Fixing diver trim and weight placement

[happy-diver]

Up or forwards...

 

[Todd Kurth]

Want to share this experiment and get your thoughts…. I have been diving LP85s and getting my trim dialed in a little at a time. Fixing the head up issue. For some reason I was focusing on getting my tank bands mounted higher instead of lengthening my shoulder straps and tightening the crotch strap. In the picture you can see I used a thinner band on the 85s to be able to move both posts up relative to the floor. I still want to move my valves towards my feet as I can reach them easily. I just bought a set of LP108s thinking I’d gain more by the longer tank and I need more gas to align more with my buddy’s lower SAC. You can see here in the pic of both setups, the lower post is almost identical height off the floor. The distance from the top of the isolator valve to the post is 1” greater for the 108s. I’m going to lengthen the shoulders and shorten the crotch strap 1” or a little more to take advantage. Any thoughts on this?

 

[makalutoo]

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:

• 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.

These two forces are always trying to align themselves. If you draw a line radiating out from the center of the Earth, the line would go through the CoG, then the CoL.

Credit to GUE.com for the image of the diver. Scribbling on it courtesy of yours truly.
View attachment 867664
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.


View attachment 867663
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.

View attachment 867662
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.
View attachment 867661

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.

Great info! Thanks!!

 

[happy-diver]

I can reach them easily

You wanna know how to reach your valves chief
when your arm glides past your head by the approved prescribed method
press your forearm, against it as you move your head, towards your valves

 

[Todd Kurth]

You wanna know how to reach your valves chief
when your arm glides past your head by the approved prescribed method
press your forearm, against it as you move your head, towards your valves

Thanks for the response! After posting, I was thinking that I could have been clearer about my question . I can easily reach the valves and so I'm looking to move the tanks down towards my butt, as I think this will help my trim also, which is my key concern. Relating this back to the original post and COG/COL...I have been focusing on moving the tanks (COG) relative to the wing (and everything else) by adjusting the band position higher on the tanks. Further, I thought switching to 108s from the 85s would give me more room to do so. Unfortunately what I am seeing is that additional length is above the top band (bands up as high as I can get) which means net change in going to 108s is moving my valves up and perhaps my COG (wrong direction). So, to adjust my now higher valve location I need to move the tanks down via lengthening the shoulder straps, which also means the wing, COL moves down to some degree? This is why I was originally focused on getting the bands higher on the tank and specifically not moving the wing. This must not be an issue for most divers as greater adjustability of the wing with to respect to the tanks is not an option. e.g., My Tecline wing has two positions, where the similar Halcyon wing only has one, so no adjustability for this at all. Just looking for any commentary on this...

 

[crofrog]

Thanks for the response! After posting, I was thinking that I could have been clearer about my question . I can easily reach the valves and so I'm looking to move the tanks down towards my butt, as I think this will help my trim also, which is my key concern. Relating this back to the original post and COG/COL...I have been focusing on moving the tanks (COG) relative to the wing (and everything else) by adjusting the band position higher on the tanks. Further, I thought switching to 108s from the 85s would give me more room to do so. Unfortunately what I am seeing is that additional length is above the top band (bands up as high as I can get) which means net change in going to 108s is moving my valves up and perhaps my COG (wrong direction). So, to adjust my now higher valve location I need to move the tanks down via lengthening the shoulder straps, which also means the wing, COL moves down to some degree? This is why I was originally focused on getting the bands higher on the tank and specifically not moving the wing. This must not be an issue for most divers as greater adjustability of the wing with to respect to the tanks is not an option. e.g., My Tecline wing has two positions, where the similar Halcyon wing only has one, so no adjustability for this at all. Just looking for any commentary on this...

where the weight is on the tank and the buoyancy of the tank is important. The % weight of the valves vs the % weight of the tanks vs the % weight of bottom of the tanks.

And yes the COL moves down as you move the tanks down, but it seems the impact of that is much less than the impact of moving the the tanks down effectively.

 

[happy-diver]

You wanna know how to move your tanks down chief
Bands go to the top, tanks in the middle with the wing and the shoulder straps loose for full arm movement
So basically everything sits where it sits, it can't sit elsewhere, and 3 inches here or there just means nothing

And then as it is you learn how to dive your rig as it is