

-The noise sources are mainly from the vessel as follows:

a) direct transmission through the hull, e.g. engine vibrations, machinery

b) water or propeller noise (partic~larly bad if the propeller is chipped or poorly designed)

c) turbulent flow across the transducer face

d) quenching - air bubbl es trapped under the hull by vessel motion

e) vessel pitching (slamming)

f) electrical interference

Other noise effects are from sea surface and sea bed reverberations and other vessel wakes.

Beam bending due to water temperature change (temperature usually decreases with depth) can have a drastic effect on range or, alternatively in the case of sharp changes, cause the beam to skip over or under the target. By tilting the beam down to reach the depth desired there will be beam bending, without thermoclines, that will have varying degrees of significance. The presence of thermoclines will always cause significant changes and may have disastrous effects on target detection.

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To overcome these problems the unit is required to be in a relatively noise free position and looking through water of ostensibly constant temperature. To achieve this, the conceptual design requires the ability be provided to place the sensor in the horizontal plane of the target where the beam is parallel to the sea bed and sea surface.

The conceptual design, therefore, became that of a variable depth sonar (VDS) in which the sonar detector is in a towed body and can be tOvJed at any depth desired within the environmental and mechanical conditions.

A view was made of the fishing requirements for the forseeable future in consort wtth commercial fishing ~roups to rlptprminp the equipment . operational requirements.

Basic requirements are that the maximum water volume search is made which is a function of sonar coverage and speed plus ease of handling. Unfortunately, these requirements have some confliction.

Fishing to a depth of 800M is considered to ' be reasonable for the near future with probabilities of greater depths within the next f~w years. At this depth, the pressure experienced on a body is about 76kg/cm so that the underwater unit must be constructed to withstand this pressure. In addition, the VDS requires sufficient weight to take it to the depth at the speed of towing required. However, for ease of handling, the system should be as light weight as possible. Similarly, to contend with the stresses involved in launch, tow and recovery in sea states up to 6, the deck handling system must be robust which in turn involves mass. For reduction in towing strains and also to maximize the depth capability for a given cable length, see Figure 1, the cable drag requires to be minimized by fairing; this requires single lay winding on the drum and the drum must therefore be large. Compensation for vessel motion must also be provided which adds to the deck · handling equipment size and complexity, yet deck space use must be kept to a minimum.

Design

To facilitate the search for fish in deep waters, the Industrial Development Branch of Fisheries and Marine Services, Department of the Environment, undertook the design of a VDS. The design is complete and described below.

The contractor selected was Fathom Oceanology Limited, Port Credit, Ontario,wh;chwas required to design an equipment to fulfill the operational requirements as shown in Appendix "AII.

The designed equipment has three basic units: the Towed Body, the Handling Gear and the Display.

The Towed Body

This unit consists of a IIFish ll skin, the transducer, transmitter­receiver, and mounting framework; the skin is, in reality, a fairing covering

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the framework which supports the transducer and electronics package.

The body is ballasted to obtain the correct weight and balance, and trim tabs are fitted to the tail fins to provide correct aspect and stability; the body tows with its longitudinal axis horizontal. The total body weight for this operation is approxi~ately 390 kg in air and about 260 kg in water.

The Skin

Two types of skin were considered, both being of the free-flooding type for use in weight depressant systems. One was the single.moulded FRP unit of National Research Council design, and the uLller was the ABS plastic split skin designed by Fathom Oceanology Limited; both units are 1.5M long and O.5M in diameter. For ease of accessibility, the split model was selected with slight modification (see Figure 2). The modified skin consists of three parts; upper and lower after sections and a nose cap. The separation is along the horizontal axis plane for the after section and in the vertical plane, at the forward end of the electronics housing, for the nose cap.

Acoustically, the skin is very good with no discernable effect on the beam pattern and the system loss through the skin is 2.5dB.

The Framework

The framework, to which the skin is affixed, is made of epoxy coated carbon steel and is shown in Figure 3. This framework is the entire strength member and accommodates the tow point in addition to being the electronic unit support structure. The single tow point is a ball joint, through which the tow cable passes, and allows ± 35° angular movement in any direction about the tow pOint.

