Master Neutral Buoyancy: The Importance of Horizontal Trim (Simple Vector Physics)

[IVC]

One problem with this picture is that it is not really addressing "trim," but "negative buoyancy."

A person can be neutrally buoyant, yet out of trim. This is a consequence of a still diver having zero torque (trim) in addition to zero total force (neutral buoyancy.) If we add thrust to an out-of-trim diver to get *horizontal* movement, then we are compensating for negative buoyancy, not lack of trim. Another way to say it is that a perfectly trimmed, but overweight diver would still have to swim in the "head up" position to maintain depth. If such a diver stopped, he would return to proper trim, but would sink.

 

[Steve_C]

You're jumping ahead to Bubble Management. I've found teaching others, that they have to get a grasp of vectors first

Ok, I think in terms of vectors at the start. Can appreciate that not everyone does. Seen many a student who does not.

 

[The Chairman]

If we add thrust to an out-of-trim diver to get *horizontal* movement, then we are compensating for negative buoyancy, not lack of trim.

It's really the other way around. People compensate for bad trim by adding more weight. You can correct overweighting simply by adding more air. On a diver that's mostly neutral to begin with, any additional downward propulsion will cause them to ascend. Ergo, they add more weight to stay down. Quite often that will exacerbate the trim problem because they have a harder time managing the bubble. More on that later. Let's just keep this thread to angles of propulsion.

 

[The Chairman]

Ok, I think in terms of vectors at the start. Can appreciate that not everyone does. Seen many a student who does not.

I have yet to meet the student who thought of the direction of his kicking as a vector naturally. There's so much new going on, and they really don't see the water moving. The four concepts take me about 20 to 30 minutes to cover before we hit the pool. The time saved as they put these principles to work from the get go is huge. My students come out of the class with a good grasp of trim and buoyancy. They certainly don't need a 100 dives afterwards to get comfortable and they won't have to take a Peak Performance Buoyancy Class. There are many old school instructors who suggest that classes are being 'dumbed down'. Not with me. They get a couple of lessons in physics and gas kinetics as we work through the class.

This past 'ScubaBoard Surge' in Curacao, I had the pleasure of demonstrating this principle to about half a dozen divers. No, I didn't get to spend a lot of time with them, so I concentrated on vectors and breathing techniques. The smiles told the story and divers who had been timid and even fearful felt in control for the very first time in their diving career.

 

[IVC]

Let's just keep this thread to angles of propulsion.

Fair enough, but then we have another problem. For the constant angle of attack (trim angle) there is only one speed that will produce true horizontal movement (no vertical component.) Go faster, you'll raise. Go slower, you'll sink. This is because the viscosity is (close to) linearly dependent on speed, so a fixed speed implies fixed propulsion force, which in turn implies fixed vertical component of the thrust that compensates the negative buoyancy.

Now you have a speed-dependent trim angle that the divers must achieve in order to move horizontaly. No problem here, except that this trim angle is not the natural "steady state" trim angle - there is only one trim angle the person would assume if you removed thrust and that's not the angle in those pictures. This brings us back to the trim during swimming not being the true "bad trim" we started with (at steady state.)

 

[fsardone]

I've changed the term to "propulsion". I doubt that anyone but a jet pilot was confused by this, but now everyone should be able to understand the concepts.

Hi Doc,
fact is (I am a non native speker disclaimer) what I understand is that the diver is propelled down and needs to add weight. If this is true it makes no sense. In both cases thrust and propulsion on the diver is up, only the water goes down.

IMHO:
It would be better to say: this position propels (or pushes) water down with a resulting force (propulsio/thrust) pushing the diver up and therefore this is compensated by increased weight/decreased buoyancy.

I dive rebreathers and, believe it or not, I need to adjust my thrust vector while decompressing since breathing does nothing good in term of buoyancy for me ...

Kudo on you for teaching this stuff to entry level divers.

Cheers

Fabio

 

[admikar]

Or try it this way: stop kicking and check for buoyancy. Once neutral, lead with your head. If you're ascending, head down and body will follow, reverse for unwanted descent.

 

[The Chairman]

Hi Doc,
fact is (I am a non native speker disclaimer) what I understand is that the diver is propelled down and needs to add weight. If this is true it makes no sense.

You're missing the constant force of gravity. If you are flat in the water, your propulsion is straight back and you are neutral, you won't have to do a thing when you stop. If you have to become negatively buoyant in order to compensate for bad trim, once the propulsion is gone when you stop, you'll sink. We could go crazy with all sorts of force vectors, but I'm really trying to keep it simple enough for divers to conceptualize while they are submerged. Too often I see people constant fiddling with their BCs. One second they're too heavy and the next they're floating to the surface. Everyone has horrible trim and simply don't realize that they create downward propulsion (upward thrust) while they are swimming that disappears when they stop. Once you get the trim right, neutral buoyancy is incredibly easy.

Rather than continue with the next topic, "How to Establish Trim" I'll probably write "Bubble Management" next. I haven't decided, but it's obvious to me by the PMs I'm getting that many divers have never considered this aspect of their diving. It's confusing and surprising the hell out of them. What would y'all like to see next?

As for rebreathers, this is in the basic section so it's about open circuit scuba.

 

[The Chairman]

Or try it this way: stop kicking and check for buoyancy. Once neutral, lead with your head. If you're ascending, head down and body will follow, reverse for unwanted descent.

Yes, you have to stop kicking in order to check for buoyancy. No, you should very rarely 'lead with your head'. Learn to breathe yourself down and up rather than screw with your trim. Manage your bubble and you'll manage your depth while maintaining trim. You'll also find your SAC rate will improve and you'll end up seeing more. You'll also be able to handle those delicate situations when you find yourself close to a silty, silty bottom to look under a ledge. Your head is way down, your feet are way up and you need to ascend gently. I'm getting off topic here. This thread is about trim and how it affects propulsion vectors. More on this in Bubble Management.

 

[fsardone]

Well now you mention it ...
The equilibrium equation in water is:
Trust + Weight = Drag + Buoyancy
these are vectors.
Drag always have same direction and opposite to velocity
Buoyancy and weight always vertical and opposite
Trust depending on how you swim.
In case you swim horizontally (velocity vector horizontal trust vector still could be anywhere)
Vertical components:
Trust (vertical component) + Weight = Buoyancy
Horizontal components:
Trust (horizontal component) = Drag

So if trust is not perfectly horizontal you buoyancy will be off whenever you chnage your swimming velocity (drag is proportional to velocity squared)

Cheers

Fabio


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