Chelonia Limited

  Cetacean Monitoring Systems




What is your experience of deploying PODs? As each deployment situation is unique, we welcome messages about your own solutions. Please contact us.

Safety at sea

Deployment of moorings requires very careful thought before going to sea:

  1. A line tangling with a person on the way out can kill if the person goes overboard.
  2. Movement of the boat relative to the mooring can create massive forces, even capsizing small boats.
  3. Plan for loss of control of a mooring when you retrieve it – make sure you can let it go without anyone or thing getting caught.
  4. Lines breaking under tension are dangerous – no one should be in the line of danger.
  5. Keep a sharp knife instantly accessible to cut the mooring rope.

These risks apply to many fisheries and 1 & 2 above regularly cause deaths in UK fisheries.

Acoustic release moorings with small anchors reduce all of these risks.

Where should PODs be moored?

Any position between 2 metres above the sea bed and 5 metres below the surface is possibly good. Relevant factors include:

  • Heavy snapping shrimp noise favours moving away from the bottom, moving to a different place, or using low sensitivity settings.
  • Sand in suspension during tidal currents may favour a near-surface position.
  • Fast current sites are very difficult as they are very noisy and flow around the POD causes noise and stress on the mooring. A ‘float through’ technique has been developed at SAMS.
  • Moorings within trawling zones are at risk, but there may be wrecks or reefs that are avoided by trawlers and deployments on the edges of these sites may be safer.
  • Boat navigation tracks can be excessively noisy with boat sonars.
  • Static fishing gear is often a good location.
  • Piers and pilings: PODs can be fixed close to these. The echoes are not usually as big a problem as you might expect.
  • Very shallow water: it is OK for the POD to be exposed at low tide and lying flat if that is unavoidable.
  • Beaked whale studies can be done from 100m deep, but deeper may be better as the animals rarely echo-locate at less than 400m depth.
  • Below 100m: standard POD plastic housings may not be reliable. An anodised aluminium housing is available (the Deep-C-POD) that goes to 2km.
  • Orientation: PODs show lower sensitivity along the long axis in the direction of the lid. They are 0.7kg buoyant in use, but in fast currents this is not enough to hold them up.
  • Towing: porpoises react strongly to boats in quiet areas and this makes the results of towed acoustics unpredictable, but these designs may help you do it more successfully.

Mooring problems





mostly by fishers, done to frustrate the project, repel intruders, for curiosity or in hope of a drug cache.

no surface buoy; cooperation or deployment by fishers themselves; heavy moorings, tidal moorings, acoustic release.


a tough problem.

protective shells; fold-down moorings; recovery labelling; avoid trawled ground; POD below navigational buoy.


can damage moorings, move moorings, or move the ground they sit on.

low drag mooring and buoy; elastic moorings; choice of anchor, acoustic release.


of lines on lines, on anchors or on rocks.

sheath ropes in hosepipe where vulnerable; steel reinforced ropes.


lines to POD with lines to surface; lines at surface tangling with propellers.

separation of lines; use weights or sinking lines (polyester or leaded) to avoid any line floating at the surface.


can sink surface buoys.

anti-fouling; regular servicing; thinner lines.

Vibration and impacts

vibration is rare – mainly seen in badly designed towing rigs.

improve design or handling procedures to avoid these.

Excessive ambient noise

as above.

move away from the source, or use low sensitivity settings.

Mobile sand

in sand wave fields your mooring can be buried as well as dragged!

buoys on the up line; weak link in case the anchor is buried too deep.


can take a long time.

German and Swedish military fears about submarine detection have been allayed by their own tests.

Notes on solutions

No surface buoy

Acoustic release: these are expensive, but allow you to use a light, fast boat to visit many moorings. Handling light gear is much less costly, quicker and less dangerous. We have used a Sonardyne LRT 7986 lightweight release transponder with good results. However, some users have had problems with the LRT releasing spontaneously in the office and at sea. In this photo there is a banana pinger, under test, on the line below the buoys. The mooring below uses no metal:metal connections and is light enough for one person to carry.

Light mooring using Sonardyne LRT Easy POD deployment (and retrieval) using a small vessel

The release is fired from a deck unit – an acoustic remote control that allows you to get a distance to the release before you release it. The rig then surfaces within less than a minute and is still connected to the anchor weights by a thin rope that you can use to lift those weights. In this rig the POD can be exchanged without untying or tying any knots. The rope is then fitted back inside the black canister and the release is reset - this process takes 15 minutes.

Grappling line: a bottom line, usually buoyant (polypropylene) and at least 20m long, set between two anchors at a known location and orientation. This requires shallow water and a sand or mud sea bed, so that you can haul a grappling device across the position of the line to catch it.

Vertical grapple: A buoyed line that floats up from the mooring but does not reach the surface. It is retrieved by circling the mooring location towing a weighted line with light grapples on it. After two circles the boat moves over the mooring position and pull in the line which, hopefully, will become tangled with the vertical grapple line.

Diver moorings: often very successful in shallow water.

Timed release: releasing a small buoy and line. No examples yet.

Tidal dive buoys: if the mooring line is not much longer than depth the buoy may be pulled during tidal currents and available for theft or retrieval only at slack water. Hard buoy required. This method limits number of moorings that can be serviced on one tide.

