Keeping Warm in Cold Water


Our guide to keeping warm in cold water…

It’s a bit like the Norwegian saying “There’s no such thing as bad weather, only bad clothes”. Diving, and keeping warm in cold water brings with it a fairly unique set of challenges, but unlike the Norwegian saying, it can’t all be fixed with clothing. Admittedly, a bad drysuit will not help, but it’s perfectly possible to have the best drysuit and undergarment and be cold in unexpected circumstances.

It wasn’t even that cold…

In December 2017, I was diving in my favourite teaching destination, Krnica Diving in Croatia. I was doing a GUE Tech 1 class, which often involves 2-3 hour skills dives. The water temperature was a gentle 14C, but the air temperature was around freezing. I froze during that week, and I couldn’t work out why. Until I stated thinking about it.

There’s another really flippant piece of advice about keeping warm in cold water, and that is “don’t get cold”. It sounds crazy, but I understand what it means. Basically, once you are cold, it’s much more difficult to get warm again. That means that in order to keep warm in cold water, I needed to stay warmer on the surface. It was around freezing, and we were doing field drills outside, riding on the boats in an icy wind, and I was quite chilled before I even got into the water.

Adapt and overcome!

So I changed a few things around about how I prepared for the dives, and things started to improve. The first thing I did was to wear a thick down jacket (overkill for the temperature), a wooly hat, and gloves too. I made sure that my feet were warm and well insulated too. You can loose a lot of heat through your head, and also through your feet. I kept myself moving as well, to generate body heat. You don’t need to go running or anything strenuous, but just don’t stand still. When I was out on the boat, the goal was to stay out of the wind as much as possible, so I sat down behind the wheelhouse in the shelter, rather than up on the bow, or on the back of the deck.

This all made a big difference. The water didn’t seem quite as cold, and I could tolerate the 2-3 hour dives a lot easier. The shivering started a lot later, and I wasn’t quite as drained at the end of the day.

Rule one. Don’t get cold!

The next thing might be a bit of a surprise to you. It’s hydration. As divers you should know that it’s a good idea to keep hydrated as it makes the blood easier to flow round the body, and that seems to reduce the risk of decompression sickness. It’s probably due to easier inert gas transport due to less viscous blood. But how can it help us in cold weather?

Firstly, how do we even get dehydrated in cold weather? We’re not sweating right? Well, no. You’re always sweating to some degree, and that causes a fluid loss. You’re also trying to keep warm on the surface, by wearing coats and hats etc. so you definitely sweat and dehydrate as a consequence. And then you get back home, turn on the heating, and sit in front of the fire for a while. It all leads to a gradual dehydrating process.

Why does being dehydrated stop you keeping warm in cold water?

Your body needs water volume in the blood – it acts like the hot water in your central heating system and the heart pumps it round the body to regulate temperature. As we become more dehydrated, then it gets more difficult to do that. The first thing to shut down is the supply to the skin, which is, funnily enough, where most of your temperature sensing nerves are. So a lack of blood supply to the skin, brought about by dehydration, can make you feel colder.

Keeping properly hydrated helped a lot. I felt warmer on the dives, and maintaining a comfortable temperature was a lot easier. There was way less shivering!

Rule two. Stay hydrated, even in cold weather.

What else could there be? I’d never been cold in 14C water before, so my next strategy was to look at what I’d been eating. I must confess that I’d been playing around with a low-carb diet as a way of maintaining my weight when travelling. Hotel food is generally not a good thing to eat for a sustained period.

The low-carb approach was really working. However, a think about what happens to carbs in the body led me to change track for a while. Carbohydrates like potato, bread and pasta are relatively easy for the body to break down and use for energy, whereas proteins and fats are much harder. Take a look at the specific dynamic action (SDA) of food to learn more of the science behind it. So most of my diet was based on proteins and fats – and it’s really hard for the body to turn that into useful energy for keeping warm.

So I switched to mountains of pasta, pizza’s and lots of sweet things too. I drank buckets of sweet tea before the dives for hydration and also the sugars needed to keep me warm.

Rule three. Eat carbs and sugars for fast energy.

