Do you want to look like a sleek diving ninja? Then you need to learn proper trim, or body position in the water. Don't be offended, but unless you have undergone technical scuba
training, you probably have poor trim. Proper trim requires dedicated practice and the application of basic principles and techniques that you will not know unless you are taught. Why should you care? Excellent trim will increase your dive time, leave you less exhausted after diving, and improve your buoyancy control. Most importantly, everyone will be jealous of how incredibly awesome you look underwater.
What Is Proper Trim for Scuba Diving?:
Proper trim is a perfectly horizontal body position, as if the diver is laying on his stomach on an invisible platform. The diver's legs are bent at the knees so that his fins are higher than any other part of his legs. The fins themselves are parallel to the floor. The diver's arms extend in front of his body and his hands are at the same level as his stomach, or only slightly below it. No other part of the diver's body or gear extends below his horizontal body line.
Why Should All Divers Want Proper Trim?:
Proper horizontal trim streamlines a diver to minimize drag. Because streamlining reduces the effort required to move through the water, it lowers a diver's air consumption rate and lengthens his dives. A diver in proper trim keeps his fins slightly elevated, which avoids stirring up sand, silt and other bottom sediments. Elevating the fins helps to prevent damage to coral and other delicate aquatic life from accidental fin hits. However, the most important reason that divers should care about their trim is that it allows them to have better control of all aspects of their diving.
Why Is Control Important For Every Diver?:
Every diver should work towards total automatic control of his position and movements in the water. Why is control so important? A diver with poor control must divide his attention between the struggle to remain stable and other important tasks such as buddy awareness, depth and time monitoring, and gas management. Any distraction from these life-sustaining tasks can be dangerous. Such a diver will be unable to focus on the pleasant aspects of diving, such as fish-watching. He will fail to master complicated tasks such as photography and compass navigation, because he loses control of his buoyancy and position every time he shifts his attention to another activity.
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.
• Buoyancy Basics for Scuba Divers
• Pressure, Depth and Diving
• Boyle's Law and Scuba Diving
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.
More: Trim Part II - Five Ways to Position Your Body for Proper Diving Trim