Tow Cable

The cable supplies two functions: the towing strength member and ­electrical conductor. A coaxial cable ~ 1.0 cm. overall diameter is used and consists of a double, reverse lay, armour strength member surrounding a polythene insulated centre copper conductor. The copper conductor is flexible with 19 strands of wire, approximating to a 16 awg wire having 50 ohms impedance; the strength member has a breaking strain of approximately 450 kg.

Design of the cable presented some problems as it required a small diameter to reduce towing drag, coupled with high strength, yet have large conductor(s) for low electrical resistance and low capacitance for good signal transmission. A further consideration for a small diameter cable is that of accommodation on the winch drum.

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To obtain good towing characteristics, a fairing is fitted to the cable. A number of fairings were examined, and the one selected as having the best characteristics is the IIFlexnose ll (*) fairing. An evaluation of this and other fairings is given in NRC report #LTR-SH-167. The IIFlexnose ll

fairing has a urethane nose piece which slides with a snug fit on the cable, and plastic tails are ultrasonically welded on (See Figure 4). All sections are linked together to prevent separation during towing and prevent angular disorientation.

This fairing has been tested for drag effects, displaying a drag coefficient of 0.12 compared with the bare cable drag coefficient of 12 .. Trail ing rubber fairings showed between 0.3 and 0.4 and strea.mer (ha.ired) fairing came out at between 0.6 and 0.8. The importance of this fairing can be appreciated by considering the effect of a speed change when using faired and unfaired cable. With 600M of bare cable, a depth of 400M would be reached at 3 knots, and if speed changes ± 1 knot, the depth would change ± iOOM. Using "Flexnose ll fairing the depth change was ± 9M for the same conditions; other fairings were affected in proportion to the drag coefficient (See Figure 5). Tension on the towing cable is almost directly dependent on the cable length and towing speed (See Figure 6).

Transducer

The transducer is basically that of the LSS-30P sonar manufactured by C-Tech Limited, Cornwall, Ontario, but redesigned to withstand the pressure at 800M water depth. The redesign provides the same 432 elements set in a cylindrical housing but now consists of 12 rings each of 36 separate elements, instead of 36 vertical staves of 12 elements, each on a common backmass. The elements now have a ~backmass each to avoid the excessive bending and possible breakage of a common mass and the loss of contact with the ceramic.

The unit is mounted, with its axis vertical, on the forward side of the vertical bulkhead. The element connections are brought out individually by cable to the rear cap of the electronics housing. Characteristics of the transducer are given in Appendix 118 11 and a sketch is shown in Figure 7.

The acoustical activity of the transducer is restricted in both transmit and receive. In transmit, only the forward 180° (18 staves) are energized because of the screening by the transverse bulkhead and the wide horizontal pattern of the individual staves. This arrangement allows ensonification of the required scanning angle of 160° forward without reflection from this bulkhead distorting the pattern. In receive, the after 100° sector (10 staves) is inactive to avoid possible interference from this bulkhead. However, to provide coverage with the formed beam it would normally be necessary to activate 28 staves. This is necessary as 12°staves are required to form a 10° beam so that 16 staves are required to cover 1600 plus 6 on each side for beam forming. Due to a packaging limitation, it was found that only 26 amplifiers could be fitted on the printed circuit board so that the end stave on each side

(* "Flexnose ll is a registered trademark of Fathom Oceanology Limited and is patented under Canadian Patent No. 896,987.)

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was eliminated. The beam therefore deteriorates slightly at each end of the 1600 sector but the degradation is slight.

The transducer elements are lead zirconate/lead titanate and operate at 30KHz with a transmitter output power of 1.8 kw. Bandwidth of the transducer and preamplifier is 1KHz with the centre frequency tunable ± 1.5KHz.

Transmitter/Receiver

Thp transmitter and receiver, plus the beam scanning' circuits, are housed in a pressure vessel inside the towed body, supported by Lilt:!

framework as shown in Figure 3. The housing is a cylinder with a domed rear end cap and the forward end is a plane plate which is also a support bulkhead. The cylinder is made from tube 12 in. internal diameter, 10 in. long and 0.375 in. wall, tested to 140 kg/sq. cm. external pressure. The after-end Cap carries the equipment chassis, and withdraws as a whole for servicing, with a spigot on the forward bulkhead to align and support the chassis' forward end.

