By Piers Boileau-Goad

By Piers Boileau-Goad

9th August 2022

Gas used. Trimix 11% O2 / 45% Helium / 44% Nitrogen (known as 11/45).

On a beautiful, flat calm, 7th August at 10:36 the horn on Sea Leopard went and two divers plunged into the murky depths of Lyme Bay above HMS M2. A tropical 21 degrees on the surface, and visibility initially a low 1m, the temperature dropped as we descended. Two divers on the line stopped with apparent problems, we later found out that this was ear related so we politely asked if we could pass by and were given the ‘OK’.
Off we went, down to the bottom of the Tower at 30.2m. The visibility here was much better, a green 4m. It was so good that we were able to stand off a few metres from the starboard side of the wreck  and see the vents for air/ballast tanks. I even imagined that I could see the huge white ‘M2’ on her tower and the conning officer giving orders below or to those on deck. This was probably the best visibility I have had on her and it was wonderful. At this stage I realised how hot it was at the bottom as I was genuinely sweating!
That said, I had a problem. Having heard loads of bubbles nearby I started to notice a pattern so cycled through my screens just to make sure that all settings and readings were ok. They were not. After our bubble check (done through my mirror due to congestion on the line) at 6m, it seems that my Dilluent cylinder valve had started to leak and leak a lot. Having started the dive with 209 bar of diluent, at minute 11 I had 76 bar remaining. Not too worry though as diluent is only used to expand the counter lung on descent so this got switched off at the cylinder valve and through the solenoid on my unit. This problem solved, we managed to crack on with the dive and see the launching catapult, and the various congers inhabiting the wreck but sadly, no lobster today.
After 24 minutes on the bottom we decided to head up, the tricky bit now begins.
Stepping back one minute..the purpose of a check dive on the 1st August was primarily to check weighting, obviously some skills and drills were secondary. 6Kg was the weight for a dip in Vobster, it might even have been marginally too heavy. So with no changes, 8Kg would have been required for the sea. Adding 2Kg is pretty much standard advice and has worked in the past.
So with 8Kg onboard and a pretty straightforward descent behind me, we started our ascent. More problems started pretty soon after as our SMB lines got tangled due to the slight current and our proximity due to visibility deteriorating above the wreck. We left the bottom at 29m (minute 24) and went up to 22m for 2 minutes while sorting lines. Once sorted, up we go to 12m…so far so good. With a bit of stress from my cylinder issue, tangling lines, I retained some gas somewhere and made my way up to the surface. My average ascent rate was 3m per minute so well within 9m recommended by most agencies and certainly PADI’s 18m/minute. My intention was to stop at 3m for 4 minutes (as required by my CCR. This would have cleared however before my arrival at 3m due to the nature of a CCR applying a fixed setpoint throughout rather than an Open Circuit PPO2 dropping until the next gas switch) but with increased buoyancy I didn’t really have much choice.
Bandits at Three O'Clock

On surfacing, the SMB was held horizontally above my head as required by the skipper on Sea Leopard and I was picked up. The skipper was in front of me while going up the lift so I told him I missed a stop immediately. Being a Technical diver, instructor and overall guru, he told me to stay on my loop and maintain 1bar manually while he prepared the O2 and took my spare computer to check the dive profile. Once ready, both the Skipper and my Buddy helped me out of my unit and into a space on the deck cleared so that I could sit down and just chill. After 45 minutes on O2 at 25 litres per minute and no symptoms, type 1 or 2, I came off the O2 and chilled. I had taken an O’Dive (doppler sensor) reading as well which came back as 75% (which is a good score) so this gave me some confidence. The second dive was skipped.

  • Total Dive time: 33 minutes
  • Max depth: 32.53m
  • Temperature at the bottom: Between 17 and 20 Degrees!

Lessons Learned

  • Its not always so simple adding 2 kg from a fresh water check dive to a salty check.
  • Stress before and during the dive could change you buoyancy due to retained gas – hyperventilation – so try to stay relaxed. This was my first sea dive since returning to Britain from my ship.
  • ALWAYS tell the skipper immediately if something like this happens. It is better to act fast than wait.
  • Things WILL go wrong at some stage (even if they have been recently serviced!) so be prepared. Just imagine if my diluent cylinder had been a single cylinder diver on air after a bimble around this wreck and doing an air sharing ascent, or worse, buddy separation at the last possible moment before ascent and a mandatory safety stop.
  • When driving home and monitoring for symptoms we tend to focus on any tingle, ache or bruise that we have rather than taking a step back, some aches are normal while others are not.
  • Gas mix matters. By diving on 11/45, I had already reduced my chances of a Nitrogen bend due to the helium and O2 content in my diluent mix. My set point was 1.3 Bar giving me a breathing gas of 30.9% (0.46 Bar O2, 1.89 Bar Helium and 1.84 Bar Nitrogen) on the bottom. My deco obligation was 4 minutes at 3m. When combining the gas used and the ‘guesstimate’ nature of current deco theory I did not feel that I was looking at a serious bend so when the skipper asked if I wanted a Helicopter evacuation I said that it was not necessary.

How Do I Feel?

Still aching, humiliated, still monitoring but overall pretty annoyed with myself for having got this one wrong, that said, I have learnt a lot. I am also very happy to have been with the skipper I was with, Al Wright at Sea Leopard (ex Salutay). Had it been another skipper, I would not perhaps have felt in such safe hands.

A Massive thanks to Al, Frieda and my Buddy. Without them I would have been more concerned than I was.