Low drag

Small buoys / pipe buoys give less wave drag. The lower limit on size is set by the risk of biofouling sinking the buoy, or tidal currents holding it down. If the buoy is pulled down for part of the tide cycle you may need to use a hard buoy so that it doesn't lose buoyancy at depth and fail to surface. Pipe buoys are vertical tubular buoys with some weight at the lower end to make them stand up.

A long mooring line e.g. more than twice water depth, reduces the lifting action with passing waves but it will increase the swing of the buoy.

Elastic moorings

A simple mooring consisting of a weight with a direct line to a large surface buoy is very risky. Waves get a direct upward pull on the weight and can make quite massive weights move long distances in small steps. A design with elasticity to take up the pull distance of each wave can be stable with a much smaller weight.

The elasticity means that each wave has to stretch out the mooring before it can pull hard on the anchor, and normally the wave has passed before the mooring is fully stretched. During the slack phase of the wave the mooring recovers.

Elasticity can be built in at several points. Working from the bottom up:

Distributed bottom weights: Chain from the anchor is an ancient method. During the wave slack the chain falls to the bottom pulling the buoys back. Chain is expensive and corrodes fast, so a cheaper method is to have one or more heavy weights on the line. Take care over abrasion risks.

Midwater float and sinker: a hard buoy on the line with a smaller weight further up creates an 'N' shape in the line that is elastic. In the drawing below a leaded (heavy) rope, blue, has been used instead of a weight. The buoys must be able to carry the weight and drag, so hard buoys may be needed.

Dive buoy – a surface sinker: a small buoy a few metres from the main buoy will pull the main buoy back towards the anchor if the small buoy has been pulled down by a wave. A simple and very valuable technique that should be used in almost every mooring with surface buoys. It has an added advantage that pairs of buoys are easier to find in choppy seas.

Surface lead lines: a length of sinking (leaded) rope between the main and dive buoys will pull them together during the slack part of the wave.

Pipe buoys: if a choice is permitted long thin pipe-buoys give elasticity as they can sink a long way in response to pull on the line where a spherical buoy would exert a huge pull.


Rope sheaths: Thread ropes at vulnerable positions through tough flexible plastic pipes or hosepipes. The most vulnerable positions are near the bottom – see Acoustic release above. If the POD is moored from the centre and barnacles, mussels, etc grow on it they can abrade the rope as the POD moves around in the current.

Bad rope joins: Ropes that are looped through each other can move back and forth and abrade each other. Use nylon '8' shaped links or make sure that ropes are joined in ways that don't allow relative movement.

Corrosion / fracture / failure

Metal components are surprisingly often the link in a mooring that fails and some yacht moorings are now metal-free. A plastic covered rope, Roblon, may be available and lasts longer than chain in many locations, but it does not have the weight of a chain that provides elasticity in a mooring.

Corrosion: avoid dissimilar metals in contact; watch any moving chain as they can corrode very quickly. The moving end of the chain in the buoy mooring below lasts at most 1 year.

Shackles: only use brass or stainless shackles and in most situations they are best wired with Monel wire or heavily taped to ensure they don’t work loose.

Knot failure: modern ropes can be slippery. The bowline knot, once totally reliable, is no longer so. If you have any doubts about your knots, strap them - especially the tails - tightly with electricians tape.


Anchors need to be matched to the type of sea bed and are not often used in long duration moorings.

On sand, digging anchors can pull out with each change of tide that causes the direction of put to reverse. Over time they can skip a long way. A better alternative is a heap of very heavy chain or a series of two or three weights.

In deep mud anchors may sink and become impossible to lift.

Stones may indicate strong currents, and bedrock may indicate very strong currents. Chain is often preferred as an anchor here, as traditional anchors either fail to bite or risk jamming. Take care with abrasion protection.

Concrete weights: a good shape is a flat disc with a central attachment and suitable abrasion protection (above).

An example

The sketch below shows a mooring, based on a design by Brett Stones, and used where the risk of theft was low. 36 of these were used on a shallow sandy/muddy sea bed and survived a severe storm that lifted boats out of the sea and onto the quay in the nearby harbour. The long lines between weights allow the weights to be lifted separately and add an element of elasticity. More elasticity is added by the headline floats, the leaded line between the buoys and the use of two buoys. The headline floats should be a little less than the water depth at low tide from the small weight. If larger surface buoys were required much larger weights would be needed.

Anchor mooring sketch

Deployment of these moorings: The picture below shows the cluttered side of the ship – the other side of deck is kept clear and clean so that the lay of the ropes and anchors can be clearly seen and isolated from snags and people. In this boat we put out a tight grid of 36 moorings and 44 PODs in 12 hours.

The blue tubes are abrasion protection on the weights. The nylon '8' piece allows ropes to be attached quickly without risk of abrasion between ropes. The visible buoys are pick-up buoys – the dive buoys are smaller.

Multiple anchor deployment for a POD array

Navigational buoys

Simulation of navigation buoy using chainThese can be used if you can get permission but the mooring needs to decouple the vertical heaving of the buoy from the POD. This can be done by making the POD neutrally or slightly buoyant at the end of rope with internal or external weighting.

By making the POD only slightly buoyant it floats up slowly and is occasionally pulled down when the buoy drops low in a trough.

The rig shown on the right needs a tidal stream or current to keep the POD away from the dropping chain as the buoy heaves. If there is no current but large waves some method is needed to keep the POD out of the line of the chain.

The POD needs to be 5m or more below the surface.