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Easy Diving | Fine Tuning

What do we mean by trim in diving, and how can you get it right? DIVE’s blogger Crowley (Mark Russell) explains

Trim – a very short word with several fairly complicated definitions. In fact, the Collins Online Dictionary lists 32 of them, if you include nautical and engineering terminology.

Most of us associate the word ‘trim’ with a haircut and, in fact, this is the definition I’d like to start with: Trim (noun) – a haircut that neatens but does not alter the existing hairstyle.

But how does a short back and sides apply to diving? Well, it makes it easier to wear a hood, for a start. But apart from that, the part about neatening but not altering is very applicable. When we fine tune our trim in the water, we aren’t adding or subtracting large pieces of equipment or altering our shape. We’re simply tidying up what we already have.

The following definitions are paraphrased from the same dictionary:

Trim.

a. to adjust the balance … [of a vessel]… to maintain an even balance, by distribution of ballast, cargo, etc.

b. to balance (an aircraft) before flight by adjusting the position of the load or in flight by the use of trim tabs, fuel transfer, etc.

c. the attitude of an aircraft in flight when the pilot allows the main control surfaces to take up their own positions

The first two definitions speak about balance by distribution of ballast (weight) and the second by using the controls of an aircraft to maintain that balance. We’ll come back to the third part of that definition in a moment.

As we already know, we use our weights and equipment to achieve neutral buoyancy underwater, but it’s important that those weights are evenly distributed so we stay in an efficient horizontal swimming position, and we can demonstrate the difference quite easily. If you need 8kg of lead to achieve neutral buoyancy, then, in terms of buoyancy, it doesn’t really matter where you place it. But if you wear all those kilos in one pocket, you’ll be swimming sideways. It makes sense, therefore, to put 4kg on either side of your body. »

However, this doesn’t mean you’ll have good trim in the water. Wearing the correct amount of weight is the start (see the Easy Diving article in the October 2012 issue of DIVE), but there’s no cure-all for positioning your weights to achieve good trim. Just like the amount of lead you need, trim is dependent on the equipment and exposure suit you’re using, as well as your own body shape and size.

And this is where Part 3 of the definition comes in handy: the attitude of an aircraft in flight when the pilot allows the main control surfaces to take up their own positions. ‘Er… but I’m a diver, not a pilot,’ I hear you thinking. ‘So how is this relevant?’ Well, the definition basically means that if the pilot of an aircraft lets go of all the controls, the plane will assume its natural position. This should be horizontal, because too much up or down when you’re flying tends to result in unfortunate interaction with the ground.

An easier analogy is the playground see-saw. All things being equal, the natural attitude, or position, of a see-saw is straight and level. When a child sits on one end, it sinks to the ground. If another child of the same weight sits on the other end, it should be level again, and then bouncy, bouncy, lots of fun. On the other hand, if you – an adult fond of pies – sat on the other end, the see-saw is no longer balanced. To compensate, you could sit closer to the middle – the pivot, or fulcrum – of the see-saw to balance it out again. Weight has more of an effect the further it is placed from the centre of gravity.

And in terms of relevance to diving? If your weights are concentrated around the middle of your body, they’re closer to your centre of gravity and have less effect on your body position – but if you held a couple of kilos in your outstretched arms, you would rotate into a head down position.

When we’re diving, we can use these analogies to determine where we need to place our weights. The position we’re aiming for is horizontal – a flat body, as if you are lying face down on the floor, with your knees bent at right angles so your fins are elevated and parallel with your body. This position has several advantages: firstly, you’re streamlined so it’s less effort to move forward, leading to better
air consumption; secondly, it will help you stay at the same depth. If you’re positioned at an angle, the tendency is to move shallower or deeper with each fin-kick, meaning you have to re-adjust your buoyancy as you change depth.

We can use this straight and level ideal to see where we need to place our weights. To do this, establish neutral buoyancy – between 5m and 10m is a good depth. Place yourself in the horizontal position and once comfortable, with controlled breathing so you’re not moving up and down too much, just stop. Let go of everything, don’t move, breathe lightly, and see what happens to your body. If you find your head rises and your legs sink, then you need to redistribute your lead towards your head. If the reverse is true, you need to place more weights further down your body to maintain balance.

For recreational diving, generally your weights are concentrated around your waist on a belt or in pockets of a BC. If you dive with a weight belt, you can easily see how much effect its position has on your trim by simply shifting the belt an inch or two up or down your torso, or even just by holding a weight in your hands. Try it: carry one kilo in your hand, then hold it out to the left, then the right, then in front of you – you’ll see how even a small adjustment can affect your horizontal position in the water.

Weight belts and pockets are not the only places you can wear your weights, however. Many jacket-style BCs have trim pouches, usually located across the shoulders or around the kidneys, so it’s worth considering their use if you find you tend to rotate up or down when you’re at rest.

Divers using bulky drysuits for cold water may use a weight harness, which allows weights to be more evenly distributed around the torso – useful for both trim and because carrying 20kg of lead around your waist is really uncomfortable! Divers wearing thick exposure suits (dry or semi-dry) often find their feet tend to float due to the increased buoyancy of the suit around their legs – in which case ankle weights are available.

One configuration that’s becoming more common is the backplate and wing set-up. Some divers on internet forums espouse this as the only configuration that allows you to dive properly, but this is simply not true – perfect trim is equally possible in a jacket-style BC. However, the backplate and wing does have advantages. Often the backplate is steel, so you don’t have to carry as many weights and, more importantly, it distributes that weight along the length of your torso. It’s also much easier to trim with additional weights by attaching them to conveniently located D-rings on the wing. But though many divers recommend them, they’re NOT a cure-all for poor buoyancy or trim control.

Something else to consider is the placement of your tank. The standard placement is such that the top of the valve is level with the top of the BC, or the tank strap is approximately one hand-span from the shoulder of the tank. Move it up or down a little and see what happens to your trim. As long as you’re not reducing the reach of your regulator hoses or finding that the first stage is bashing you in the back of the head, you do have a certain amount of play with the position of the tank. You could also consider wearing weights around the tank strap itself. Some BCs come with weight pockets already attached there, or you can buy some, or use a bit of imagination and a spare weight belt to strap them on.

It’s not practical to do a lot of this stuff midway through a dive of course, but find a buddy, go to the pool, the lake or the shore dive on holiday, and work your way towards the goal of perfect trim. Maybe take a buoyancy class or workshop if that’s available. Correct weighting, controlled air consumption, self-awareness, good trim – these are what make your diving easier. And easy diving is so much more fun than flapping around like a daddy-long-legs in a hurricane. It just takes a bit of practice and experimentation, that’s all.

 

Do

  • Make sure you’re correctly weighted first, otherwise the whole exercise becomes pointless
  • Experiment. Perfect balance takes practice
  • Have a look at other divers' configurations. Ask questionsν
  • Enquire about buoyancy classes if you can’t find an experienced buddy to help

Don't

  • Add extra weight if you don’t need it. Reposition what you already have
  • Be disappointed if it doesn’t work first time!
  • Accept that there’s ‘only one way’ to achieve good trim. There are lots of configurations, and everybody is different
  • Panic. If you’ve got good buoyancy, good air consumption and the correct mental attitude, good trim is only a few dives away

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