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

[markmud]

Hi NetDoc,

Thanks for this discussion and keep going with your next topic, please!

I understand your strategy and enjoy reading your posts regarding this issue. KISS is a great teaching method especially when trying to communicate U/W with people who are not hydrodynamic or aerodynamic experts.

However, ISV and especially Farsdone are incredibly knowledgeable and I enjoy reading their posts. Fighter pilots are highly trained and very smart. For a wonky guy like me, they are highlights.

thanks again,

markm

 

[CuzzA]

I'm not understanding why all the criticism. It's pretty freaking elementary the way NetDoc illustrated it. Whatever your trim is will be the direction you go, up or down. Once you establish neutral buoyancy, stay horizontal and you should not have to adjust your buoyancy again unless you ascend or descend to a different depth. Pretty freaking simple. Use your breathing and lungs to make minor changes to your buoyancy if you want to get closer or further away from something.

 

[fsardone]

I'm not understanding why all the criticism.

Neither do I ... actually I do not see the critism you are referring to.

For my part I am only contributing some physics which in itself does not critise anything, just is :shocked2:

Fabio

 

[CuzzA]

To add a little more to the trim subject I can offer a little technique I like to use (perhaps this is well known, self taught for me), especially if I'm working with a current or stopped or slowly finning with the current. I feel much more comfortable keeping my body horizontal or slightly inverted and my legs up and fins above head level. One it keeps me from kicking the bottom if I'm hugging the seabed exploring. Two, when I want to start moving it's much easier to curve up while taking a breath and start kicking and then regain horizontal trim, compared to what I've observed with many divers where their bodies are vertical and they let their feet drop below them and have to somewhat curve/dive down while propelling up (kicking the reef) to regain horizontal trim to move.

Edit: A good way to illustrate this is to watch a fish, like a tang, explore some rock work for algae. You'll notice they're always inverted with their tail up. They're never facing up toward the surface. When they want to move on they bring their head up and begin to fin. Their swim bladder would be the equivalent of us using our lungs to adjust up or down buoyancy and trim.



---------- Post added December 23rd, 2015 at 08:04 AM ----------

Neither do I ... actually I do not see the critism you are referring to.

For my part I am only contributing some physics which in itself does not critise anything, just is :shocked2:

Fabio

I understand, but we can derail a good thread over semantics or we can add constructive tips and techniques that apply to trim. fsardone, my comment wasn't directed specifically toward you, just the overall nature of the replies to the thread. Vectors, victors, hectors, whatevers... The concept is simple.

 

[Colliam7]

Pete, THANK YOU for the illustrations. They are very useful. I also discuss trim and the consequences of bad trim – particularly in OW and AOW classes – from the perspective of vectors, but have heretofore used only crude, hand-drawn illustrations on a white board. These visuals are great. Directing new students to this thread on SB can serve a combined purpose, of educating them and introducing them to SB. Several thoughts occur to me as I have read the comments.

This is a slight angle as I've seen many, many divers with a 45^o angle or worse!

In teaching, I refer to these divers as the ’45-ers’, and make it clear to students that I do not wish to see any ‘45-ers’ in my classes.

Unfortunately, once the badly trimmed diver stops, he has no upward force to keep him neutral, so he starts to sink. This is where people start to scull with their hands or they add some air to their BC.

And, the next, logical consequence is that, when they start swimming again they begin to ascend, so they purge air from their BCD. The composite process is therefore characterized by a continuing series of series of gas additions to, and purges from, their BCD, leaving divers frustrated, and searching for solutions that too often involve unnecessary weight additions (or subtractions) and equipment solutions to skills problems – not going off topic here, just adding comment on the practical consequences of the student’s failure to understand the physics of the problem.

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.

True. But, (I think) Pete’s point – with which I fully agree - is that bad trim is a primary initiating factor that creates a cascade of problems. Divers adjust their buoyancy (i.e. adjust air in their BCD) while swimming to achieve what they perceive to be neutral buoyancy while moving forward. Because they are not in good horizontal trim, and part of their propulsion is directed downward, they add less air that would be ideal for the depth.

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.

Also true. In flying we set power, and adjust trim, to maintain level flight (and target airspeed). Unlike aircraft, most divers do not have a very flexible 'throttle'. Yes, they can swim ‘faster’ or ‘slower’ at times, but most divers use a relatively constant finning rate (be it flutter or frog), most of the time. So, when swimming (i.e. the state condition that Pete based this thread on at the beginning), they adjust their buoyancy to maintain their depth.

The comments illustrate yet again the conspicuous similarities between flying and diving, and the fact that more than a few SB members are also pilots is not surprising.