The electronic circuits include 6 transmitters (one per two transducer rings), 26 preamplifiers (one per vertical column), and the electronic scanning switch.

Operation of the receiver system is basically that described in the paper on the LSS-30 sonar *(1). New techniques have been used to improve operation, such as active phasing networks instead of delay lines, and the noise level has been improved by 6dB plus bandwidth reduction to 1KHz from 2KHz. The Sum Beam is now obtained by adding all twelve interpolator outputs and the difference by differentiation instead of leading and trailing half beams being added or subtracted respectively.

Interface circuitry is provided both in the Control-Indicator and in the Transmitter-Receiver for transfer of data through the coaxial tow line. Synchronism of the display and transducer scan is frequently multiplexed both ways through the tow line. The data transferred is:

(a) To Control-Indicator

(i) Sum Beam ( i i ) Sca n Sync

(b) To Transmitter-Receiver

(i) Transmit frequency data (ii) Transmit timing pulse (iii) D.C. Supply

*(1) "Advanced Design of a Fishing Sonar" - Oceanology International '72, H. M. Johnson and L. W. Proctor.

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In receive, the Sum Beam is transferred to 30KHz simultaneously \'iith the Sweep Sync data at 555Hz. The Sweep Sync controls the display and starts the helical trace at 0° bearing of the transducer sweep. At transmit time a 940Hz pulse train is sent to the T/R unit concurrent with a 30KHz pulse train. The 30KHz signal contains control data for the power level, pulse duration and frequency ta drive the transmitter. Detection of the 940Hz signal provides the switching action for the T/R unit to transfer from receive to transmit.

The DC current required for the T/R unit is present on the cable at all times. The peak requirement is 2 amps with the average at 0.72 amp (0.6% duty cycle on the transmitters). The voltage is regulated to provide 4BV at the fiR unH wh leh b Lltell l'egul ated to 34V for the

, transmitters, ± l5V and f 5V for the logic and signal processing.

Connections are made through glands in the end cap and internal connectors are provided to allow chassis removal.

The Handling Gear

The handling gear for launching, towing and recovering the sensor had to be a substantial unit to cope with the loading of the sensor weight, plus shocks under towing and recovery in seas up to Sea State 6. As the operation has to be over the side, the tow point can be subjected to vertical movement of over 7M. In addition, the winch drum may only have single layer winding due to the fairings being unable to accept multi­layer pressure. When stowed, there could not be any portion of the system overhanging the side (because of docking), so that the launch/recovery cycles had movement in two planes (as shown in Figure 8).

The winch has two drums where the outer drum is approximately 2.5M diameter, and the inner 2M. The drums are fluted to accept the fairing nose piece diameter and facilitate winding. Guides for the cable have flumes to ensure that the fairings are perpendicular to the drum surface when winding. The cable is wound on the inner drum through a slot in the outer drum surface, this drum being locked at this position. When the last turn on the inner drum is made, the lock is released and the outer drum starts winding on. The two drums hold approximately 870M of faired cable, about 60 turns on each drum (see Figures 9 and 9A).

The cable runs over a series of sheaves through the compensatory unit which allOivs for the cable length difference caused by the.tow point movement as shown in Figure 10. The compensation is achieved by a gas/oil spring actuator, controlled by manual adjustment for speed and sea state, to eliminate snap loads.

To avoid the body twirling as it lifts from the water, a cradle is provided to which the body fits (as shown in Figure 10). The cradle is lowered into the water prior to release or recovery. This also prevents the body from acting as a pendulum and swinging against the ship's side with probable damage, or placing great bending strain on the cable.

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Ideally, the sensor would be over the stern and a relatively simple hoist would be used to bring the fish aboard for stowage, i.e. a single arm would move vertically through some 1800 and rest with the body on top. To facilitate side launching this arm (an IAI Frame) is mounted on a horizontal support arm which projects from a turntable on the side deck. Once the body is raised above level, the whole support mechanism is turned inboard for stowage.

