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

[The Chairman]

Who hasn't admired those effortless divers. They're in perfect control and seem to use no energy during their dives, adjusting their depths magically, never creating a silt out, never scull and their air seems to last forever! Some say, this takes at least a hundred dives, but I've got a few lessons that will make you feel and look a lot better on your very next dive. This will be a multi-part discussion about how to master Neutral Buoyancy. They will be "The Importance of Horizontal Trim", "How to Establish Trim", "Bubble Management" and "Breathing Techniques". Consider this my Christmas present to all my students, past, present and future.

It's my personal opinion that these discussions and demonstrations should be a part of every open water class, but alas they are not. However, this thread is only to discuss the physics of trim and how it affects neutral buoyancy and not the importance of when or where it should be taught. PLEASE KEEP ON TOPIC.

Disclaimer... if the following picture epitomizes your concept of neutral buoyancy, then this thread may not be for you. In fact, it might just piss you off!

Horrible Diver Buoyancy/Trim​

Almost everyone knows the physical law that for every action, there is an equal and opposite reaction. This is the beginning of understanding physics for divers. Don't worry, this won't be painful. I've taken a bit of time to create some animated GIFs to demonstrate what I'm writing about. As for testing your comprehension, your final exam will be on the reef.

For our purposes today, we'll be quantifying kick propulsion as a vector. A vector has both amplitude and direction which can be broken down to indicate both it's vertical and horizontal components. We're not going to worry about left or right in this discussion so we can keep this simple. All we are worried about is propulsion as it relates to vertical and horizontal components.

So, let's look at a nicely trimmed, horizontal diver. Of course, we're not talking about Christmas ornaments, but their attitude in the water, which should be mostly flat and horizontal.

Notice the propulsion vector being generated by the diver's kick. It's going straight behind him with no upwards or downwards element. This diver will control their depth by using their lungs rather than adjusting their propulsion angle and we'll cover how to do that in a couple of installments. Now let's look at a badly trimmed diver. We can see that his propulsion is not horizontal anymore but rather has a downward element to it.

While this is pretty as a picture, it's not giving us a real clue as to what is really happening. In order to do that, we are going to separate the resultant vector into its horizontal and vertical component vectors. They are smaller, but when added together, they become the bigger resultant vector seen above.

Notice the small vector representing the downward propulsion happening here. The diver must compensate for this downward propulsion by adding more weight or letting out air in their BC to keep them moving horizontally. This is a slight angle as I've seen many, many divers with a 45^o angle or worse! The larger the angle in regards to horizontal, the more force is being directed downwards rather than rearwards. That means you have to overweight yourself even more and that you're wasting a lot of energy pushing yourself up instead of forward like the horizontal diver. Good exercise, sure, but what happens when you stop? Here's a side by side comparison of these two divers at rest. The horizontally trimmed diver is on the left.

See? When the horizontally trimmed diver stops, he just stays there. 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. Sculling and playing with your BC all the time are two major indicators that your trim is off: way off. That horizontal diver can just relax and look at whatever he stopped to admire. The badly trimmed diver has chased away whatever he wanted to look at with his flailing hands. Even worse, he gets the 'rototiller' award for kicking the crap out of the reef when he starts up again. While it's still your choice, it's pretty obvious from a physics standpoint that getting horizontal will improve your diving. You'll use less energy, which means your air will last longer and your drive will be far more enjoyable. I'll post the second installment in a day or two on how to establish horizontal trim. I bet you have a number of ideas already!

Comments, questions or insights? Please post them here: The Importance of Horizontal Trim Discussion​

 

[stuartv]

This 'reaction' can be quantified as a vector. A vector has both amplitude and direction.

Cool graphics. Nice illustration of the concepts.

A minor nit. Technically, a vector has a magnitude and direction.

 

[archer1960]

I think you need to add a bit of discussion that specifies the actual direction of motion in your first two graphics. #1 of course looks nice, but I didn't see anything wrong with #2 until I read on and realized that you intended for the diver to still be traveling horizontally. I look like #1 when I'm traveling horizontally, and like #2 when I'm following an upward slope or have to go over an obstruction. And then I point downward when I want to go down on the other side of the obstruction. The bottom in my area is such that I am very rarely traveling horizontally for any significant length of time, and am constantly adjusting my attitude in the water so that I am streamlined in the direction that I am moving, whether that is up, down, or level.

