Finding Proper Trim | Scuba Diving

Finding Proper Trim

November 2007

By Eric Douglas and Mike Ange

One of the biggest keys to being able to truly relax and enjoy your time underwater is achieving pinpoint buoyancy control. The basic buoyancy control skills taught in open-water class-fin pivots and hovering-are only the beginning. Ultimate buoyancy control, and all the benefits that come with it (lower air consumption, less fatigue, longer and more comfortable dives) are earned with practice and experience, but you can accelerate the learning process with our tips and an easy in-water test.

Weighting and Trim

Effortless buoyancy control really comes down to two key factors-finding the just-right amount of weight, and finding the best way to wear that ballast on your body and gear to achieve proper trim.

Most of us understand the importance of finding the right amount of weight, but if you watch any group of divers in action, it's easy to spot the ones who have yet to realize the importance of proper trim. They're the divers who muscle through the water in an almost upright position instead gliding along in a smooth horizontal one. They may find themselves constantly fighting to keep from rolling to one side, or to keep their feet from floating toward the surface. Many of these divers may be unaware of their trim problems, either by accepting them as part of diving or because they've adopted an inefficient swimming style-learning to lurch back to the center every few kicks, for example-to overcome them.

It often takes new divers about 20 dives or so to get comfortably weighted with the right amount of ballast, but because trim issues are less conspicuous, divers can go for years without realizing them. For the purposes of this article, we'll assume you've already sorted out the correct amount of weight you need, but if you need a refresher, check out "Four Steps to Perfect Weighting" (opposite page). It's also a good idea to run this weighting exercise whenever you change gear configurations or exposure protection, gain or lose weight, and even when shifting from fresh to salt water. The goal of the weighting exercise is always the same: You want to find the smallest ballast load that will allow you to achieve neutral buoyancy at the end of the dive when your tank is low and you need to make a safety stop.

Why Trim Matters

A diver can have perfect weighting and still swim through the water like a large truck. The idea, of course, is to fine-tune your profile so you swim more like a sleek sports car. With traditional BCs and weight belts, however, all of the ballast weight is centered at the diver's waistline, while most of the buoyant force of the BC bladder is concentrated near the shoulders. As the diver moves into a swimming position, his shoulders are pulled to the surface while his waist is dragged to the bottom, forcing him into an upright profile. This position dramatically increases the diver's resistance in the water, which increases his workload and air consumption. If the diver is wearing too much weight, this problem is exacerbated on both fronts as the diver adds more air to counter the downward pull of the excessive weight-but even with proper weighting, the problem can be very pronounced due to the physics involved.

In traditional scuba gear your torso basically becomes a lever with a force at each end-buoyancy at the top and weights at the bottom. Levers magnify force, so even a slight imbalance can have a substantial impact. Try this demonstration: lay a two-pound weight on the palm of your hand and hold it at waist level for one minute with your elbow bent 90 degrees. Now extend your arm straight out in front of you and hold it there for another minute. Feel the difference? That's why proper trim is so important. A pound or two of extra lead in the wrong position can have a dramatic effect on your swimming position.

Test Yourself

Here's how to diagnose your current state of balance and trim. Descend to 15 feet and get neutral. If your weighting is correct, you should be able to hover in the water for one to two minutes with minimal movement. Without changing depth, put your body in a face-down, swimming position with your arms clasped in front of you and your knees bent only very slightly. Using small breaths to avoid rising or falling in the water column any more than necessary, try to hover without moving any part of your body for 30 to 60 seconds. Most divers will find that their body has a tendency to shift or roll during this exercise. Don't fight it. Let it happen and note exactly what the problem is.

The most typical problem that divers find when completing this test is that their feet have a tendency to move toward the bottom while their upper body moves toward the surface. This means that the center of gravity is too low on the diver's body and some of the weight needs to be repositioned toward the shoulders. In the event that your feet have a tendency to go toward the surface, you have exactly the opposite problem. You need more weight closer to the lower half of your body. Finally, if your body has a tendency to roll to one side, some weight needs to be redistributed to offset this tendency. Many divers underestimate the impact that a heavy console, a clip-on dive light or some other piece of gear may have on their trim and balance. It may be necessary to shift a pound or two of lead to one side of the body to offset this tendency to roll.