All power is provided by hydraulic actuators supplied from an auxiliary hydraulic unit. This unit supplies all drive power and has a capability of allowing reLt·jevdl or the sensor from 800M depth within 20 minutes.

Electrical connection is maintained, without the use of sliprings, by means of a special winding mechanism on the winch drum which winds the flexible cable on a separate drum, feeding on one end whilst winding off the other end (see Figure 11).

Control Indicator

The Control Indicator is similar to that of the LSS-30 unit modified by removal of the Audio and Rotating Directional Transmission (ROT) features of searchlight operation, limitation of the transmit pulse duty cycle and addition of interface electronics. The data is displayed on a 10-in. long persistence phosphor, orange/orange tube and the control of all functions is from this unit.

Operations

There are ~ number of considerations which must be given to operating a Variable Depth Sonar, and it is not a simple case of putting a sensor on the end of a cable, placing it over the side and towing.

The VDS can be operated in sea conditions worse than is possible for a hull mounted unit. However, the sensor must be launched and recovered without damage in sea states up to state 6, when fishing would normally stop. The handling gear must therefore be substantial, and also capable of operation under these conditions of sea and winter environments (ice, etc.). Similarly, manual work has to be minimal, and in fact, operation of this equipment only requires a console operator, i.e. no physical labour.

A common comment made is that of an extra line over the side and the possibility of entanglement in the net. This situation would be difficult to achieve as the VDS tows at a point almost straight down at speeds at which a net is towed. With reference to Figure 1, it will be seen that at a speed of 4 knots, with 800M of cable, the trail is only 220M and, at 2 knots, the trail would be less than SaM. The sensor and cable would, therefore, be well ahead of the net.

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When towing a VDS, forward motion should be maintained to prevent a possibility of the body rotation, as may occur when stationary, or as would happen if going astern. The former may occur from a small t(nsional imbalance from the two strength members which are steel wire outer layers o~ reverse lay; moving astern would, of course, cause an 1800 realignment of the body which may become 3600 when assuming forward motion again.

In operation, the body depth may be judged by cable length as described by the tow curves. However, by installing a small vertical sounder, a direct reading may be obtained, and this data could be multiplexed up the tow cable.

So far, comments have all considered trawling, and a different situation would arise with a purse seiner. These vessels must stop, in which case the VDS would have to be wound up to the cradle in case the net drifts under the vessel and envelops the VDS, in addition to the basic stopping problem. This would present little difficulty for the operator, as a purse seiner is usually fishing in waters less than 100M and recovery would be very rapid. In addition, for shallower depths, a lighter equipment could be used with a single drum to hold approximately 100M of cable, i.e. one eighth of the drum capacity of the described unit.

Tilting of the beam has not been considered for two reasons~ Firstly, the tilt control would require a more complex system, and necessitates either a multi-core cable or considerable circuitry for the control. Secondly, the sensor itself can be raised and lowered to obtain a depth check of a target. Experience will determine the movement necessary to check depth by observing the change in signal strength; e.g. if signal reduces when raising the sensor, the target must have been near centre of the beam. At half range (2000M) the vertical coverage will be approximately ± 175M, about centre, so that the actual target depth can be determined with reasonable accuracy, certainly far better than from a hull unit.

By judicious positioning of the sensor, bottom reverberation is minimized; i.e. the beam edge skims the bottom and fish close to the bottom will be more easily detected. This capability will be particularly useful in ground or semi-pelagic trawling. In pelagic trawling, surface reverberation can be minimized for clear target observation and target position assessment made within sufficient time to position the vessel for sweeping the harvest concentrations.

The VDS is considered, in this deep water version, as a fleet tool whereby one vessel would search the deep water and direct other vessels to the scene, giving them a line of tow. The cost of the equipment renders installation on each vessel uneconomical, apart from the space considerations. For shallower (400M) waters, lighter equipment can be used for a cost reduction, but this would still be about four times that of a high quality hull unit. Over-the-stern launching models would be easier to make and, again, show a reduction in cost.

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Conclusion

A Variable Depth Sonar can, undoubtedly, detection capability than any hull-mounted sonar. the potential ;s very great to the point, perhaps, satisfactory method.

provide a better search For deep water search, of being the only

Although this equipment has been designed for fish detection, rotating the axis through the 90 to a vertical scan plane, and narrowing the beam, would allow precision survey work to be undertaken. As the sensor is stable, accuracy would be hiqh. and. as long range is not required, the equipment could be scaled down for easier handling.

