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THE "PERFECT ( being horizontal ) TRIM" HOAX

Discussion in 'Basic Scuba Discussions' started by BLACKCRUSADER, Dec 27, 2020.

  1. wetb4igetinthewater

    wetb4igetinthewater Instructor ScubaBoard Supporter

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    Yeah, I'm working right now on a 3-part blog series that I'll be submitting to SDI to publish where I discuss the center of mass vs. center of buoyancy in the discussion on weight distribution. I think I picked up the term center of buoyancy from a cave diving book that was a series of articles.
     
    John C. Ratliff likes this.
  2. MichaelMc

    MichaelMc Working toward Cenotes ScubaBoard Supporter

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    I am fine with 'center of buoyancy' as where your volume center is. Though trying to think through the physics, buoyancy is a force from an absence of as much mass affected by gravity just gets messy. The center of volume is very clear. But if you need buoyancy to get an opposing force arrow, that works for me.

    But my quest is not so much between those terms. But the relation of that and center of mass. Your center of buoyancy is just dictated by your shape.

    What is an easy term for 'your mass balance is out of whack or unhelpful'.
    Mass balance
    Mass distribution (ETA: with respect to volume/displacement/buoyancy center)
    balance
    'relation of centers of mass/gravity and volume/buoyancy'

    And we are under a constant gravitational field, so center of mass is the same as center of gravity. Plus you do not have a center of gravity. You have a center of mass. Which is the center of the force that gravity has on you.
    (Sorry, math major with a good bit of physics as well.)
     
    John C. Ratliff likes this.
  3. wetb4igetinthewater

    wetb4igetinthewater Instructor ScubaBoard Supporter

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    Maybe center of displacement?
     
  4. MichaelMc

    MichaelMc Working toward Cenotes ScubaBoard Supporter

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    Center of displacement is fine for volume, or buoyancy.

    Center of displacement may be better than volume as it ties into Archimedes' principle definition.
     
  5. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    I think the reason you see recreational divers swimming in that familiar 30°-45° posse is that is how they were taught to dive. Oh, the words used in the classes convey the importance of diving in horizontal trim, but the methods used to instruct contradict those words.

    Most OW instruction is still done on the knees, and that requires overweighting. That is typically done with a load of weight on a weight belt or waist pockets. When they swim, they have to put a large amount of air in the BCD. Lots of weight on the hips plus lots of air at the chest level equals a diver swimming with chest up and feet down. Since the kicking they do drives them upward, they must dive while negatively buoyant to keep from going to the surface.

    That is a simple fact, but it is not really the topic of the thread.
     
    John C. Ratliff, -JD-, Ayisha and 2 others like this.
  6. Coztick

    Coztick Manta Ray

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    I agree, the BALANCE that most tune with "trim" weights and fin selection.
    Diving a single tank in sidemount, that balance point moves side to side as well.
    Diving sm doubles, you can feel a difference of 400psi between tanks.
    Because we are submerged, this balance is very sensitive. When diving singles, I start with 2x3lbs on the left side and finish with both on the right to maintain that balance as the tank empties.
    However you prefer to be in the water, the idea is it should be easy!
     
    MichaelMc likes this.
  7. MichaelMc

    MichaelMc Working toward Cenotes ScubaBoard Supporter

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    Among the terms related to volume, the three below all describe the same location but emphasize different aspects.

    Center of volume is the cleanest. If we all understand that Archimedes creates an upward force there. It focuses on a simple concept, our volume and our volume's shape as a diver at that moment. It is about what is under our relative control and what we can all see. Among working informed divers, I would use that term. It also matches at a core physics level with mass. Objects have mass and volume, and centers of each. Gravity acts on our center of mass, buoyancy acts on our center of volume. (But I have a year plus of physics at a top engineering school.)

    Center of displacement might be better for teaching. As it makes the link to the force created by Archimedes explicit yet retains that our shape dictates where it is.

    Center of buoyancy is the most explicit in why we care. Because we have one force going up and one going down. The further apart they are the more forcefully gravity tries to rotate us. You could tie it back to shape with 'center of buoyancy created by our shape', then it might be best for teaching.

    On their relation to the center of mass:
    I like balance for their relation to the center of mass. Neutral, butt heavy or head heavy balance. (And left/right, back/front.) Neutral mass distribution vs buoyancy center balance.
    Unfortunately, there is also balanced rig, as in (roughly) not too heavy to swim up.

    But still:
    Neutral (mass/weight/gravity) balance (with respect to the center of displacement/buoyancy.)
    Though I'm happy for a better term for that relationship.
     
  8. Angelo Farina

    Angelo Farina Marine Scientist

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    This thread did perhaps not start very well, and at some point it derailed a bit.
    But in the end it resulted very informative for me and here I want to thank the contributors.
    English is not my mothertongue, and this thread exposed the need of proper technical terminology for describing the various physical phenomena involved in buoyancy, rotational force balance, attitude (in both of his meanings), passive and active trim (or static and dynamic, if you prefer).
    I apologize for when I did use some terms improperly.
    I also apologize if, when describing how was my first-level basic training in the seventies, I gave the impression of advocating that such a paramilitary traing approach was the good one, which should be used also today.
    Actually my position was exactly the opposite.
    I was one of the few CMAS instructors who pushed for shortening the first basic course from 6 months to 4 weeks.
    The other one was Gianni Bertieri.
    Together we shaped super-short basic diving courses to be taught in holiday resorts, in two weeks in villages in the Mediterranean, and in just one week at Maldives.
    Of course, shortening the training so much, the ARO training had to be suppressed, together with the long exercises about buoyancy and dynamic trim control.
    On the other side, the equipment of the time (an heavy 15 liters steel tank on the back, and an horse-collar BCD) made it very difficult to get the coincidence between mass and volume centers. All divers were invariantly feet-heavy.
    So what we were teaching, in such short courses, was a subset of dynamic trim control based on two concepts:
    - neutral buoyancy (not negative)
    - proper finning technique
    This allows to swim horizontally, or even with fins slightly above the body, creating the rotational momentus which counter-balance the uneven weight distribution.
    But as soon as the diver stops finning (I do not use the word kicking by purpose) he comes back almost vertical.
    It was not possible to do anything better in a short time and with that sort of equipment.
    Of course nowadays we can do better, and I agree that, particularly with beginners, providing them with properly balanced equipment is the way to go.
    Unfortunately I still see many divers who are still strongly unbalanced.
    So I agree that the first goal in a basic course is to get the students properly balanced.
    Starting there, the trim control will come easily with some experience and some guidance.
    The completely opposite training I had in 1975, with a strong weight unbalance and the need to learn powerful active trimming control techniques, is definitely an obsolete approach.
    I did just find useful to explain how this was the standard here in Italy for basic scuba training in the seventies.
    Rebreathers were the most common scuba systems in diving schools. It was not "technical training". It was considered more basic than using an OC system...
     
    Lostdiver71 and MichaelMc like this.
  9. BLACKCRUSADER

    BLACKCRUSADER Manta Ray

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    I saw this at the end of one of my dives. Needless to say I have no clue what this instructor was trying to teach. I found out who it was later on and where he works. But the dive shop didn't care even though XXXX 5 star lol. I wonder what certificate these divers were getting. Peak Buoyancy class?

    There are many instructors from Korea and China who teach this and you wonder why their divers always hold onto corals to take photos, or walk across reefs or break stuff with their fins.

     
    DiveClimbRide likes this.
  10. UCFKnightDiver

    UCFKnightDiver ScubaBoard Supporter ScubaBoard Supporter

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    Bizarre
     

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