How to Properly Trim Out in Sidemount – Solving Common Issues

One Solution May Lead to Another Problem
Why Is Trim So Different in Sidemount?
What Are You Weighting For?
What If You Don’t Need Any Additional Weights?
Common Mistakes Trimming Out

There have been a lot of discussions on how to trim out in a sidemount rig. The argument has long been whether to trim out using air in your wing or to trim out using weights placed somewhere on your rig, usually around the shoulders. Many of the commercially manufactured sidemount rigs are designed with this in mind. The original Dive Rite Nomad was designed so the bungees would trap air in the bottom area of the wing to create more lift closer to the feet. Donut wing systems were designed with 360 wings with the largest area at the bottom so air would migrate in that direction and provide more lift closer to the feet. Pillow shaped wings are designed to be positioned lower along the torso to shift the center of gravity toward to feet. While these are a good ideas in concept, they could lead to a huge mess should the wing fail.

One Solution May Lead to Another Problem

If a wing that is designed to keep the diver in trim fails, that will result in the diver being out of trim. With no air in the wing, the diver will revert to the position created by the center of gravity of the rig, which is usually lower along the main mass of the body. What this means is that without air in the wing, the diver will be in a feet down orientation.

In open water, this isn’t much of an issue. If your wing fails, you end the dive and ascend to the surface. However, in a cave, this isn’t an option. If your wing fails and you aren’t able to maintain horizontal trim, you may end up leaving a mess behind in the cave.

I’ve experience a couple of wing failures, both in very silty cave passages. Had I depended on my wing for my trim, I would have destroyed the visibility in the cave, and in some areas, I would have left quite a few marks on the floor since the floor to ceiling height would not have accommodated me out of trim making contact with them.

Why Is Trim So Different in Sidemount?

When you use backmounted cylinders, they are positioned along your spine directly behind your torso. In sidemount configuration, the cylinders are moved closer to the feet, shifting the center of gravity in that direction. This is the reason most of the manufacturers try to create more lift at the bottom of the wing.

I’ve trained and mentored a lot of divers in sidemount diving. I start all of them without trim weight. We get in the water and I have them get neutral and relaxed. A majority of them trim out at about a 30-45 degree angle with their feet down. This occurs even after the sidemount harness and wing and the cylinders have been set up and rigged properly. The cylinders are positioned in the armpits. Any farther forward and the valves and first stages would have to go in front of the shoulders and throw the trim of the cylinders off.

We can make modifications to direct the air in the wing toward the feet to raise them up. This works if the wing isn’t already over half inflated to compensate for the weight of the cylinders (typical in wet suit divers). This also works if the wing is working properly (hasn’t failed). With fully inflated wings or failed wings, this method won’t fix anything.

What Are You Weighting For?

You must consider the entire sidemount diving configuration as a whole, not in parts. If you need weight to achieve neutral buoyancy, why not position the weight strategically to play double duty? Weights don’t fail. No matter how much air is in or isn’t in the wing, you will remain in horizontal trim. Any change in the amount of air that is in your wing will also affect your overall trim. Weights don’t move. When positioned properly, you will remain in horizontal trim regardless of how much air is in your wing.

What If You Don’t Need Any Additional Weights?

This is a rare circumstance. Most divers need some weight to compensate for the gas being breathed from the cylinders. The gas weighs one pound for every 400-600 psi (depending on the cylinder size). A typical cave dive will result in the use of about 2000 psi, which equals 3-5 lbs/1.4-2.3 kg per cylinder for a total of 7-10 lbs/3.2-4.5 kg of lost “ballast”. That doesn’t take into account the 1-4 lbs/0.5-1.8 kg per cylinder (2-8 lbs/1-3.6 kg total) worth of gas that could be lost or used in emergency situations.

As you learned in your open water class, you are supposed to be neutrally buoyant with an empty cylinder. We have learned to check for proper weighting at the end of the dive with 500 psi in the cylinder and then add 1 lb/0.5 kg to compensate for that 500 psi of gas. This doesn’t change in technical or cave diving. We still need to be neutrally buoyant with empty cylinders. Many divers don’t think about this and are positive with empty cylinders, even if only by a few pounds/kilograms. Adding trim weights not only trims these divers out, but it also allows them to be neutral should they end up underwater with very little gas left in their cylinders.

Common Mistakes Trimming Out

Relying on the wing for trim
Not having sufficient weight to counteract loss of breathing gas
Not having a plan for a wing failure
Incorrect equipment configuration
Incorrect hose lengths and routing

Equipment configuration, including hose lengths and routing, can also affect your ability to maintain stable trim. That should be addressed before you make adjustments to anything else.

Whatever method you decide to use to trim out, be aware of the possible consequences. If you’re strictly an open water diver, then it doesn’t much matter either way. But if you do any overhead diving, you need to consider what can happen to your trim if your wing fails. While you may be able to get out of the cave or wreck, it may not be without doing damage and destroying visibility.

For a complete overview of how these elements work together, see The Complete Guide to Sidemount Diving Configuration.

Readers of this article can receive $10 off the book Sidemount Diving or $5 off the book Recreational Sidemount Diving when purchasing directly from the website.

Use code TRIM10 for the Sidemount Diving Guide and TRIM5 for the Recreational Sidemount Diving Guide during checkout.

Proper Sidemount Cylinder Rigging: What Matters Most
How to Set Up a Sidemount Harness
How Sidemount Bungees Work (And Why they Matter)