sits on an even keel in 64 metres of water in a deep channel in the middle of the Firth of Forth, which is so wide here that the land seems very distant – you feel almost as if you are in open water. She is in a depth that is well within the modern technical diving range. So I determined to dive her and see this fascinating piece of naval history for myself.
My regular dive buddy, Paul Haynes, and I booked ourselves onto a technical dive boat that runs out of Eyemouth, and with a fully laden jeep filled with two full sets of technical diving rig and two underwater scooters (diver propulsion vehicles – DPVs) we drove the three hours down from Stonehaven on a Friday evening to stay overnight locally in Eyemouth and be ready for an early ropes off the next morning to catch slack water – the Holy Grail of diving, the time when the tide would go slack on the wreck and there would be no current to fight against.
The next morning, we were up early for a full breakfast – I always like to stock up well first thing when I am going to be out at sea all day. Next came the laborious task of ferrying all our dive kit, rebreathers, weights, bailout cylinders and scooters along the jetty and onto the dive boat.
Finally, after working up a bit of a sweat, it was done, and it was time for ropes off. Our skipper skilfully took the boat away from the jetty and we moved slowly north-east out of the quaint, ancient fishing harbour. As we left the harbour, the mainland was to our left and local skerries to our right. To our north and east lay the North Sea.
Once in open water, we turned to the north-west and began to motor up towards the Firth of Forth – towards the last resting place of Pathfinder. It was a warm calm day, the early morning sunlight sparkling off the blue water and casting long shadows. We passed the rocky foreshore and cliffs of the famous St Abb’s Head National Nature Reserve on our port beam, before leaving the land behind as we headed out into the open expanse of the Firth of Forth. Our destination lay far offshore.
As we neared the site, I popped into the wheelhouse and watched the echo sounder as the boat slowed on our approach to the site. On the first pass, the familiar multi-coloured silhouette of a wreck far below, rising a good 5–10 metres off the seabed, appeared on the sounder. We were in business – and the crew readied the shotline, a weighted line with a large buoy at its other end.
The UK has semi-diurnal tides, which means that the seawater flows in one direction, say south, for roughly six hours, before turning to move in the opposite direction, north, for another six hours. The current gets progressively stronger from the beginning of the six-hour period until midway through the cycle, after which the strength of the current begins to drop away and lessen towards the point when the tide begins to turn, at which point the water goes slack; there is little or no tidal flow.
The actual strength of the tide at any one time in the cycle depends on celestial mechanics and the alignment of the sun, earth and moon. There are two types of tides - the stronger are called spring tides, whilst the weaker are called neap tides.
Spring tides: When the sun, moon and earth are in a line and so there is a new or full moon, the gravitational pull of the sun on the earth’s water adds to the gravitational pull on the water by the moon. This causes the water on the earth to bulge outwards towards the sun and moon. As the earth rotates, the bulge, locked towards the sun and moon, appears to sweep around the earth in the form of a long-period wave. We get the highest high tides and the lowest low tides, and the tidal flow each way is strong. Spring tides are nothing to do with spring or the seasons – they occur naturally twice each lunar month, all year long.
Neap tides: When the sun and the moon are at right angles to each other respective to the earth, the bulge of the ocean caused by the sun is partially cancelled out by the bulge of the ocean caused by the moon. We get weaker neap tides – with lower high tides and higher low tides. The tidal flow, and its rise and fall, are not as extreme as with spring tides – and just like spring tides, neap tides occur twice a lunar month, all year long.
At the moment when the tide turns to run in the opposite direction, the current, which relentlessly ebbs and floods in these six-hour cycles in UK waters, drops away and lessens to almost nothing as it swings around to begin to move in the opposite direction for the next six hours. It’s the magical time called slack water.
Divers in tidal waters always aim to arrive on site well before slack water to give time to shot the wreck by dropping a line with a heavy weight tied to one end that has a buoy on the other end to keep it afloat. Sufficient time is always allowed for divers to get kitted up, everything being timed so that the water is just going slack as you enter the water to begin your dive. (In diving we say that you can never be too early for slack water. It will always come – but if you are too late for slack water, slack water won’t come around for another six hours.) Divers descend down the shotline, often called the downline, and then for safety, at the end of a deep decompression dive or a dive in an exposed location, will tend to return to the downline to ascend.
The DSMB fully inflated on its reel. It is common to write the diver’s name on the very top (which will protrude above the water) in large letters so those on the dive boat can identify who is below. © Bob Anderson
Delayed surface marker buoy (DSMB) rigged for diving. It is clipped or stored somewhere convenient on the dive rig.
In the north-east of Scotland we get slack water of about 20 minutes at springs – and almost two hours at neaps. So, if we are diving a wreck on a spring tide, we aim to get that precious slack water whilst we are down on the wreck itself, in the knowledge that as we begin our ascent the tide will have turned and the current will be picking up.
But in the North Sea at springs we can get currents of 1–1.5 knots, and it is not feasible for a group of divers on ascent to all try to hang onto the shotline down to the wreck for perhaps an hour of decompression: it would be a rough hour with the water whipping past you at about one knot. (A knot – one nautical mile per hour – may not seem very fast, but when you’re immersed in that water its force is considerable.)
As a result, technical divers ascending from moderate depths often carry out a free ascent, hanging on a reel under their red 6-foot-tall sausage-like delayed surface marker buoy (DSMB), which is inflated and sent to the surface as they ascend so that topside know where they are – in an hour of decompression in UK waters, divers will drift perhaps half a mile or more away from the dive boat. When the skipper of the dive boat sees DSMBs coming up, he knows to leave the fixed downline and shadow the DSMBs until the divers break the surface.
The alternative way of doing this sort of free decompression ascent in tidal waters is to deploy a free-drifting decompression station. This can be a decompression trapeze, or at its simplest, a weighted line, both of which get carabinered to the downline at 20–30 metres and have their own big surface buoy(s).
The trapeze is simply three long aluminium tubes that are horizontally secured to vertical ropes at either end of them, the tubes being positioned at depths of 12, 9 and 6 metres. The ropes at either side are tied off to their own large buoys, which suspend the whole contraption.
Either way, the trapeze or separate weighted buoy line can be laced with spare bailout cylinders of breathing gas at different depths to make sure everyone has enough gas if there is a problem. As rebreather divers, we all carry our own bailout cylinders under our arms, which hold sufficient breathing gas for us to do the whole dive open circuit if the rebreather malfunctions and we have to bail out off it onto our spare cylinders. So, in theory, no one should need any gas. But the unexpected often happens … as divers we say you can never have too much gas underwater. But you can have too little – and then you are in big trouble.
