SCUBA dive shop owners who have been in business since the 1970s all share first-hand the devastating impact the 1975 release of the blockbuster summer film ‘Jaws’ had on their business. Almost 4 decades later, most of us have seen this iconic film and if honest, confess its impact on our own ‘healthy’ (or unfortunately, misinformed unhealthy!) fear of sharks. At the end of the movie, Chief Martin Brody (played by Roy Schneider) manages to blow Jaws to bits by shooting a common SCUBA dive cylinder (presumably fully charged with air) lodged in Jaw’s mouth.
Haskell W. O’Brien III, a Systems Administrator for the University of Missouri, Columbia, MO has a fun blog where he has done the maths that estimates the potential energy from the pressure of an air-fill inside a common SCUBA dive cylinder. A fully charged 230 bar (3336 psi) 12 litre (86 cubic feet of compressed gas) scuba cylinder maintains approximately 1,242,000 joules of potential energy. A joule is ‘1 watt of electrical power for one second.’ O’Brien goes on to explain that 1,232,200 joules is enough wattage to run two 1500 watt hair driers for 7 minutes each. That doesn’t sound so impressive in it of itself. However, when compared to O’Brien’s calculations of the potential energy of TNT, the release of 1,242,000 joules in an instant in time would equate to the detonation of about 300 grams of TNT. Again, sounds like a 4th of July firework; surely nothing dangerous? Until you realize that a hand grenade contains ‘composition B’ explosive equivalent to approximately 150 grams of TNT. Therefore, assuming the bullet from Chief Brody’s gun caused a catastrophic failure of the cylinder and completely ruptured it in an instant, the explosive energy released would have been the equivalency of not one but TWO hand grenades going off at once inside Jaw’s mouth! Yep…more than enough destructive power to make fish chowder out of him.
When an ordinary SCUBA dive cylinder is full, the pressure from the compressed air inside is roughly equal to the pressure water exerts on an object 1.5 miles deep in the ocean. In short, the air inside the cylinder has been compressed to tremendous pressure! To reduce that pressure to something safe and ‘usable’ in the dive environment we add a ‘1st stage regulator’ to the cylinder valve. The 1st stage steps this pressure down to approximate 5/100’s of the cylinder’s fill pressure. In other words 95% is safely held back by the 1st stage and only a steady 11 BAR (160 psi) is released to the regulator mouthpiece we breathe from. The mouth piece further steps down the pressure to ‘ambient pressure’ (the pressure of the surrounding water we find ourselves in) and allows us to safely and easily take a normal breath at any depth.
A high-end (expensive!) and finely-tuned mouthpiece will release air into our lungs at just a tad higher than ambient water pressure resulting in making it exceptionally easy to breath under water. Imagine being entirely weightless… completely suspended in animation without having to flex a single muscle in your body (like an astronaut). Now proceed to take a breath and even your diaphragm muscle that enables you to breathe receives a ‘slight assist’ from a mechanical device! It is the most peaceful relaxing state of being you will ever experience and this relaxed and peaceful feeling is one of the joys of recreational scuba diving.
However, does maritime archaeological diving mimic the lovely picture I’ve just painted ? Generally speaking….NO! In my experience, archaeological divers always seem to find themselves in one of two scenarios: 1) either diving in wonderfully easy dive conditions (like warm clear calm water… a recreational diver’s vacation dream spot) but task-loaded with difficult objectives and the added emotional pressure to ‘accomplish those objectives’ and please/impress the project leader, or 2) given the ‘simplest task’ and then expected to perform it in the WORST diving conditions imaginable!
I recall a dive in 7 meters (23 ft) depth in the Solent in 2014 off the Isle of Wight in the English Channel on the HMS Invincible wreck site. Project Director Dan Pascoe of Pascoe Archaeological Service gave me and my dive buddy our dive briefing wile anchored on site in a little RIB (rigid inflatable boat…rubber ‘blow up’ boat). Dan explained: “We’re anchored just off the wreck here…shot line has been deployed (heavy weighted rope with a large float at the top) to descend and ascend on… you’ve got your water proof laminated map of the site. There are 16 control stakes on the map stuck in the sand about 70 meters apart in total spread. Go find them and measure each with a tape measure from tip-to-sand and record the measurements on your slate. Be back in an hour or less. Any questions?” Now to be fair, I am a certified SCUBA dive instructor with a specialty rating certified to teach ‘Underwater Navigation.’ I have dove the Solent and dove the Invincible site before so I’m both familiar and comfortable. (There was nothing inappropriate about the brevity of Dan’s briefing for this site formation process sediment deposition measurement dive objective.) However, navigating by map and compass across the better part of a football field-sized area underwater when visibility is reduced to 2 meters (6 feet) compounded with massive amounts of seaweed carried by the Solent’s strong and unusual ‘double tide’ cross-current (that fully changed direction within the hour long dive!) puts your underwater navigation skills to the test! (I found 15 and got measurements on 14. I was a little disappointed in myself but he seemed pleased.
I find that underwater archaeology requires only common recreation dive skills, but those skills must be honed to a very high degree of proficiency. Archaeological divers must remain horizontal (like a fish…you never see a fish upright in the water, do you?) with PERFECT buoyancy skills. Practice laying on a picnic table– lay horizontally flat on your belly, bend your knees, arch your back ever so slightly, and get those feet and fins up in the air, because you’ve got to be able to maintain that position in the water at depth for 40 minutes or more while operating water/air dredge excavation equipment (those ‘suction tube’ things that suck all the mud off the wreck site.)
A final word about sharks. It is very rare to see them. You could spend a whole life time recreationally diving and never see one. One of my mentors taught me “You wanna see a shark? Be the first one off the boat and immediately stick your face in the water and scan out away from the boat… you might get lucky and catch a glimpse of one quickly swimming away.” To be fair, there are places where you’ll find them but almost never on archaeological dive sites where equipment is in use and team dive ops are under way. On those exceptionally rare occasions when I have found myself in the presence of a curious bull shark in Florida waters, I do what an old commercial fisherman boat captain taught me years ago… “Ya don’t swim towards him, ya don’t swim away from him, and ya don’t turn your back on him. Just let him know by your calm purposed behavior that you are neither a threat nor his next meal.”
David J. Selmo
MSc Maritime Archaeology (Distinction)
University of Southampton