Fixing diver trim and weight placement

[atestani]

B is the same meaning as A/C, but with the opposite phrasing. B says adding weight down low helps. C says adding weight higher up hurts. They all implicitly assume being head heavy when horizontal, which leads to the diver compensating by going head-up (which better aligns the centers of lift & mass).

A 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.
B If your head starts to drop, you need to move weight toward your feet.
C If you misdiagnose the problem and keep adding weight higher on the diver, you’ll only make them more head-up in the water.

A and C say head-up, add weight low
B says head-down add weight low (assuming "head-down" and "head-dropping" are the same thing)

Sorry, but I'm still not seeing how these are the same when they both say move weight low.

 

[atestani]

Yes, it is a bit counter intuitive that a diver in a head-up orientation can be head-heavy. The opposite is also true: a foot-heavy diver can be stable in one of 2 orientations: head up (intuitive) OR head down (not intuitive).

That's not the part I find counter-intuitive. I am OK with "head-heavy" as a term to say there is more weight than needed toward the head.

I need to better understand your description with vertical lines thru the CoM and CoL as a way to visualize this. I think my problem is that I think of it as my body is a lever that pivots at the CoL so if I put weight on one end of the lever it is that end that goes down. This is apparently not the right way to think of this because that is not what happens

BTW, I dove for years with a jacket BCD and a weight belt and never had any trim issues, never even thought about it. After having been out of diving for some years, I decided to get all new equipment and get back in. I bought another jacket BCD with integrated weights which came with trim pockets on the cam-band. I proceeded to reduce my integrated weight by 4 lbs and put 2 lbs in each trim pocket. Needless to say I was "trimmed" at best at a 45 degree angle! (see C above!) So I traded in the jacket for a BP/W and am setting it up now. So never having had to think about trim for some 600 dives, I am now right in the thick of it, and so far, a bit confused.

EDIT: Is the following correct (including terminology)?

(Legs Down)---------- |
HEAD-UP orientation | -- move weight DOWN (toward feet)
(head-heavy)--------- |

(Legs Up)------------------- |
HEAD-DOWN orientation | -- move weight UP (toward head)
(leg-heavy) -----------------|

 

[inquis]

I need to better understand your description with vertical lines thru the CoM and CoL as a way to visualize this.

Look at the images in the OP for a bit. You actually rotate about the CoM, and the CoL determines the direction and speed. Looking at the diver from the side, if the vertical CoL line is to the right of the vertical CoM line, they will rotate anti-clockwise and the spacing between the two vertical lines will decrease. When that spacing is 0, the rotation stops.

 

[inquis]

A and C say head-up, add weight low
B says head-down add weight low

A/B/C assume being head-heavy, which is corrected by moving weight lower. "Head-heavy" is NOT synonymous with "head-down", though many people think it does. (That was the main reason for the OP -- to point that out.) The term head-heavy means that when oriented horizontal (a prerequisite), your head will subsequently move/rotate downward.

You can be stable in a head-up OR head-down orientation when "head-heavy". Read that sentence again; it's not a mistake. Both of these conditions are pictured above.

Alternatively, you can also be stable in a head-up orientation when foot-heavy OR head-heavy. That's why attributing a head-up orientation to either foot or head heavy would be a mistake.

All that being said, I'd guess that at least 9 out of 10 single-tank divers who are head-up are that way because they are foot-heavy. OTOH, I'd guess 9 out of 10 double-tank divers who are head-up are that way because they are head-heavy. Regardless, you don't have to guess. It's trivial to determine head or foot heaviness by going flat, stop kicking, and seeing what happens.

 

[atestani]

Is my edit above correct?

 

[inquis]

(Legs Down)---------- |
HEAD-UP orientation | -- move weight DOWN (toward feet)
(head-heavy)--------- |

I'm not sure what you mean by the first line. If that's the direction your legs move when horizontal, then no, it's backwards.

Bottom line: shift weight toward whichever end rises after getting horizontal and going motionless.

 

[atestani]

Bottom line: shift weight toward whichever end rises after getting horizontal and going motionless.

I apologize if I am being dense about this but I think there must be a terminology/semantics issue or things wouldn't seem contradictory.

The OP said (which there is general agreement on):
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.

That statement corresponded to this picture:

I thought that is what I was saying when I said:
(Legs Down)---------- | <<< yes... I meant if the head is up, the legs are down as in the picture
HEAD-UP orientation | -- move weight DOWN (toward feet)
(head-heavy)--------- |

You said just the opposite, i.e. "shift weight toward the end that rises" which, using the "head-up" terminology, says "head-up, shift weight up" which totally contradicts the OP and what I wrote. Please explain.

 

[inquis]

You said just the opposite, i.e. "shift weight toward the end that rises" which, using the "head-up" terminology, says "head-up, shift weight up" which totally contradicts the OP and what I wrote. Please explain.

The critical part that I believe you're missing: first go horizontal and stop kicking. The SUBSEQUENT MOVEMENT will tell you what you want to know.

You cannot reliably base the corrective weight shift on the stable orientation alone because a heads-up orientation is stable for BOTH head-heavy AND foot-heavy conditions. In other words, the stable orientation tells you *nothing* about how to improve the situation. (This is the non-intuitive part that the OP was trying to highlight.)

 

[FishWatcher747]

The critical part that I believe you're missing: first go horizontal and stop kicking. The SUBSEQUENT MOVEMENT will tell you what you want to know.

You cannot reliably base the corrective weight shift on the stable orientation alone because a heads-up orientation is stable for BOTH head-heavy AND foot-heavy conditions. The stable orientation tells you *nothing* about how to improve the situation. (This is the non-intuitive part that the OP was trying to highlight.)

During the online learning of OW there should be a multiple choice question that you may not pass the course without answering correctly. If one answers incorrectly one may do further learning and answer the question correctly and then pass.

If a diver goes horizontal without moving and his head goes up and feet go down

a. He should move his weights towards his feet.

b. Leave the weights the same.

c. take an optimal buoyancy course.

d. move his weights towards his head.

 

[atestani]

The critical part that I believe you're missing: first go horizontal and stop kicking. The SUBSEQUENT MOVEMENT will tell you what you want to know.

I totally get that but if I go horizontal and motionless and my head rises you say "shift weight toward the end that rises" so I should move weight toward my head. Really!?