Trim

In last month’s article I looked at buoyancy control and now this month I am going to move on to talk about trim. These two topics are often discussed together as they are often inter-related and struggling with one may show up as a problem with the other. The importance of buoyancy control is well known to most divers and is emphasised in the entry-level training programmes of all the major training agencies. Trim, on the other hand, is less well-understood and is rarely mentioned, if at all, in many training courses. Many divers are not even sure what trim even means.

My Collin’s English dictionary has 26 meanings of the word trim, including the following three. ‘The difference between the draught of the vessel at the bow and the stern. To maintain an even-balance of a ship or aircraft by adjusting the position of the load or cargo. The attitude of an aircraft in flight when the pilot lets the main control surfaces to take their own position’. Each of these has a bearing on the meaning of the word ‘trim’ when we use it in diving. The most basic use of the word trim is similar to the first definition above and refers to our position in the water. If a line is drawn from the shoulders to the hips or even on to the knees of the diver then trim can be used to mean the angle of this line. This angle can be anything from completely vertical in the water to completely horizontal. This usage of the word makes no judgement about the ideal angle, but is simple a measure of this angle. It is this measure that is then used in the other uses of the term.

A slightly different usage is related to the second definition above. This assumes that ‘flat trim’ - that is, where the diver is as close to horizontal in the water - is desirable and upright trim is less desirable. In this case the diver can move equipment or use specific techniques to try to maintain an even balance and move into a horizontal position.
The last usage of the word trim is related to the third definition above. Here it is used to mean the position in the water that the diver takes when they relax and do not use hands, fins or any other technique to maintain their position in the water. Here it is primarily the distribution of weight and equipment rather than technique that will determine the position in the water.

As I mentioned above, buoyancy and trim are often considered together because although they are completely different effects, they are often used together by the diver. For example, many divers never achieve neutral buoyancy and yet still maintain their position in the water. They do this by having a head-up trim position and then finning upwards. Their negative buoyancy is offset by the upwards finning, which makes it appear that they have neutral buoyancy. In some cases, typically during the ascent, the diver becomes completely vertical and starts to fin harder in order to move upwards. In this case the diver is forced to have vertical trim in order to ascend. Less common is for divers to fin downwards in order to counteract positive buoyancy. There are many arguments among divers about whether the drysuit or BCD/wing should be used as the primary source of buoyancy. However, for many divers, it is actually their fins that are their primary buoyancy control device.

All of the major training agencies emphasise the importance of buoyancy in their training, even if the practical side of it is less well-emphasised. However, there is very little emphasis on trim and many divers are not even aware of the concept. The reason for this is that for recreational diving buoyancy is very important and becomes even more important at the technical diving level. Trim on the other hand is not considered to be as important at the recreational level and only starts to be emphasised at the technical level. Despite this, I believe that having good trim can also provide benefits for the recreational diver.

The first reason for this is that focusing on trim also forces the diver to focus on their buoyancy. With upright trim it is possible to get away with poor buoyancy because you can fin to compensate, but if you try to get flat in the water you must also focus on getting your buoyancy control right. Of course this isn’t easy. Trying to change the way you dive by changing two factors at the same time requires time and effort and many divers do not want to spend this amount of time on improving their technique. One of the main reasons why it is worth investing this time and effort is that in a difficult situation the fact that the diver is maintaining their position in the water by finning rather than buoyancy control can make the situation worse. The reason for this is that when a diver is stressed and is focusing on some problem they are not focusing on their buoyancy and may stop finning while they deal with the problem. If this occurs then they will start to sink as it was only the finning that was keeping them in position. In this case the diver is still dealing with the original problem, but now also has the added complication of sinking deeper at the same time. It doesn’t even need to be a major problem or incident. Divers who are sending up a DSMB will often start to sink once their attention switched from buoyancy control to the DSMB. So by improving your trim and reducing your reliance on finning to maintain buoyancy, you can avoid this problem.

Another reason to improve your trim and reduce your reliance on your fins for buoyancy control is that the effort of finning uses up energy and will increase your breathing rate. So improving your trim you can also reduce your breathing rate. This also has the effect of making you feel more comfortable in the water as you are no longer fighting to stay in the same position. This makes the whole experience of diving more enjoyable and comfortable, which again is likely to reduce your breathing rate.