It’s strange that I started with thinking about what you do on the surface isn’t it? Keeping warm in the dry, maintaining hydration, and eating the right kind of foods. Most people jump into thinking heated vests are the solution. But often they are not needed. I dived over 2 hours in 4C water in the Baltic Sea without heating, and while it’s a nice thing to have you can easily get by without it. If you follow the three rules above, but also take care with your in-water thermal protection, then keeping warm in cold water starts to get easier.

How do we protect ourselves when we’re in the water? Keeping warm in cold water starts simple, like all things. Make sure your drysuit is actually dry! Remember that water conducts heat 25 times better than air, so even the mildest leak will cool you down very fast. My experiences with working with Scandinavian divers told me very quickly that they do not tolerate leaky suits. Any signs of a leak, and it gets repaired. These divers will operate for multiple hours in 2-3C water, especially as the snow starts to melt, and leaks can become dangerous very quickly.

Rule four. Get your suit serviced and stay dry.

BZ400 the best undersuit for keeping warm in cold water

At some point you do need to consider what you’re wearing under the drysuit. In a trilaminate suit, this is the bit thatinsulates you from the cold. From my experience, the feature that makes most difference is the distance between your skin and the trilaminate shell of the suit.

Fourier's Law is the thing that will prevent you keeping warm in cold water

If you’re a physicist, this is a direct consequence of the heat equation. The further you are away from a cold thing, the less heat you loose to that cold thing. You need a thick undersuit that doesn’t compress when you’re in the water. It really is that simple.

Avoid puffy “down jacket” type undersuits. Run screaming from anything that allows you to squeeze it down to paper-thin dimensions just with your fingers. Don’t believe the marketing nonsense about this weeks new undersuit and how it works much better than last weeks. I find that 400g thinsulate is the minimum for keeping warm in cold water. Something like a Santi BZ400 is ideal.

Rule five. Get a proper undersuit.

A good hood really helps in keeping warm in cold water

I’m sure your mum told you to put a hat on when you went out in the cold. Mine did. It used to be said that you can loose 40% of your body heat through your head, but that turns out to be not true. However if your head is not insulated well, then you will definitely loose more heat than if it’s not…

One thing to consider is the neck area. You have two big arteries at the front of your neck (the carotid arteries) and these carry approximately 20% of the cardiac output. They run very close to the skin surface, so a bare neck will allow more cooling than you might think.

A good 10mm thick hood, with a neck collar is a perfect solution. Make sure it fits and doesn’t allow water flushing, which will cool you off quicker than an ice cube melting in a cup of coffee.

Rule six. Wear a good hood.

Your hands are also a very weak point in the system. Some people (myself included) really suffer with cold hands. They get painful, loose all of their strength, and can be the most distracting thing on a dive. Even if the rest of your body is warm. Good wet gloves, with no holes are OK if the dive is short, but eventually your hands will seize up. One of the reasons for this seems to be related to tight wrist seals. I find that the seals restrict blood flow to my hands, and make it impossible to keep warm in wet gloves.

For keeping warm in cold water, dry gloves make a world of difference. Hands stay warmer for longer, they rewarm faster, and you keep your strength and dexterity throughout the dive. You can also increase the insulation inside to match the exposure you expect. I currently use wool/thinsulate mixture that you can buy from any outdoor shop for a few pounds. Fleece gloves compress too much and wear out quickly. The liners that come with many dry gloves are more or less useless. If you want to boost the gloves warmth then use a silk liner, like skiers and mountaineers do. It’s yet another small but significant improvement for keeping warm in cold water.

Rule seven. Wear drygloves.

Some people get cold feet. I don’t fortunately, but it’s not at all uncommon. Again, layering and proper insulation are the key to keeping warm in cold water. The best thing I’ve found is to wear 2 pairs of socks – a thin base layer and then very good quality walking socks with a high wool content. Walking socks are built to withstand being pounded and crushed, so will resist the compression inside a drysuit. Wool has amazing insulation properties, and lasts a long time as well.

Rule eight. Wool socks.

Without delving into heating systems, that’s about all I know about keeping warm in cold water! There is a lot to think about, and you need to consider all of these factors as the temperatures drop. Once you’re into single-figures, then much more attention needs to be given to all of these things. Below 6C that everything is important, and every degree lower is another significant step required in preparation and proper equipment. It really is worth the effort though, some of my best dives ever have been in very cold water, and this knowledge will keep you diving through the coldest winters.

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