Fixing diver trim and weight placement

[Sascha314]

Thanks for this. Let me add a comment especially for recreational divers. The described situation is stronger for BPW with a dry suit. A Wing has a specific form, which I believe allows more air to fill at the "bottom". Also by having a dry suit you add more lift to your legs, which are long. Basically that induces a torque, creating the effect as described nicely above.

If you have a regular BCD, the air does not shift as extreme. Moreover the tank is often just attached by a single strap. So here the jacket itself trys to pull you upright by design.
Now using an AL tank it is negatively buyont full, however gets about positively buyont and half pressure. So while negative it pulls down and creates a force on the whole jacket. If now the bottom gets positively buyont it creates a torque, such that the divers feet get pulled to the top.

What I am often see at vacation destinations is, that most put the strap very much at the most top point you can do. So at the start they are head up, and then get tilted more and more during the dive.
Its better to align the strap of the tank in this situation more close to the center of gravity as also outlined above to start with. Then the shift during gas consumptions is less pronounced.

 

[inquis]

If now the bottom gets positively buyont it creates a torque, such that the divers feet get pulled to the top.

I disagree with this view, due to the critical aspect that it's rigidly attached to the backplate and you guys the crotch/waist strap. To illustrate, one has only to strap a few lbs of lead to the tail of the tank, and there would be no torque by your logic. However, if you move that lead to your body at the same position (head to toe), your trim would be identical in spite of that "torque". And if you'd say, "sure, but the lead counteracts the torque", then by similar (and equally as flawed) logic, there would be an opposite torque before the gas were consumed, trying to push your legs down.

I vastly prefer the view that you switched to at the end of your post -- mass is lost as gas is used which shifts the center of mass. If it becomes misaligned from the center of buoyancy, a rotational moment is induced.

The simple answer is to position lead for stability with an empty tank and wing. Then position the wing for stability with a full tank (and without moving lead). This ensures the mass & buoyancy centers remain aligned as gas is consumed.

 

[lostsheep]

I disagree with this view, due to the critical aspect that it's rigidly attached to the backplate and you guys the crotch/waist strap. To illustrate, one has only to strap a few lbs of lead to the tail of the tank, and there would be no torque by your logic. However, if you move that lead to your body at the same position (head to toe), your trim would be identical in spite of that "torque". And if you'd say, "sure, but the lead counteracts the torque", then by similar (and equally as flawed) logic, there would be an opposite torque before the gas were consumed, trying to push your legs down.

I vastly prefer the view that you switched to at the end of your post -- mass is lost as gas is used which shifts the center of mass. If it becomes misaligned from the center of buoyancy, a rotational moment is induced.

The simple answer is to position lead for stability with an empty tank and wing. Then position the wing for stability with a full tank (and without moving lead). This ensures the mass & buoyancy centers remain aligned as gas is consumed.

Y’all really need to draw a free body diagram to remove ambiguities

 

[inquis]

Y’all really need to draw a free body diagram to remove ambiguities

Sorry to have been confusing. Simply put, the issue I had is looking at the tank in isolation (i.e., the light tank butt somehow "levers" the diver into a head-heavy situation) as an explanation. A FBD would consider the whole system, so yeah, it might help.

I just didn't want other readers to go down the rabbit hole of looking at only part of the system, especially when the big picture view is so simple: 1) Get balanced with empty tank/wing. 2) When you add gas mass to the situation, you need to add buoyancy in the same place (via the wing) to stay aligned. I think that would be common sense. There's no need to worry about light tank butts or tanks sometimes negative/sometimes positive (in isolation).

 

[The Chairman]

@crofrog Thanks for the lesson in vector physics on the two pulls. That was a fresh look at it that I had not seen, but often pondered.

 

[MB NZ]

My thoughts may be wandering, but early on in my diving, I realised that my BCD was pulling me up for want of a better description, my weights were pulling me down and I was the medium for these opposing forces. I quickly changed to a BCD with integrated weights.

 

[grantmac]

Move the mass, including tanks, until trim is perfect with empty tanks.
Then move the wing until it's perfect with full ones.
Most BP/W have enough adjustability for this.

 

[The Chairman]

My thoughts may be wandering, but early on in my diving, I realised that my BCD was pulling me up for want of a better description, my weights were pulling me down and I was the medium for these opposing forces. I quickly changed to a BCD with integrated weights.

Integrated weights do help, but not as much as a crotch strap. I only carry a few pounds of ditchable weight if any, preferring to use steel tanks for my buoyancy needs. The steel tank provides a long length of ballast too, making it easy to match COG with COB. I can park myself 6 inches off the floor, close my eyes, and staying motionless, I can maintain that for a good bit. The goal is to establish a natural trim, where you use minimal effort to maintain your trim during the dive.

So many have to keep fighting their trim and so end up sculling, which ruins the feng shui of natural trim. This also consumes large amounts of gas. The less you move, the less gas you'll need to breathe. Start off with five or ten seconds with your eyes closed. Move weights around and try again. Work up to a minute, then two, then five. It's going to take a lot of adjustments and balancing skill.