The design phase was completed in March 1975. The manufacture phase can be commenced when funds are available.

APPENDIX "A"

Operational Requirements

1. Towed unit to operate to depths of 800M with less than 20% deterioration in performance from that at surface levels.

2. Sonar range to be 4,OOOM; detection of 100 ton .school of herring at 3,000M at surface level.

3. Scan forward 160°.

4. rlithin pulse scanning.

5. Azimuthal resolution better than 10°.

6. Vertical resolution 10°.

7. Plan Position Indicator (PPI) display.

8. Operate at towed speeds up to 12 knots.

9. Launch and recover tlnit in Sea States 5 and 6 respectively.

10. Towed body handling gear to allow launch and recovery over side or stern of a vessel from a maximum height of 15 ft. a.s.l.

11. Operate from a 110 v 60 Hz supply; winch may be 230 v 60 Hz 3 phase.

Performance and Operating Parameters

Operating Frequency

Pulse Length

Transmitter Power

Beam Width (-3db)

- Vertical

- Horizontal

Receiving Sensitivity

Indicators

Plant Position Indicator (PPI)

Power "

Controls

Power

Brightness

STC

Pulse

Clutter

Width

Recycle

Gain

Mode

Power

Range

APPENDIX liB II

30KP.z -: 1.5KHz

3 to 30 ~s elicited t o O.6~ dutv eyel.·)

1.8 Kilm.:atts

All modes - 9 degrees

m'INI Transmit - 200 degree~

Receiving - 10 degrees

-35 db/l microbar (PPI)

10 inch (25cm) Cathode Ray Tube with long persistence phosphor.

"ON" Indicating lamp.

Transmitter power

Brightness of CRT display

Sensitivity Time Control (adjusts slope of time varied gain character­istic) Transmitter pulse length

Video Threshold

Width of CRT display - 5 to 10 degrees arc

Vernier Range Control from 50 to

100% of range scale

Receiver gain

Mode Selector: O}mI

Power On/Off Switch

Range Selector: 250, 500, 1000,

2000 or 4000 meters

900r-----~------~----~~------,-------~----~------~------~----~

800~--~~=_--_T~~--r_----_r----_4------~----_r----~----~

7QO~----~------~------+_----~~----_r------+_----_1------~----__1

600~-----+--~~4-------r------4------~~----~~--~------~----~

500~----~~----_+------_r------1_~~--+_----~~r_--_+------_r----~

~

I 400~--~~r_----~~----~~----_1------~------~----~~--=---~------~ .... a. l.IJ o

300~--*--+----~+---~~--~~~----~~-----r------~~--~----~

200~~--~----~+-----~--~--~----~r-----~------~--~-+----~

o 100 200 300 400

TRAIL(M)

500 600 700 800

------------------------, F'i gur e Depth & Scup e vs. Trail at VariOU3 Speeds

-------------_._--- --- -----... ---- . _.--._-- -.--- - -- -----_ .. _ .. _---_ .. _-_._ .. - -_._--_._-_._--

NOSe: SKIN

00

00

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0"

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TOW POINT

UPPER AFT SKIN DIHEDRAL

LOWER SKIN AFT FLOODING a DRAINING TRIM TABS

1·5K

SLOTS

WEIGHT-IN AIR 300 Kg

-IN WATER 260 Kg

TRANSDUCER

FAIRI:.D lOW (;A~LI£

TOW POINT

-- -----------------~~

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EPOXY COATED CARBON STEEL FRAME

TRANSMITTER- RECEIVER

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BASED ON C DR V ES FISH

O. 5M DI~~ TER 90KG WT. IN DIA.

CABLE lCA~ER O. 6KG/M WT. IN W LE SCOPE bOOM CAB

'I_ 300M

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Piollrp R HOlst ArT2, :::c:ni::nr on Ship

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CABLE TENSION COMPENSATION

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--_ .. _-------Figure 1 1 - Signal Tran:;[er lvlechd.nism (Elirninates Slio Rings)