Physics disagreement with GUE instructor on trim, buoyancy, and center of gravity – looking for physics input
I’m currently going through GUE training and recently had a fairly intense disagreement with my instructor about the underlying physics of trim and stability in scuba configuration. I’d like to sanity-check my understanding with people who have a strong physics background.
Context:
I had to do Fundies twice because I got sick during my first attempt. At that time I was consistently very foot-heavy and couldn’t fully compensate for it with my beginner skill level.
Configuration during that period:
- Wetsuit + Jetfins
- Aluminum cylinders Doubles 11.1 with weights on the bottom if the cylinders
On my second attempt:
- Similar setup, but steel cylinders instead of aluminum (only option available), no weights
In both cases, I was extremely foot-heavy.
During the second attempt I was just barely able to pass the tech rating, but only by holding a very constrained body position:
- Legs fully flexed / tucked to counter heavy Jetfins (size XXL)
- Arms tightly pulled in because they were buoyant (brand new 5 mm neoprene suit + double 12s)
In short: my “neutral” position was not actually neutral. To stay level I had to actively maintain a very rigid posture. I couldn't compensate for any more offset weights towards my feet. Was at absolutely my maximum. Could go all the way into the other direction but not a single bit further into the one i already am.
My interpretation of the situation:
To return to a relaxed, stable horizontal trim with a proper position (knees at 45 degress not 90, slightlybend arms etc.), I believe I need to shift my center of gravity forward (e.g., by moving weight/lead or redistributing mass toward the head). Otherwise I’m always fighting a torque imbalance between buoyancy and weight distribution.
My instructor’s view:
He argues that this is not primarily an equipment/weight distribution issue but a skill issue. He suggested that what I perceive as imbalance is actually related to:
- Center of gravity shifting dynamically
- Gas in the wing transferring kinetic effects
- And that proper technique should resolve it without changing configuration
He framed it as something that can be solved through skill refinement rather than physical redistribution.
Where I’m stuck:
To me, this seems like a classic static stability problem: a torque between center of buoyancy and center of gravity. Similar to how a ship trims bow/stern heavy depending on mass distribution.
I don’t see how “skill” alone can permanently change a persistent rotational moment caused by mass distribution, unless I’m misunderstanding something fundamental about how buoyancy + gas shift interacts in real diving conditions.
For reference, this is within the context of Global Underwater Explorers training, which tends to emphasize standardized configuration and physics-based explanations.
Question:
From a physics perspective (rigid body mechanics + buoyancy systems), is my interpretation correct that persistent foot-heaviness is primarily a center-of-mass / center-of-buoyancy alignment problem?
Or is there a valid mechanism where skill alone can realistically compensate for what appears to be a static torque imbalance in a stable horizontal hover?
I’m specifically looking for input from people with physics or engineering background, not training doctrine.
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Edit: Thanks for all the replies. Honestly, this discussion saved my sanity a bit.
I work as a mechanical design engineer in heavy steel/machine construction, and after that conversation I was seriously starting to question myself to the point where I joked I might need a new career.
I’ve since talked to another Global Underwater Explorers instructor who explained the issue using center of buoyancy (where the lift effectively acts, in my understanding) and center of mass / center of gravity. His first recommendation for improving trim was either:
- adding a trim weight directly below the upper tank band
- or reducing weight at the feet
So this aligns much more with what I originally thought.
Please avoid turning this into “GUE bad” comments. This was one individual instructor, not the organization as a whole.
I also spoke to a higher-level GUE instructor responsible for a specific area within GUE, and one of his suggestions was simply lighter fins. I won’t mention names because I don’t want to expose who said what.
Unfortunately the whole discussion eventually became emotionally exhausting and honestly pretty unpleasant. It turned into a situation where every objection was reframed into “you just don’t listen to the instructor” or “you think you know better.”
At some point words were put into my mouth, and my actual arguments got exaggerated into absurd versions of themselves. For example, I got responses like:
«“10 kg on your ankles won’t solve the problem.”»
…even though that was basically the exact opposite of what I was arguing.
Near the end, during a dive, he started moving a 1 kg weight around in his hand and said that proved he could change trim without changing equipment. But to me that literally is changing mass distribution.
The whole thing affected me more emotionally than I expected. It became less about physics and more about pride and not wanting to lose face.
Personally, I think this has nothing to do with GUE itself and everything to do with one instructor.
I’ll stay with GUE, but I’ll avoid this instructor in the future.
Also: does anyone have good reading recommendations on the physics/mechanics of diving trim and buoyancy? I found Technical Diving in Depth during a quick search. Is it any good?