 

[The Chairman]

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

Now you're getting it. Your propulsion has to be horizontal to not have an effect on your buoyancy. Drop those feet and now we have to mitigate the introduced downward propulsion by decreasing our buoyancy if we want to continue in a horizontal direction.

FWIW, if you're going fast enough for drag to really be an issue, you're going way too phreakin' fast! Slow down! No need to be jetting around under water!

As for 'all' the vectors, the point of this thread is to keep it simple. This is not a treatise on the all the physics of diving, but rather a treatise on why horizontal trim is the best way to scuba.

As for some of the apparent nitpicking... I don't see anyone trying to be mean about it. It's not that they're being critical, it's that they aren't used to thinking about their diving in terms of vectors. Unfortunately, I've used a term wrongly. I'm not a pilot and think of thrust as the flames coming out of the tail of the jet and not the resultant direction and speed it's going. Fair enough. That's the great thing about ScubaBoard, you get to learn even while you're teaching. I find the feedback invaluable in refining my teaching techniques. Every student benefits from the students before them and they'll help the next one. I keep and refine what works and toss what doesn't. My 20/30 minute lecture (usually at breakfast) about vectors, trim, bubble management and breathing techniques cuts down on my student's frustration in the pool considerably. After all, Scuba and learning Scuba should be fun. That doesn't mean it shouldn't make you think or be a dumbed down class. For me, it means that I want to continually evolve as an educator. How can I turn out a better student while not wasting their time and keeping it fun?

Recently, I received a PM from a lady who has signed on to SB just to sell her gear. She told me why and I couldn't blame her. She was all over the place during class and in her dives. It was obvious to me that she was never in control of her buoyancy and ergo always felt like she was in danger. That's no fun. Getting in control of your diving will make it fun, fun, fun. Isn't that why we dive? To have fun? This is written for all those divers who either aren't having fun or want to have even more fun. That's why I teach the way I do. That's right, the first skill my students master is trim and neutral buoyancy followed by breathing techniques. There's no kneeling, standing or lying on the bottom of the pool to learn or demonstrate skills in my class. I find it easier this way as I don't have to break any bad habits at the end. We start neutral and build from there. This is a part of my process, that's all.

 

[fsardone]

FWIW, if you're going fast enough for drag to really be an issue, you're going way too phreakin' fast! Slow down! No need to be jetting around under water!

As for 'all' the vectors, the point of this thread is to keep it simple. This is not a treatise on the all the physics of diving, but rather a treatise on why horizontal trim is the best way to scuba.

Doc,
when OC I dive twins on my back and 2 or 3 deco bottles ... drag is my way of life

I am going to PM you in order not to derail the thread.
Merry Xmas

Fabio

 

[dumpsterDiver]

Seems that some people are nitpicking. My conceptual model is pretty simple...If your trim is off, putting you at a 45 degree angle, then when you swim, your body is plaining upward, since the angle of attack to the water makes your body like a wing and is lifting you toward the surface. It is not simply that the diver is kicking downward, to compensate for the being heavy.

So to fix the situation, try to adjust your weight distribution (NOT amount), so your natural trim is horizontal. If you are close to that, then you should be able to swim efficiently.

However, even if your trim is perfect, your buoyancy can still be off and you will not really know it if you are swimming at a good pace. If you are too heavy, you will naturally and subconsciously adjust your kick to a slightly downward angle (for flutter anyway) and if you are a tiny bit too light you will be kicking to help hold yourself down. This is actually hard to detect and only serves to waste your energy.

If you are riding a scooter, you can get your buoyancy even more out of whack without realizing it, because the scooter is so powerful it is really easy to compensate for a few extra (or too few) lbs of air in the BC (by adjusting your fin angle a tiny bit).

So as people mentioned, you just have to stop kicking (or scootering) every few minutes for like 10 - 12 seconds and "test" if you are moving up or down.

 

[Steve_C]

I have yet to meet the student who thought of the direction of his kicking as a vector naturally. .

Just means that you have not had a new student who had taught vector calculus and differential equations and worked with a number of engineers and applied mathematicians for 30 plus years PRIOR to starting scuba. My instructor was also trained as an engineer so we were vectors from day one. Probably not the typical situation.

 

[admikar]

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.

I totally agree. What I mean is, if you're neutrally buoyant and still going up/down, your trim is off, so you need to change body position relative to movement direction. It doesn't need to be a large change, hence lead with your head statement.

However, even if your trim is perfect, your buoyancy can still be off and you will not really know it if you are swimming at a good pace. If you are too heavy, you will naturally and subconsciously adjust your kick to a slightly downward angle (for flutter anyway) and if you are a tiny bit too light you will be kicking to help hold yourself down. This is actually hard to detect and only serves to waste your energy.

That's why I use to stop every now and then to check for buoyancy.