I added this to a post in the cave diving forum a while ago, but it seems common enough that I wanted to post it here in basic scuba as well, to hopefully prevent people from chasing their own tails as I did for a while.
One common issue I encounter with divers working on their trim, especially in a double-tank configuration, is an unintentional "head-up" posture. In almost every case, if a diver’s trim is head-up, it’s often due to being "head-heavy." The solution is to move some weight down toward their feet.
Here are some definitions to clarify:
Credit to GUE.com for the image of the diver. Scribbling on it courtesy of yours truly.
In this diagram, the diver’s CoL is positioned above their CoG, but the diver is in a head-up orientation. When the diver tries to flatten their trim, the misalignment between CoL and CoG causes a downward head tilt due to gravity pulling on the misaligned CoG.
The forces from CoL and CoG are no longer balanced because the bubble in the diver’s wing or BCD moves toward their feet as they flatten out. In a dry suit, gas also shifts to the feet, exaggerating this effect, resulting in the head dropping due to static imbalance.
This diagram represents the other position of static stability, where the CoG is directly below the CoL. In a dry suit, this often results in the diver hanging by their boots as the dry suit bubble shifts.
By moving the weights toward the diver’s feet, you can create flat static stability.
In my experience, with doubles (a manifold and two first stages), the CoG is almost always too close to the diver’s head, though this can happen with single tanks as well.
To assess for this condition, position yourself completely flat in the water and remain still, without any sculling or hand swimming. If your head starts to drop, you need to move weight toward your feet. In a backplate and wing setup, start by lowering the tank (if it’s a single tank). Then, loosen the shoulder straps and tighten the crotch strap to allow the plate to move down your back. Next, consider adding more weight to the waist belt or the bottom of the tanks, using trim pouches or something like this:
Review - Instructions for ditchable trim weight pouches for single tank diving on Halcyon backplate with Halcyon trim weight pockets | ScubaBoard
If you misdiagnose the problem and keep adding weight higher on the diver, you’ll only make them more head-up in the water.
One common issue I encounter with divers working on their trim, especially in a double-tank configuration, is an unintentional "head-up" posture. In almost every case, if a diver’s trim is head-up, it’s often due to being "head-heavy." The solution is to move some weight down toward their feet.
Here are some definitions to clarify:
- Center of Gravity (CoG): The diver’s mass center. This is the point at which gravity pulls on the diver.
- Center of Lift (CoL): The point where the sum of the buoyant forces acts on the diver.
Credit to GUE.com for the image of the diver. Scribbling on it courtesy of yours truly.
In this diagram, the diver’s CoL is positioned above their CoG, but the diver is in a head-up orientation. When the diver tries to flatten their trim, the misalignment between CoL and CoG causes a downward head tilt due to gravity pulling on the misaligned CoG.
The forces from CoL and CoG are no longer balanced because the bubble in the diver’s wing or BCD moves toward their feet as they flatten out. In a dry suit, gas also shifts to the feet, exaggerating this effect, resulting in the head dropping due to static imbalance.
This diagram represents the other position of static stability, where the CoG is directly below the CoL. In a dry suit, this often results in the diver hanging by their boots as the dry suit bubble shifts.
By moving the weights toward the diver’s feet, you can create flat static stability.
In my experience, with doubles (a manifold and two first stages), the CoG is almost always too close to the diver’s head, though this can happen with single tanks as well.
To assess for this condition, position yourself completely flat in the water and remain still, without any sculling or hand swimming. If your head starts to drop, you need to move weight toward your feet. In a backplate and wing setup, start by lowering the tank (if it’s a single tank). Then, loosen the shoulder straps and tighten the crotch strap to allow the plate to move down your back. Next, consider adding more weight to the waist belt or the bottom of the tanks, using trim pouches or something like this:
Review - Instructions for ditchable trim weight pouches for single tank diving on Halcyon backplate with Halcyon trim weight pockets | ScubaBoard
If you misdiagnose the problem and keep adding weight higher on the diver, you’ll only make them more head-up in the water.