It is important to note that many divers do not have a horizontal position in the water, but this does not mean that they have poor buoyancy control. Some of the most-accomplished recreational divers and instructors in the world have a trim position that is far from horizontal and yet have perfect buoyancy. So a horizontal position is not essential for good buoyancy control, although it is a good way to improve your buoyancy. For this reason a horizontal position has not been viewed as essential in recreational diving, but rather is a useful teaching tool or an optional extra. For technical diving, on the other hand, horizontal trim becomes much more important.

There are a number of ways that we can change our position or trim in the water. The second and third definitions of trim give us an idea of what we can do and what we are aiming for. ‘To maintain an even balance of a ship or aircraft by adjusting the position of the load or cargo. The attitude of an aircraft in flight when the pilot lets the main control surfaces to take their own position’. We have two main areas that we can focus on to improve our trim. The first is to move, change or modify our equipment in order to change our trim, and the second is to change our diving technique to adjust our trim.

When we look at our diving equipment we have a number of items that are negatively buoyant; cylinders, weights, torches, etc and a number of items that are positively buoyant; BCD/wing, drysuit, etc. For neutral buoyancy we just need to ensure that all of the negative buoyancy is counteracted by the positive buoyancy. As long as the two are equal then it doesn’t matter where on our body we wear the equipment, we will still be neutrally buoyant. For trim, on the other hand, the correct positioning of the positive and negative pieces of equipment is essential in order to ensure a horizontal position. The positioning of a weight can make the difference between being feet heavy or head heavy. Switching weight from a weight belt to a v-weight (which is positioned between the cylinders and so moves the weight higher up the diver’s body) will result in the diver becoming more head heavy, while switching it to a tail weight (which is positioned below the bottom bolt of the twinset and so lower down the diver’s body) will result in the diver becoming more feet heavy. Short dumpy cylinders will sit higher on the diver’s back and make them head heavy, whereas taller and thinner cylinders will spread the weight along the length of the diver and make them more balanced. Many divers who have got into the habit of using ankle weights will find it very difficult to maintain a horizontal position as the ankle weights pull their feet down. The first step to achieving a horizontal position is therefore to lie completely still in the water without moving your hands or feet. You will then see if the distribution of weight is tipping you head down or feet down. You can then look at the distribution of the weight of your equipment and see if weight needs to be shifted around in order to compensate for the tipping.

The second way to control your trim is to modify your technique in the water. As well as our equipment, our trim is also dependant on the position of our body. By moving our body we can have a significant effect on our trim. Many divers do this by constantly moving their hands and feet. Divers with ankle weights usually fin continuously in order to keep their feet up. Others continuously use their hands to control their position. The disadvantage of both these methods is that when distracted the diver may forget to make these compensations. In addition, for technical diving, these options are no longer viable. Constant finning on decompression stops or movement of hands and feet in an overhead environment are to be avoided, as we will see below. However, there are other techniques that we can use to control our position without constant movement. For example, if you are head heavy then by pushing your arms forward your will move the positive buoyancy of your arms forward and bring your head up. Bending or straightening your legs as well as arching your back into a freefall parachutist position will affect your trim. Ultimately, the goal of the technical diver is to be able to hold position in the water without any variation in their buoyancy and without tipping head down or feet down, all without any movement of the hands or fins.

During decompression dives constant finning in order to maintain a deco stop is not practical. In order to do this the diver will have to maintain constant vigilance to ensure they do not drift down or up and any distraction, such as sending up a DSMB, switching to deco gas or checking their decompression schedule may result in the diver moving up or down. In addition, constant finning on decompression stops means that the diver will be continuously drifting forward into their buddy, which can be very frustrating to both, as well as likely to lead to twisted DSMB lines if there is more then one in the water.

In an overhead environment such as a wreck or a cave then good trim becomes absolutely essential. In many cases it is essential to be completely flat in the water in order to fit through small entrances and exits. Swimming through a doorway of a wreck that is lying on its side is impossible unless you are completely horizontal. A diver in an upright position will simply not fit through the gap and even a slightly upright position is likely to result in the diver banging their cylinders on the top of the entrance or their knees and fins on the bottom of the entrance. The overhead environment diver cannot fin continuously or use their hands to control their position. By finning continuously they will be constantly moving, which makes it impossible to stay stationary long enough to tie off their guideline. Drifting forward is not an option when there is a silty obstruction in front of you, and moving your fins or hands to keep position can kick up additional silt.