Are trim and buoyancy fundamentally related?

[HantsDiver]

After an interesting discussion with @The Chairman in another thread, I wanted to see if you guys think trim and buoyancy are fundamentally related to one another.

I'll start and say that I do not think they are. I can see how people assume they are - but the simple idea that you must have good trim to be neutrally buoyant is wrong. Nor do you need good buoyancy to be in trim. That is my opinion and experience of diving.

An example not related to trim is airplanes. They are clearly not neutrally buoyant. However, they manage to fly in trim position. They do this using thrust from the engines and the angle of the wings. The idea of this being neutral buoyancy is total nonsense.

In diving, our moment of inertia is controlled by where we place out weight and our body position. This is what controls our positioning in the water. Thus we are able to hover in a number of positions whilst we dive if we are neutral. We are able to also use thrust from our fins to exert a force which keeps us in a body position. However, none of this has anything to do with buoyancy in my opinion.

Whilst it is true that people who are severely negatively buoyant counter this using upwards thrust from their fins. This is simply a reaction we make rather than a fundamental connection. If they were to change their body positions they would still be in trim however they would sink.

Please let me know what you think.

 

[RyanT]

I'm going to say yes and no. Your premise that you do not have to have good trim to achieve neutral buoyancy is correct. You could be really foot heavy (e.g. vertical in the water column) and still achieve neutral buoyancy. On the other hand, having good trim will really increase your comfort level and make maintaining neutral buoyancy much easier.

 

[guruboy]

No, they are not.

 

[HantsDiver]

I'm going to say yes and no. Your premise that you do not have to have good trim to achieve neutral buoyancy is correct. You could be really foot heavy (e.g. vertical in the water column) and still achieve neutral buoyancy. On the other hand, having good trim will really increase your comfort level and make maintaining neutral buoyancy much easier.

I'm less than convinced that for most divers being in trim is more comfortable. Arched neck. Arched back. Clenched bum cheeks. I'd personally find it more relaxing to do deco stops slightly out of trim. Admittedly you can get into trim without doing that using weight distribution. But fundamentally a totally relaxed person is not forcing themselves into any position.

I also don't think that makes them fundamentally related. Better performance regs make me more comfortable. Better thermal protection makes me more comfortable (dreaming about a santi heated undersuit). Diving with people I know well makes me more comfortable. But none of those things would generally be considered fundamentally connected to buoyancy.

 

[Jack Hammer]

Yes and no. You can be neutral In any trim however it is easier to stay neutral while you are in horizontal trim. The arching and butt clenching is more typical of someone learning to be horizontal and overcompensating.

 

[DevonDiver]

Trim is fundamental to buoyancy.

You can't be truly neutrally buoyant if propelling yourself forwards with your feet down.

If a truly neutral diver were hovering, with legs at 45 degrees down, then kicked themselves forwards... then 50% of their propulsion would move them forwards and 50% would move them upwards. They'd have to dump gas from the BCD to compensate, and would then be negatively buoyant.

If the same diver was kicking along in 45-degree trim, they'd stay at a constant depth. When they stopped kicking, they'd lose that 50% upwards propulsion. Without kicking they'd be negatively buoyant and would sink.

If a diver was in flat trim, 100% of their propulsion would be horizontal. They'd be neutrally buoyant. Their depth wouldn't change whether they kicked or not.

Diving out of trim allows the diver to shortcut proper neutral buoyancy...to compensate through propulsion.

Achieving good, reliable and consistent trim removes the ability to take the shortcut. It means the diver improves their ability to achieve neutral buoyancy much, much quicker.

10 Tips to Attain a Better Horizontal Hover | Scuba Diving Techniques

Trim & Position | 5of9 | Scuba Buoyancy Masterclass

99 Problems....and your TRIM is one!

 

[HantsDiver]

@DevonDiver I agree with everything you are saying. My point was more along the lines of you can be neutrally buoyant without being in trim. And you can be in trim without being neutrally buoyant. They are separate things that come together to produce the desired result. I believe as you can have one without the other so we don't need to always equate them.

I'll be totally honest and say that in my personal diving circle we talk about trim as the position we are holding when we are trying not to move. It's sort of a given that people will be flat when they are propelling themselves forward. So what I didn't really explain was that for me, trim = shut downs, drills, deco etc... Moving is just swimming. That wasn't clear on my part. And I do understand lots of people have different definitions of trim.

I think I'll change my opinion to - trim is the position we hold in the water. Buoyancy is how we maintain our depth. Propulsion is how we move forwards and backwards. Whilst they are independent of each other they can combine to give us the result we want.

 

[DevonDiver]

In this photo, I am neutrally buoyant...

 

[MichaelMc]

Without good no-fins buoyancy (/depth) control, doing anything is really hard as you are either always messing with your BC or will soon run out of water column. I think being able to hold depth is essential first, and basic, whether it be in a buddha hover or a mostly flat trim with legs crossed, the later being the preferable goal. The only way to get time to focus on trim is with buoyant control. Sure, once you’ve mastered trim, you could hold perfect trim during a negative sky dive to depth, but it is hard to learn by doing that again and again until you learned to hold the trim.

Once you start moving, depth is affected by trim, even horizontal trim. I snorkel in the pool to practice hovers and various kicks -- frog, reverse, helicopter. I maintain stationary flat trim and depth rather well, there and on scuba. On breath hold, I find any sort of forward speed drives me up or down, and I believe more from my body pushing through the water than any angled nature of my body’s kicks. I do not as much notice this on scuba, but may just be compensating automatically with breath.

The frog kick starts above your body and ends inline, so it pushing you perfectly horizontal is a least a bit problematic. Your body moving through the water is an odd shaped plank that likely has some wing or at least wedge effect pushing you up or down.

I think the kick-glide rhythm of frog kick separates a bit in time the thrust/planing effects from the buoyancy effects on depth. The flutter kick’s continuous nature seems to turn me into more of a slow jet that goes whichever way I point, drowning out any small buoyancy imbalances.

 

[ajduplessis]

No, they are not. The addition of forward thrust can impact buoyancy in relation to trim.