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.
The practice is that the trapeze is carabinered to the downline at a suitable depth with a transfer line – that is, a line that allows divers to transfer from the downline to the trapeze.
As the last diver comes up from depth at the end of the dive, when they arrive at the point where the trapeze or deco station is carabinered to the downline, the transfer line can be unclipped from the fixed downline. Everyone then goes for a drift, holding on to one of the trapeze bars. Drifting with the current in a fixed body of water, you now feel that you are stationary in the water – whereas, in reality, you are speeding over the seabed far below at anything up to a knot.
My group has a tag system to assist in knowing where everyone is. On the way down the shotline, at the beginning of the dive, we clip a plastic tag with our name on it onto a fixed ring on the shotline beside the trapeze carabiner that is to be unclipped to allow us to drift and ascend. On the way back up, each diver removes their name tag from the ring – so if your tag is the last one on the ring, you know everyone else is above you and that it is safe to unclip the trapeze and go drifting.
Like most technical divers, we also have a system that only red DSMBs are fired up on ascent if all is well. This tells topside boat cover that all is good. We also each carry a yellow DSMB and reel, which is only deployed to the surface to tell topside that there is a problem.
As a result of the area that Pathfinder lies in, although the underwater visibility in the shallows above the wreck can be quite good, the silty seabed can be stirred up as the tide runs over the seabed, and it is common to find that down on the wreck the particles in suspension filter out all light coming down from above. As a result, there is little or no ambient light – the wreck usually has the feeling of being very dark and moody. Divers are reliant on their torches, the rusty red brown metal of the ship being covered in the soft coral known as dead men’s fingers, which flares white in the torch beams.
For UK technical diving on wrecks like Pathfinder, where you expect it to be pitch black with often poor, silty visibility in torch beams, each diver also carries a small strobe which is clipped to the downline a few metres above the wreck. The downline itself would be next to impossible to find without it, and doing a free ascent from great depth on a tidal wreck which is known to have many nets snagged on it is not the best idea. But 5–6 strobes flashing away in the darkness can be seen from a long way off. By the end of the bottom time, a diver’s night vision will have kicked in and you often see a fuzzy halo of light from the strobes flashing well in the distance.
On this visit, the skipper having positioned his boat to take account of the tide, he then gave the command for the shotline to be deployed over the side of the boat, intentionally placing the shot on the seabed just off the wreck. Skippers are very sensitive to not dropping weighted shotlines on war graves – particularly on fully munitioned warships like Pathfinder.
Our group of divers had dressed into our drysuits some time before on the approach to the site; pee valves (or should I say, offboard urination devices) were already all connected up. With the wreck shotted and slack water approaching, we began to wriggle into our rebreather harness webbing, pulling on fins and mask, clipping on bailout cylinders under each arm, connecting suit inflation direct feeds and switching on our rebreather wrist computers to let them start going through their boot-up self-check menus. Finally, fully rigged, we simply sat still carrying out our rebreather final pre-breathe