Scenarios

Returning to the Shot-line in a Current

You are diving from a boat that has anchored on a flat reef top. There is a strong current running; about two-thirds of a knot. The skipper has asked that you return to the shot-line to avoid having to go and chase after surfacing divers. On descending, the current dies away and you observe that the reef is flat at about 13 metres, with many corals but no real identifying features. After 25 minutes you realise that you are lost and cannot find the shot-line. The skipper wants you back at 40 minutes. What would you do ?

Plan the dive; dive the plan. Even before entering the water, contingency plans should be considered. Is it essential to return to the line ? If yes, then reeling off is the only option. Otherwise use navigation. Take a compass bearing on the surface current before descending. In this case the current was running to the west. This is important information. If we can’t find the shot, then at about 30 minutes into the dive, we should start to swim east (hence it’s worth knowing the direction of the surface current) for five minutes. It takes you five minutes to surface from 15 metres (one to the safety stop; three on the stop, and one to surface). Start to ascend but do not release the DSMB; the surface current only starts at about 6 metres and you don’t want to be dragged along by it. Rise to the safety stop and then deploy the DSMB. You are now committed to drift with the current for another four minutes. (A knot is 0.5 metre/sec). Our two-thirds knot current will carry us 80 metres. On surfacing we should still be up-current of the boat. If you are off to one side of the boat DO NOT SWIM AT THE BOAT. This guarantees that you will miss it. Swim at right angles to the current and drift down to the boat.

What equipment do we need to execute the plan ? A compass, reel and DSMB are the minimum required for this dive.

Decompression Stops - Conservative Suuntos

Suuntos are conservative, particularly on repeat dives. This is no bad thing, but be aware of it before you purchase and when you use it. You won't be the first diver that traded their Suunto for an Aladin. So what to do when, at the end of a dive, your buddy's Aladin says clear to surface, and your Suunto requires 10 minutes of stops. (This is not unusual, I've seen 8 and 24 minutes). Firstly, don't try to rationalise it underwater, just do what you've always been taught - Stay with your buddy and obey the most conservative computer or table.

Once you've surfaced, together, then figure out what happened. Were your profiles as identical as you thought ? Were your residual saturations the same ? Some computers are sensitive to air consumption rate (air-integrated) and temperature, some are not.  More often than not it simply comes down to the difference between two computers.

What not to do. Don't leave your buddy alone just because your computer is clear. Divers have drowned only three metres below the surface. What's going to be worse, the wrath of the marshal for overstaying your time and surfacing on less than 50 bars or missing stops. If the marshal knows that you are on the shot-line, there's no major drama.

What to do. If your buddy has cleared and you haven't, rise to just below your deco ceiling (three metres on our sort of dives). Your computer clears at the advertised rate there. It's at least 50% slower to clear at six metres.

But what if you're drifting away in a current and you're not sure that the marshal will have seen you. This a whole new ball-game and requires a judgment call. If you are really sure that you and your buddy have made identical dives and his computer is clear and yours is not, practically speaking you're sitting in the error bar of a complex and frankly not that well understood algorithm. Surfacing may be the correct and safer option, but your computer won't work for 24 hours and carry a huge penalty for 24 hours after that. Not that this should dissuade you from surfacing. It sounds like this situation has arisen from a dive plan that didn't quite cover all eventualities, but there are times, like when all the air is gone, when you just have to surface.