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

[LowVizWiz]

... I would love to get a real videographer on this project. Let me know if you're available, or maybe we can do it early January at Jacobs Aquatic Center in Key Largo after the first of the year.

My availability varies, but this sound like a fun project to do!

 

[Steve_C]

Personally when drifting along slowly looking for little critters I prefer to be slightly head down with body straight keeping awareness of what lies ahead of course. In the same way it is easier on my neck when searching ahead to be slightly trim up and looking for big stuff.
Point is I dial in my buoyancy first at depth and then I adjust my trim to be easiest on my not so flexible 70 year old body, protect the surrounding environment, and make it easiest to look at whatever it is I want to look at.

 

[The Chairman]

I personally think the changes in the weight of the tanks makes optimizing trim more challenging and the best solution might be something that works perfectly toward the middle of the range.

Once you get the concepts down, it's a piece of cake. Crumb cake.

My availability varies, but this sound like a fun project to do!

My Man, Wys comes through! I might be up your way for a rebreather class in January too. Do you have access to a pool up there?

 

[Eric Sedletzky]

If you're willing to essentially "waste" a couple of dives, you can probably get this done relatively quickly. Rather than swimming around exploring and looking at things, you'll have to be willing to get in the water, see how your trim is, get back out of the water to move weights around, and go back in to test. Probably multiple times. In my case, I didn't do that; I dove once per week for 3 or 4 weeks in a row, making sure to spend some time in each dive just hanging motionless to see how my trim and overall weighting were, and then I made adjustments during the week before the next dive. AFter about 4 dives, I had it dialed in pretty well. Now I can hang motionless head up, head down, or horizontally as required, and hold a 6 ft depth with an empty BC at the end of the dive using just my lungs. I chose to use 6' instead of 15' because so much of my diving is in the shallows near shore, and I didn't want to have to fight to stay down to look at things.

But at least you know what weighting you need for 6'. Using 15' establishes a starting point and sets the minimum weight combo needed for any dive. After that it's just a pound here or there to adjust for 10' or 6' or whatever. The problem is not many people have a clue whatsoever about what weight they need to achieve anything, they just figure they have a BC to cover for it. This is a typical instructing problem, many times instructors themselves have no clue about what weights are needed, all they know is they don't want their students to cork so they weight the hell out of them to keep them planted on the bottom. However this bad info never gets rectified until the student either takes a PPB class or learns it on their own somewhere somehow.

 

[DelrayDM]

As a new diver, I *think* what I need to know next is how to Establish Trim.

Same here.

When I go in the water wearing nothing but shorts, I sink to the bottom (fresh water or salt water). So what I have been trying to figure out for 20 years is how to get my legs not to sink.

 

[Jim Lapenta]

Same here.

When I go in the water wearing nothing but shorts, I sink to the bottom (fresh water or salt water). So what I have been trying to figure out for 20 years is how to get my legs not to sink.

Several options present themselves for this. Lighter fins, more buoyant boots (my 5 mil pinnacles are crazy buoyant), and decide to wear some form of exposure protection. I don't like to go in any open water with more exposed skin than I have to. Even a 1.5 mil full suit helps.
You can also move weights around on your body and get them off of your hips. A BPW helps with this as it not only puts weight over your lungs where it's needed but the harness allows you position weights pretty much anywhere. Including on the shoulders.

The other thing that seems to help is adopting a modified frog kick or scissor kick with the feet up. This puts the weight of your legs more towards your center instead of way out behind you. Then you can extend your arms a bit to add to the shift.

 

[freewillie]

Technically, a vector has a magnitude and direction.

From Despicable Me

 

[boulderjohn]

As a new diver, I *think* what I need to know next is how to Establish Trim. The importance was reasonably clear from my OW training, and made clearer from your first post, so thanks! But the path to getting comfortable with Trim looks like a lot of trial and error over many dives, made more complicated by vacation diving and rental equipment. If there's a way to be more efficient about it, I'd love to know.

Rental gear is indeed part of the problem. When you rent gear, you usually get nothing but a weight belt, with no easy options for putting weight anywhere else. When I used a back inflate BCD with trim pockets in the shoulders, I found that putting the same amount of weight in the shoulder pockets as I did in the integrated pockets was perfect for me. You will rarely get that option with rental gear.

I certified two close friends when we went on a vacation to Mexico together, the dive shop we used assuring me that they would let me do that with their gear. Knowing that they would have only the most basic rental gear, I brought four bungee loops with me and used them to attach weights to their cam bands, two each. We were able to get them in such beautiful trim that the new DM we had the day after they were certified assumed they were veteran divers with at least 100 dives.

I was just cave diving in Mexico, and my buddy and I were using a sidemount system with aluminum tanks. Neither of us had ever used that system with aluminum tanks, so we weren't sure how we would weight ourselves for trim. We used those same bungee loops, which enabled us to put them almost wherever we wanted. My buddy attached them to the tops of his shoulder straps. I only needed one weight, and I put it on the small of my back.

We also had zip ties with us, and we could have used them as well.

When I go in the water wearing nothing but shorts, I sink to the bottom (fresh water or salt water). So what I have been trying to figure out for 20 years is how to get my legs not to sink.

One thing to do is to put as much weight high up as you can (see the part of my post above), which will partially balance the weight. Another thing to do is to adjust the length of your levers. The effect of weight is not just the amount of weight you have, but how far it is from your center of gravity. You can shorten your legs by bending the knees 90°, so the feet are up. You can lengthen the front levers by holding your arms out in front of you.

You can also make a change with gear. If I am diving with a wet suit, I prefer to dive with the lighter of my fin choices for the reason you describe. A few pounds can make a big difference when they are out on the ends of your feet. When I am diving with a dry suit, I prefer my heavier fins, both because I want those extra pounds (less need for lead) and because I can adjust buoyancy by controlling the bubble of air in my dry suit.

 

[markmud]

Hello NetDoc and BoulderJohn,

Where do you like to position the tank on ones back?

I see many divers with their tank half way down their butt. I trim-out best with the tank centered on my back, with the first stage and tank valve at shoulder level, but below my neck (my head can't hit it if I arch my head backwards).

If my tank is hanging of my ass, I need trim weight in my shoulder pockets as Boulder John described in post #48. I don't shift weight from other pockets, I have to add weight.

The tank is the biggest and heaviest object that is a part of our SCUBA kit. It can be negative, positive, or neutral; depending on many factors.

How much emphasis do you place on tank placement for any particular diver?

Thanks in advance,

markm

 

[The Chairman]

We're way ahead of ourselves here. There will be a thread about getting trim... This is only about why to get trim. Until you understand the need, it will never be much of a priority. Just be patient. The discussion will come in due time.