Optimum BCD bladder shape & location for perfect trim

[Doctor Rig]

How does a diver find the “right” wing shape and set-up for perfect trim where one’s buoyancy CG (or center of force) matches their weight CG?

I dive with the Zeagle Covert XT BCD which has a U shaped wing. Since getting that BCD “perfect trim” has been my goal, but I’m not there yet. The Covert has trim pockets on the upper tank strap and smallish dumpable pockets on the belt. As my “perfect trim” efforts have evolved, I’ve started using a zippered pocket weight belt to carry my dumpable weight and find that if I wear the weight belt high on my waist, I’m the closest to nirvana I’ve been, but not perfect. (I have a slim build and weigh about 155 pounds.) I’ve considered buying an adjustable/tune-able weight harness like DUI makes to force my weight CG higher, to lower my upper body and raise my feet into better trim.

After further thought, I’m now wondering if lowering my buoyancy CG would be a more effective approach than raising my weight CG.

Any suggestions on effectively lowering my present Covert BC buoyancy CG (without compromising something else)?

Since a U shaped BCD bladder inherently raises a divers buoyancy CG, I’m guessing a donut shaped BCD would be better for me than a U shaped BCD. Make sense?

Would a BP/W set up inherently give the diver the best versatility in optimally aligning their buoyancy and weight CG? If yes, Is there something specific I should look for in a BP/W?

 

[lexvil]

Dry or wet?

 

[tbone1004]

@Doctor Rig this is VERY complicated...

"fixed" values on your kit. Obviously the body CG is not fixed because it changes every time you breathe, and depending on your body position during the kick cycle, but if we look at it while hovering and over the duration of a breath it's fixed. The tank is also not fixed because it's different at full pressure vs. empty, but it's not changing a whole lot and it's enough that changing the angle of your knee bend a couple degrees will offset naturally, so that's fixed.

Humans will naturally go feet down in the water based on our physiology, but it's close enough to center and if you are in the "skydiver" position like you should be, then it's about flat, especially with neutrally buoyant fins. This gets weird with drysuits that tend to make you go head down because the air travels to your feet.

Tanks will NORMALLY go head down because of the valve and first stage. There are some rather rare tanks that are really foot heavy *some steels in Europe have a flat bottom that make them trim out funny, but they don't exist in America*. The tank counters the natural feet heaviness of most people, and if you use heavier fins like Jet Fins, then it balances out perfectly.

This brings the last and only true fixed factor which is your added ballast and where you put it. If you have a steel plate, then you are distributing that basically in the middle of the CG of the body and the tank so it does essentially nothing. If you wear all of your ballast on your hips in a weight belt or pockets, then you are adding all of that weight below your CG and you'll go feet down. This is the decision that you have to make for yourself before you can look at a wings shape. Things like the Ranger have an exaggerated U shape because they assume that because you're wearing a soft backpack that does nothing for your CG, and/or have foam padding that is already moving it down, that you need more lift at your hips. They are designing that to counter the 16lb weight pockets on the hips, and I'm willing to bet that if you fully load that bc the way it was designed, it will probably trim out near perfectly.

So, once you figure out where you want to put your lead and how much lead you're going to put on there, you can choose your wing shape. Now, the issue with that, is the wing can't defeat physics, so if you are head up, it will always fill the top first. If you are diving a balanced rig, then there isn't that much air up there. Say an AL80 with a 5mm wetsuit, that's MAX 15lbs or 1/4 of a cubic ft. Based on your size, I assume you don't have an XXL so that's 26lbs capacity, so you're only about halfway full at this scenario. You will always have to force the air to go to the back of the wing to get it to trim you head down and it won't just naturally do that.

Not knowing the specifics of what you're wearing for exposure protection, which tank you're using, and how much lead you're wearing, I can't make a recommendation on what to do. I can tell you that going to a stainless backplate will certainly help, and for you it sounds like using those trim pockets up high will help, but don't focus on ditchable weight because it's not something that really matters except for at the surface.

 

[johndiver999]

Ditchable weight only matters on the surface? I don't agree with that.

If you want to get your head down, a very simple and cheap method is to add some (non-ditchable) weight to the neck of the tank by affixing a 2-3 lb weight there. You might be surprised about how significant of an effect that will produce.

 

[tbone1004]

Ditchable weight only matters on the surface? I don't agree with that.

If you want to get your head down, a very simple and cheap method is to add some (non-ditchable) weight to the neck of the tank by affixing a 2-3 lb weight there. You might be surprised about how significant of an effect that will produce.

not getting into a ditchable weight argument, but if you are properly weighted, then you should NEVER have to ditch weight at the bottom. End of discussion. Problem is most people are overweighted, but that's a different thread.

 

[rongoodman]

If you can get some video taken of yourself in the water, do so. Changes in your posture might be more effective than moving weight around. A little arch in your back and a little tension in your glutes will help keep your body straight from shoulders to knees.

 

[Doctor Rig]

Thanks for the comments so far! In response, here is some additional detail.

My trim efforts have all been wet. Shifting weight from the BC side pockets to a soft pocket weight belt had two purposes, One, to isolate some of that weight so if it was dumped I wouldn’t go to the surface like a bobber. Second, and primary, was to give me more adjustability in where the weight was located (up or down) so I could see evaluate the impact on trim, which it did. Moving the weight belt (up) towards the apparent buoyancy CG improved trim.

As I tried to say earlier, moving the buoyancy CG down would likely have the same affect as moving the weight CG uo. My U-shaped BC bladder CG would be higher than a donut’s Buoyancy CG (assuming they were the same length).

On a BP/W can the diver move the BC bladder up and down the plate to change the divers buoyancy CG? Do wing manufacturers make and market different bladder configurations to assist diver’s in obtaining improved buoyancy trim?

 

[dmaziuk]

I don't know about you, but I sometimes hang upside-down when taking a photo. Or swim face-up watching stuff above me. Optimal placement of the air bubble, when there is one in my wing, is subject to change.

 

[rongoodman]

On a BP/W can the diver move the BC bladder up and down the plate to change the divers buoyancy CG? Do wing manufacturers make and market different bladder configurations to assist diver’s in obtaining improved buoyancy trim?

Some wings have multiple sets of mounting holes, some don't(e.g. Halcyon). There are some variations in wing design and shape among manufacturers but I don't know how often people take their effects on CG into account when choosing what to buy. As one of the more experienced divers around here likes to say(sorry, I can't remember who right now) "You dive the gear, you don't let it dive you".

 

[Centrals]

Scuba diving is a recreational sport for most people.
Human being come in all shapes and sizes.
Tank makes from various materials and different sizes as well.
Thermal protection is another variation.

Horse for the course. Just go put and enjoy the fun.