Can you have proper weighting if you are not in a horizontal trim?

[Lorenzoid]

. . .
None of my original post was to proselytize, it was to explain the physics involved. . . .

So far I haven't read a post on this thread that disagrees with my physics explanation. I have seen posts where the poster didn't read, or carefully read what was written (shame on you); other cases where implied diving philosophy was disagreed with (not a problem except it is off topic, but what else is new on SB and it leads to some fun reading); and some that verified my understanding of the physics. At this point I'll step out of the discussion unless someone finds fault with my physics.

I don't think anyone disagrees with your physics analysis. And I believe you sum up the replies accurately. If you posted it for confirmation of your own understanding, I believe you've got it.

 

[DBailey]

Since people are looking for all the subtle exceptions, rather than at the broad principles, let me further clarify:

A lot of divers believe they can KICK in a head-up trim, be NEUTRALLY buoyant, and MAINTAIN A SPECIFIC DEPTH at the same time.

They believe it because it is possible. Considering the physics of diving, you can do exactly what you typed. The diver would be constantly kicking, but the diver could maintain a specific depth.

It isn't that kicking to maintain neutral buoyancy is bad, it is just wasteful. The diver is wasting energy which means the diver is wasting air. The diver is wasting energy to move a non-streamlined form through the water which means the diver is wasting air. The diver is tiring themselves out by having to work to be neutral instead of letting the gear do it which means it is probably a less enjoyable dive.

Edit: I'll concede that I may be reading your threads differently than you mean.

If you mean...Diver is able to hover in a heads up position, then when the diver starts kicking (while still in the heads up position) the diver will not maintain the same depth.

To me, neutrally buoyant and hovering are not the same thing. One can be neutrally buoyant while kicking, but one can not hover while kicking.

 

[Cave Diver]

A recent thread “Drop the freaking weights” got me thinking about proper buoyancy and trim in terms of simple physics. I’ve come to the conclusion that to be properly weighted you MUST be in horizontal trim.

Ergo, IMHO to be properly weighted you MUST be in proper (horizontal) trim. Let the discussion begin…

I'm not debating your physics, I'm taking issue with your conclusion. Myself and several others have already explained why it's faulty.

\
Since people are looking for all the subtle exceptions, rather than at the broad principles, let me further clarify:

A lot of divers believe they can KICK in a head-up trim, be NEUTRALLY buoyant, and MAINTAIN A SPECIFIC DEPTH at the same time.

Can, and have.

Once you learn to achieve neutral buoyancy it's not terribly hard to kick in a head up trim and still maintain depth. It's just not terribly efficient, which is why a horizontal plane is preferred.

If you want an example of how that works, try this experiment...
Hold your hand out in a 45 degree angle to the floor and walk around the room. See if your arm is forced up or down by the force of the air pushing on your hand.
Now try holding your hand at a 45 degree angle to the road as you hold it out a car moving at ~30+ mph. Is the result different?

Same principle applies under water.
You can be neutral in a heads up attitude.
You can move forward in that attitude and maintain the same depth (plane). The difficulty with which you achieve that will be dependent on the speed of your forward motion.

 

[Bob DBF]

Neutral buoyancy and trim ARE different. My point, which several posters have agreed with, is that the only way to KICK and stay at a STABLE DEPTH is to be in a horizontal trim.

Which is why I use my trim to change depth until I feel the need to actually change my buoyancy, no sense adding and dumping air constantly to move over an irregular bottom.

Why am I bringing this up? A lot of divers believe they can KICK in a head-up trim AND be neutrally buoyant at the SAME time. These divers are therefore inadvertently over-weighting themselves. How much over-weighting depends on how hard they kick and how head-up their trim is (and probably a few other minor factors). I see a lot (majority?) of divers swim in a head-up and fin down (coincidentally a silting) kick - including a lot of instructors. This means that a large number of divers are over-weighted. *sarcasm* a completely new thought *end sarcasm*.

A diver with a positive (up) trim angle and is kicking but maintaining the same depth only shows that he is negatively buoyant to some degree.

Although many divers are overweighed, you can display the symptoms you describe without having too much weight for the rig and conditions you are diving. Unless you want to redefine proper weighting to something other than enough to hold a safety stop @ 500# tank pressure.

Weight, buoyancy, trim, and propulsion are separate issues that have some overlapping effects. I was neutrally buoyant and in quite good trim bringing a 20# anchor back to my exit point a couple of weeks back, I was however overweighed once I picked up the anchor. I have found that working on these factors individually is a lot simpler and more productive than changing multiple factors at once, which can put you into the confusion you describe.



Bob
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"If you don't like it, go on the internet and complain." Brian Griffin

 

[Lorenzoid]

I think the assumption here when we say "kicking" is that the kicking is done with sufficient thrust and that the diver's body is oriented at a sufficient angle of attack that the planing of the diver's body theoretically gives rise to an upward force vector. Sure, you could pick apart the OP's physics analysis by contemplating various kicking thrust and angles of attack that, under real-world circumstances, wouldn't cause the diver to ascend. Ah, what SB post does not get picked apart? Have at it.

 

[Cave Diver]

I think the assumption here when we say "kicking" is that the kicking is done with sufficient thrust and that the diver's body is oriented at a sufficient angle of attack that the planing of the diver's body theoretically gives rise to an upward force vector. Sure, you could pick apart the OP's physics analysis by contemplating various kicking thrust and angles of attack that, under real-world circumstances, wouldn't cause the diver to ascend. Ah, what SB post does not get picked apart? Have at it.

My issue is that the OP seems to be confused about what they're trying to convey.
The initial post claimed that:

I’ve come to the conclusion that to be properly weighted you MUST be in horizontal trim.

What physics are involved? For starters I simplified the problem by ignoring any hydrodynamic forces, the diver moves too slowly for these to play an important role. Secondly this applies to a moving diver. A diver who is not moving need not be in any particular trim to stay neutrally buoyant.

A lot of divers believe they can KICK in a head-up trim, be NEUTRALLY buoyant, and MAINTAIN A SPECIFIC DEPTH at the same time.

Finally if you can kick in a purely horizontal plane with your fins either above or below your CG, without rotating around your CG, then more power to you.

These two statements contradict each other. The first says that divers move to slowly for it to really have any bearing. The second implies it's not possible to move at a constant depth while being in head-up trim.

If someone is going to present inconsistent ideas, then yeah, they're probably going to get called to either clarify or defend them. This is especially true in the Basic Scuba Discussion Forum, where many of our users may not have the experience or training to cull through what they're reading.