20x Faster Than in Air

cold thermoSo, recently Sean McGahern attempted to beat his own record or remaining underwater for 15hrs in St. George’s Bay, Malta. The attempt was prematurely terminated when he went into hypothermic shock. As a frequent cold water diver, in both wet and drysuits in all seasons, this got me thinking. Quite reasonably, he called this attempt when his drysuit ruptured a few hours into the attempt. Why? The water at Malta was 13˚C. His suit quickly filled to the waist with cold seawater. Fortunately his support staff were there to pull him out and get him to hospital where he was treated for exposure. So… what went wrong and what things should have be considered when diving in cold water?

First up, a quick look back at some basic diving science. Water is a phenomenal conductor. Sound travels 4x faster in water than in air, and it conducts heat about 20x faster than air does. It does this by the process of convection. Our protections against this consist of limiting exposure, using a thin layer of trapped water (wetsuit) or a layer of air/gas (drysuit), and of course, being active in the water. Water temperature is important too, as anything below 20˚C is considered to be a “cold water” dive. Reasonably, any prolonged or record setting dive (especially depth or bottom time related) should be treated as a cold water dive, just because of the amount of time you’re going to be exposed for. Unless you’ve sprung for an electrically heated undergarment or you’ve got a hot water suit, water will eventually wick away your warmth.

The main things I consider before a dive, particularly in cold water, are depth and duration. Depth is important because cold water is frequently also dark water, and it can be easy to become lost or disoriented. Additionally, the temperature lowers as you get deeper, and every degree counts. Duration is the big one though. It’s recommended that to 10˚C or less, you dive in a drysuit. Can it be done wet? Yes. It isn’t comfortable, and it is vitally important to plan the dive to be shorter, as your endurance will be sapped more quickly as your body burns energy trying to keep you warm. Generally, I’ve found that if the dive is 25 minutes or less, and not part of a repetitive plan, wet can work. Otherwise, drysuit all the way.

Now, I said he called the dive at the point his suit failed. This is not unreasonable. Drysuits are a pretty specifically designed piece of life support equipment, and they are not designed to operate properly when fully or partially filled with water. Being waist deep in water inside a suit will not only ruin your buoyancy (not an issue in this case, it sounds like they were walking), but will also cause a significant loss in heat retention. Undergarments for drysuits are hardly optimal when saturated by seawater, and the drysuit will hold much more water around you than a wetsuit. As a commercial diver, we were instructed that the failure of your drysuit was the end of the dive. This is not a bad policy, and one he followed.

So what went wrong for Sean? Equipment failure. Some might argue that equipment maintenance may have been an issue, but without personally being there to see it or to see how well he maintains it, it’s a a non-starter issue and a moot argument. Sometimes gear seems to be in perfect shape… then it just goes. Most experienced divers will have had this happen at one point or another with some piece of gear. Sometimes, a plan doesn’t survive contact with the enemy. Sometimes you have call a dive for safety reasons. Hypothermia can have a very rapid onset, as Sean discovered, and can quickly become lethal in an underwater context. It’s not over yet though, Sean plans a new attempt for next year, and hopefully it’ll be smooth sailing!

About the author

Graeme is a professional diver, qualified as a PADI and SDI Divemaster, DCBC 40m Unrestricted Commercial Scuba Diver, 30m Restricted Surface Supply diver, and CAUS Scientific Diver Lv.2. Graeme has an Associate Degree of Arts in Environmental Studies, where he focused on archaeology and physical geography, and an Advanced GIS certificate.