How weight affects horizontal trim position?

[RXTdiver]

1. Being overweighted can cause you to inflate your BCD and ascend, but how does being overweighted cause your orientation to be head up/feet down?

2. How does being underweighted cause your orientation to be head down/feet up?

3. When your swimming head up/feet down, is this positive or negative trim?
4. When your swimming head down/feet up, is this positive or negative trim?

 

[BoltSnap]

Being overweighted can cause you to inflate your BCD and ascend

No, if you mean to use the BC as an elevator/lift to get to the surface, this is very wrong. You should be compensating for loss of buoyancy as you go deeper. You should always strive to be neutrally buoyant. You should NOT have to inflate your BC to go up.

 

[inquis]

how does being overweighted cause your orientation to be head up/feet down?

It doesn't necessarily: positioning the center of mass lower (footward) than the center of buoyancy does. Of course, most overweighted divers put all their weight near their waist, which tends to lower the CoM, and there ya go.

How does being underweighted cause your orientation to be head down/feet up?

Because you have to swim downward somewhat to avoid ascending.

 

[Kimela]

There are so many factors involved, IMO.

• Placement of the weights
• Whether your BC fits properly and stays in place when you change position (I like a crotch strap to keep things in place)
• Whether you have air trapped in your BC (some BCs trap air in odd spots easily and are challenging to fully evacuate)
• Whether you're over-weighted or under-weighted
• Buoyancy skills
• Whether your fins are heavy (old ScubaPro Jets are super heavy) or light (my SeaWing Novas kept my feet a bit floaty)

I knew a guy who would try a different 'trick' with each dive - whatever the last person he talked to suggested - and never got it 'right'. I think that if your buoyancy skills are pretty good you can compensate for most of the other issues. But this is just my opinion.

 

[Jcp2]

Even with proper weighting you change your head up to head neutral to head down position simply by positioning your feet and hands. If you have your legs straight out behind you and your hands clasped at your waist, you’ll tilt up and if you kick, will ascend. If you bend your knees and put your hands out in front of you, you’ll tilt down and if you kick, will descend.

 

[--tom--]

I will try to answer questions.
1. First, if you are over weighted, you will add more air to your compensation bladder (BCD or maybe drysuit). Because air doesn't care, it will go to the highest point in the BCD. If you are even slightly heads up that's where the bubble will go. Feet up? just the same, ask a drysuit diver how awkward that becomes. Just like if you are on your side, the bubble goes to the highest point. Since you have more air in the bladder, it will expand quicker as you ascend, so buoyancy control becomes more difficult.
2. Underweighted, feet up? The diver is trying to overcome buoyancy by propulsion. I often do that when snorkeling. I naturally float in salt water, so I do a surface dive and kick to stay down.
3 & 4. I am unfamiliar with the terms positive or negative trim. Essentially, trim is to become as streamlined as practical; the least amount of drag. Creating a larger front mass means more work.