What Is Proper Trim for Scuba Diving?:
Why Should All Divers Want Proper Trim?:
Why Is Control Important For Every Diver?:
How Does Good Trim Improve a Diver's Control?:
Proper trim is part of the holy trinity of good diving: buoyancy, trim, and propulsion. Each is necessary for control. A diver's trim effects his buoyancy, and therefore his ability to maintain a stable position in the water.
How does trim effect a diver's buoyancy? A diver without proper trim swims with his body at an angle to the floor. A diver who points his body up will swim up, a diver who points his body down will swim down. As he changes depth by swimming up or down, the air in his buoyancy compensator (BCD) will expand or compress, causing him to lose neutral buoyancy.
Consider the example of a diver who swims slightly feet-down, which propels him slowly upwards. Because he feels that his is moving up, he dumps air from his BCD. He can maintain a consistent depth by kicking upwards while negatively buoyant, but he is exerting extra energy and will sink the moment he stops kicking. A diver who swims feet-down may erroneously believe he is under-weighted because he tends to float up as he swims. If the diver adds weight, he shifts his center of gravity towards his lower body. This forces him further out of trim and causes him to drift upwards even more rapidly.
A diver who swims with his feet too high has the opposite problem. He generally adds too much air to his BCD to compensate the fact that he tends downwards. This increases his profile and drag and causes him to float whenever he stops swimming.
A diver who swims out of trim will have poor control. He loses buoyancy every time he stops moving, and must a continuously fight to maintain a uniform depth. He will never learn to correctly control his buoyancy using his lungs and BCD, because he swims himself up or down whenever he is neutrally buoyant.
At this point you should be itching to work on your trim. Great! The first thing to do is to hop in the water and experiment with your body position, weight, and equipment configuration. Work with a buddy who can direct you into a horizontal position. If possible, have someone film or photograph you so that you can see how your trim is evolving as you take your diving to the next level.
Coming Soon: Trim Part II - Exercises, Techniques and Equipment to Improve Your Trim.