How does trim effect buoyancy?

[billt4sf]

I have been trying to be more often horizontal, and recently I noticed that if I am neutral in an upright position, if I rotate to horizontal I will slowly sink.

I can think of two reasons for this: a) if I rotate from vertical to horizontal, I would be rotating about my center of mass (approx navel) and therefore lowering the wing, compressing it; and b) I might be kicking less.

Other ideas or further explanation ?

- Bill

 

[tracydr]

The best explanation I've ever found concerning trim and buoyancy is in The Six Skills. Bill, I think you would really enjoy this book and find a great deal of help from it.

 

[Tortuga68]

In theory, if you rotate around your centre of gravity, there shouldn't be any change

In practice, you probably aren't doing that (your COG isn't alway the middle of your body/navel) and/or you're changing your breathing (lung volume)

Any finning will mask your true buoyancy, providing either postive (usually) or negative impact; most newer divers unconciously fin and mask what is actually a negtive buoyancy, thinking they are neutral

Practice not finning/moving at all and achieving neutral buoyancy, then try rotating - you shouldn't sink (or float)

 

[TSandM]

Well, the physics is pretty simple -- you need to balance gravity with lift. If you are vertical, are you kicking at all? If so, you're providing lift. If, when you rotate to the horizontal, you end up with your CG at a slightly deeper depth, you've lost lift. Also, if you breathe differently when you are vertical compared with when you are horizontal, you will change lift.

I've never noticed this with a change from vertical to horizontal, but I know if I roll over on my back, I sink. I've assumed that is because I now have the wing below the tank, and it's slightly compressed, but it may also be that I can't roll over at exactly the same depth, or that I change my breathing. (I play around with this a lot when I'm DMing in the pool.)

 

[Tortuga68]

I know if I roll over on my back, I sink. I've assumed that is because I now have the wing below the tank

Above?

 

[eelnoraa]

Trim shouldn't affect buoyancy. Unless you are in very very shallow depth (near the surface kind of depth), the change in the compression of the wing when you rotate from vertical to horizontal is very insignificant compare to your lung capacity. I highly doubt that is the cause.

I think when you are in veritcal up right position, you are unconsciencely kicking slightly to maintain at the same depth. So you are actaully negative. When you are in trim, your kick no long help you to encounter your negative buoyancy, so you sink slowly. I think this is NOT uncommon at all. Next time you dive, have your wift video your with a gopro, then take a look at yourself. Video review is very useful.

 

[DevonDiver]

As I understand it, there are two issues with trim and buoyancy - both dependent upon the effect of propulsion when in bad (feet-down/head-up) trim.

Firstly, when in feet-down/head-up (bad) trim, a significant proportion of the diver's fin thrust is downwards (amount depends on angle). That creates upward force on the diver. If the diver is subject to that upwards propulsion force, but remain at consistent depth, then they will not have attained a state of neutral buoyancy. Thus, when the thrust/propulsion ceases, they will sink...

Secondly, the attitude (physical angle, not 'mindset') of the diver whilst moving through the water will channel water around/across their body. This effect may be comparable to that of an aircraft wing in flight. The angle of attitude, and consequent effect of water movement against the diver, may (hydro-dynamists plz correct me if wrong) create a significant force upon the diver. In a feet-down/head-up trim, this force is upwards..

 

[victorzamora]

Devon, your first picture and explanation is spot-on. As for the second picture, I'm an Aerospace Engineer who specializes in fluid dynamics. So I feel qualified to say that the second picture (and your explanation of it) is mostly correct. The upwards force is partially a component of drag and a component of "lift" in the airplane-wing sense. My suspicion is that it's more similar to the physics of a kite than that of a wing....but I don't know that for sure.

Lynne, one thing you said is that if you rotate about your CG then your CG would be lower. This is by definition not true. However, if you rotate about your CG (as best as you can tell), then the center of buoyancy of compressible air cells becomes lower. Rotating about a point SPECIFICALLY DEFINES that the point doesn't move. However, your overall point was right that by rotating to flat your wing gets lower, compressing it and making you less buoyant.

Having said that, I feel the need to clarify that trim and buoyancy are completely unrelated in all but some nit-picky scenarios using mixed jargon. A diver with good buoyancy control should be able to move around in all orientations while maintaining perfect bouyancy without any sort of propulsive force. I'd bet if we got Lynne in a deep pool, we could prove this. If she were in proper posture and closed her eyes while I spun her around and oriented her weird.....I bet she'd be able to hold accurate buoyancy in that orientation. Perfect neutral buoyancy is simply a matter of weighing exactly the weight of the water you're displacing. Bill, I have a suspicion that you were compensating for poor buoyancy with vectored thrust (Andy's pictures and explanations) which is something I was doing up until recently due to bad trim in SM. If not, I'd be willing to bet that you're not pivoting around your heart but around your knees. Pivoting at the knees is what most divers feel comfortable with. If that's the case, your wing would be MUCH lower, compressing more, and making you less buoyant. If this sounded harsh, please realize I didn't mean to be offensive at all....I just meant it as a completely cold and scientific explanation of the mechanics behind buoyancy and trim.

PS-- I already defined buoyancy, but I forgot to define trim. In the paradigm of scuba, trim is defined as the center of gravity lining up with the center of buoyancy. Any difference in location (as seen from above in proper horizontal trim/posture) creates an arm for a force to apply a moment about. Distance differences in the Z direction (up and down in the water if you're in proper horizontal trim/posture) effect the stability of the system. If you're really interested in it, hang an extra 15# of lead from different parts of your body and you'll eventually find your center of pressure. You'll know it because you'll be forced into a stable mode where you have to fight to no longer be in good horizontal trim/posture. If you take your wing and sit on it and put weight on your back, you'll find the opposite effect of stability. I know that was long-winded and possibly technical (read: too detailed)....but that's the Engineer in me talking.

 

[g1138]

In a sense it's not trim, but the act of trimming yourself out that can effect your bouyancy.
Looking at it strictly from a static hovering position, no finning, breathing, or nothing; yes the act of trimming yourself out will effect your bouyancy.

One big thing people forget about. Where do you rotate when you shift from horizontal to vertical (or vice versa). It's not center of mass, it's at your hips. So your wing actually pivots at the bottom, not at the center. You end up putting the top of your wing about 3ft higher than it was before.

In the shallows this 3ft swing can have a big effect on bouyancy, depending how much air is in your wing. For those of us who don't put much air in, it doesn't effect us as much. For new divers (or divers who overcompensate with a lot of air or delayed venting of air), there's a huge effect.

Keep in mind top of the Wing is usually at level with the top of our necks or even higher for us shorter folks. There's also a huge volume of air that can be concentrated there.
Where as with stock BC you often have the top of the bladder below your shoulder line and it also tapers greatly, so there's a very small volume of air that can actually reach the top of the bladder.
So in reality, air can have a huge depth shift in a wing, where as with a jacket BC it's pretty much minimal. That can effect your bouyancy if you're shallow and have air pumped in. Remember air expansion (volume changes related to depth) is greatest in the shallows.

Same concept for rolling. You're not always rolling on the same horizontal plane. If you're bouyancy isn't already spot on you then often roll off your plane and end up a foot below where your chest was before (if doing an full invert roll). Your tank is a huge mass and centrifugal forces do pull you off a bit as you roll. Those of us who do hold an even plane when we roll, I will guess we all breath in a bit deeper when starting a roll, to offset this.