 

[The Chairman]

Cool graphics. Nice illustration of the concepts.

Thanks, I simply combined some I found on the net to add vectors to the diver. They appear to be free, so I'm using them!

A minor nit. Technically, a vector has a magnitude and direction.

Magnitude and amplitude convey the same concept to me: POWER! However, since fins tend to oscillate rather than provide a constant thrust, I think amplitude works nicely! They do happen to be synonyms.

I look like #1 when I'm traveling horizontally, and like #2 when I'm following an upward slope or have to go over an obstruction.

I look like #1 pretty much all the time unless I'm inspecting something like a critter. I don't like to mix my vectors, Victor! The fourth installment will cover the breathing techniques that will allow you to do this, but you probably already know them.

 

[tomfcrist]

Staying in trim all the time makes sense the majority of the time. The obvious exceptions include things like passing through vertical restrictions and such.

 

[archer1960]

I look like #1 pretty much all the time unless I'm inspecting something like a critter. I don't like to mix my vectors, Victor! The fourth installment will cover the breathing techniques that will allow you to do this, but you probably already know them.

Yup, but often they are too slow in low-vis water; I often don't have much warning before I have to quickly adjust my depth by a few feet. I'm still a pretty new diver, but now that I have my trim fairly well sorted out, it's pretty nice to be able to hang motionless upside down to look under a rock for a lobster, without needing to use my hands to hold myself off the bottom, or to fin to stay down.

Looking forward to your next installment!

 

[Steve_C]

"The diver must compensate for this downward thrust by adding more weight or letting out air in their BC to keep them moving horizontally."

I understand what you are saying but this seems backwards to me. The diver is over weighted to begin with and uses the downward thrust portion of their kicks to compensate. Actually in the vertical direction you have 3 vectors:, force of gravity, lift due to archimedes, and the vertical component of the kicking. The kicking is due to an imbalance between the lift and gravity. You are stuck with gravity once the dive starts. It is lift and kicking that you can play with. Even if they are a bit overweighted the kicking components is not needed by using archimedes.

 

[The Chairman]

"The diver must compensate for this downward thrust by adding more weight or letting out air in their BC to keep them moving horizontally."

I understand what you are saying but this seems backwards to me. The diver is over weighted to begin with and uses the downward thrust portion of their kicks to compensate. Actually in the vertical direction you have 3 vectors:, force of gravity, lift due to archimedes, and the vertical component of the kicking. The kicking is due to an imbalance between the lift and gravity. You are stuck with gravity once the dive starts. It is lift and kicking that you can play with. Even if they are a bit overweighted the kicking components is not needed by using archimedes.

You're jumping ahead to Bubble Management. I've found teaching others, that they have to get a grasp of vectors first and how they can make you seem lighter than you really are. Bubble management is all about getting weighted correctly and how to compensate for that shifting bubble on your back. In the field, I'm using my fingers rather than the graphics to demonstrate thrust and a hot sauce bottle to demonstrate bubble management.

 

[fsardone]

Notice the small vector representing the downward thrust happening here. The diver must compensate for this downward thrust by adding more weight or letting out air in their BC to keep them moving horizontally. This is a slight angle as I've seen many, many divers with a 45^o angle or worse! The larger the angle in regards to horizontal, the more force is being directed downwards rather than rearwards. That means you have to overweight yourself even more and that you're wasting a lot of energy pushing yourself up instead of forward like the horizontal diver. Good exercise, sure, but what happens when you stop? Here's a side by side comparison of these two divers at rest. The horizontally trimmed diver is on the left.

Hello Doc,
what the arrows are depicting here are not the horizontal and vertical thrusts but the displaced mass of water propelled by the diver's fins.
The thrust is the reaction (second law of physics) from wiki https://en.wikipedia.org/wiki/Thrust:
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system.

Therefore, in this case you have correctly indicated the consequences but erroneously indicated the direction, you have a vertical UPWARD component of thrust which while swimming will need to be equalised by reduction of buoyancy (either dump BCD, deflate lungs if OC, or increase weight) and this in turn will require adjustments every time you increase, decrease or stop your finning action.

I fly jets which expel high speed hot gas from the tailpipe and thrust, trust me (pun intended), is forward

Hope this helps

Cheers

Fabio

 

[The Chairman]

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.