There are a number of ways to move weight around to achieve perfect trim. Many newer BCs have ballast weight pockets located well above the diver's waist and generally positioned along each side of the tank precisely for better trim control. Ankle weights can be very effective at offsetting positive buoyancy in the legs, or, when placed around the tank valve, to add weight to the upper body. It is also possible to get weights in increments as small as a half-pound, which allows more precise fine-tuning than the typical two- to five-pound block weights. Finally, some weight-integrated BCs are designed so that the ballast weight is higher on the body and more in line with the buoyancy cell. Safety tip: While it's OK to distribute significant amounts of weight in order to achieve better trim and buoyancy, the majority of your total ballast load still needs to located where it can be ditched quickly and easily in an emergency. Always test your final weighting configuration to be sure you can achieve positive buoyancy and easy surface flotation.

You will need to repeat this test with any equipment changes. For example, your 7mm wetsuit or dry suit will have a distinctively different impact on your buoyancy than your 3mm tropical wetsuit. Likewise, if you use different size tanks, or carry other optional equipment on your gear, your center of gravity may shift. If you are a photographer or videographer, you may also want to conduct this test while holding your camera in position to ensure you will remain stable when shooting.

Gear Solutions

There are a number of gear modifications that may help you achieve better buoyancy and trim. A backplate and harness BC, for example, will automatically shift three to six pounds of ballast to your torso where it extends along the same line as the buoyancy in your air cell. Whenever possible, you might also consider switching to steel cylinders. These cylinders have different weight and buoyancy characteristics when compared to aluminum tanks, and they are particularly useful for moving your center of gravity higher up on your body and in line with the buoyancy provided by the BC. With both a backplate and negatively buoyant steel cylinders the weight of the objects is moved to the same area of the body as the lift in the air cell, and they are actually attached to the air cell or bladder. This means that the air cell directly impacts the buoyancy of the tank and backplate instead of floating your body, which then must float the weight system typically used in recreational diving.

The Payoff

It may take several attempts to fine-tune your trim, but when you finally achieve this final measure of total buoyancy control, diving will become a new and much more pleasurable experience. Many divers find themselves more attuned to the dive environment and are able to detect small changes in the current, surge and visibility-even the performance of their equipment. But the final, and perhaps the most important, reward is simply longer dives thanks to lower air consumption. For the first time after perfecting their trim, many divers find that air is no longer the limiting factor for their underwater excursions.

Four Steps to Perfect Weighting

Step One: Leave the scuba gear on the dock. Go in first with just mask, fins, snorkel, wetsuit and your best guess of how much lead you need. Float around and relax-after a few minutes, you'll have tamed all unnecessary movement and you'll be holding less air in your lungs. Now add or subtract lead in small increments until you float with a full breath and sink when you exhale. When you've added enough lead to sink, start taking it off again until you float. Then add a pound. You should now be able to float at eye level while holding your breath.

Step Two: Now that you've got the right amount of lead to compensate for the buoyancy of your body and your wetsuit, you'll need to adjust it to account for the BC, tank and regs. Even empty, some BCs have a tendency to float, so you might need two pounds to offset that positive buoyancy. Add one or two pounds negative buoyancy for regs and your console and another three pounds to offset the positive buoyancy of an aluminum 80 cylinder down to 500 psi at the end of the dive. That's when you want to be neutral. When you enter the water with a full tank, it will add about five pounds of weight, so submerging at the start of a dive should be easy.

Step Three: Now it's time to dive and test your weighting. Make sure all the air is out of your BC by stretching the inflator hose upward so that its attachment point to your BC is as high as it can be. At the same time, dip your right shoulder and squeeze the BC against your chest with your right arm. Next, rock backward a little. Many BCs trap a bubble of air just behind your head. Rocking backward moves the exhaust hose over the bubble and lets it escape. Remember to relax. Add lead only as a last resort if you can't sink easily.

Step Four: The best time to fine-tune your weighting is at the 15-foot safety stop with 500 psi in your tank. You should be relaxed and breathing normally. Make sure all the air is out of your BC and keep your hands and fins as still as possible. Remove or add weight in small increments until you can stay neutral at 15 feet.

Control Your Breathing

Another major component in pinpoint buoyancy control is your breathing. This was demonstrated in the second or third pool session of your open-water class when you completed a fin pivot exercise and discovered that by simply inhaling you could make your body rise through the water column. While diving, you always want to maintain full, even breaths, but the timing of your regular breaths during the course of your dives can be an effective way of compensating for small changes in depth without using your BC. For example, as you drop over the edge of the reef, a deep inhalation will slow your drop through the water column or stop it completely. If you want to descend to the bottom, a full exhalation may give you just enough negative buoyancy. Learning to control your buoyancy by controlling your breathing is also the first step to taking smaller breaths when the buoyancy demands of the dive call for them. Hovering at a neutral depth in shallow water, for instance, is easier when you can comfortably use smaller breaths than you might normally use.