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How does trim change with a change in BCD?

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#1 tickledone

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Posted 17 August 2020 - 12:59 PM

Optimizing your trim and streamlining are so important for many reasons - both perfect your buoyancy - which in turns saves you air, which means longer bottom times. 

 

We all learn the basics about trim and how distributing weight. For example, head up --> maybe distribute some weight to tank trim pockets; feet up --> consider ankle weights.

 

So, I am just trying to wrap my brain around this - does one's trim change with a change in BCD style? Does it matter if it's a jacket inflate, back inflate, harness and wing, or backplate/wing? 

 

Disclaimer: I am not afraid to admit that I don't know the answer to this question, and it's personal. I am moving from a jacket BCD to a harness/donut, and I was trying to figure out where I want to put my tank trim pockets. Yes, it would be easy to figure out if I just got into the water, but COVID, and I am trying to optimize my kit before I get in the water.  :mellow:

 

 

:thankyou:



#2 Cmdr. Clownfish

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Posted 17 August 2020 - 05:26 PM

The real question isn't which is better.  The real question is why are you switching to a harness/donut?  This is not required by anyone except some tech classes and there are alot of drawbacks.  I have used all types of BCs and I can tell you that you can get excellent bouyancy out of any of them.

Unfortunately nobody can tell you where to put your trim pockets but you.  That is kind of like asking what fins are the best.  If you really want to ruin your self worth then ask that one on scubaboard but I won't advise it if you get upset easily.

 

The problem with bouyancy is very few divers actually understand it so they say things like BP+wings put the weight on your back where it belongs so they are better.  That may be true for some people but if I tried that logic in my engineering mechanics classes 30yrs ago I would have failed since it is not that simple nor universally correct.  Its possible that with a BP+wing you may actually end up very overweighted depending on who you are.  This is particularly true for women and less so for men.

The backplate was invented many decades ago as a means of securing steel doubles for tech divers. The idea is that if the structure is steel then its not going to break in a cave. Before then people just put harnesses around the doubles and put them on their back.  At that time it was mostly men diving and the BP sat directly on top of the male CG which is usually near the bottom to the sternum.  Since the weight of the BP and doubles was right on top of their CG it made them appear heavier in the water didn't really mess that much the bouyancy and they were happy with it. They could just trim it with a few lbs and on their on their way.  Women however have a CG near the hips.  When they wear a BP it tends to tip them over and their head goes down.  So they have to put significant weight on the other side of their hips to counter it.  Then they are overweighted but that is the only way to be level.  Men not so much. So that is one way it differs.

 

So what do you do?  Unless there is a technical reason for the harness/donut then don't consider it a panacea for stability.  It is not, weight distribution is not what they are built for nomatter waht the DM, instr, or dive shopt owner tells you and you may very well end up too heavy. 

 

Think of yourself in the water like an airplane in the air.  The pilot wants the airplane to be level in the air but there is weight in the front of the wings and behind the wings.  That weight is not equally distributed on each sides of the wing so the plane has to be trimmed or it will tend to be nose up or nose down. The center of lift is where the all the pressure under the airplane is essentially concentrated. This is what holds the airplane in the air. This is kind of like you can take all your weight and concentrate it at your center of gravity to represent your weight only with pressure. Now the center of lift should be right near the center of the wing. Ideally you want the CG of the airplane to be right on the center of lift.  If they overlap the weight and the lift cancel each other out and airplane is balanced and will sit flat in the air.  But that doesn't happen so the pilot has to adjust weight and the tail stabilizers to move the CG there.  That is what he/she is doing when they adjust the weight and you look back and see the horizontal staiblizers at an angle but the plane is level.  That is to move the CG to the right spot on the wing. (So I dont get called an idiot only the CG is actually moved by adjusting the weight. A force is added by the horizontal stabilizers to counteract not being able to move the CG enough but you get the idea.)

 

As a diver you do the same thing.  Your center of lift is under the bladder (that is what is lifting you).  Your natural CG may be near there if you are a man but is closer to your hips if you are a woman. So you have to move your CG to be under the bladder. Since your CG is not under the bladder you have to put more weight under the bladder to shift it in that direction. That is what your trim pockets do for you. Exactly where to put them near the bladder is dependent on the individual. If you understand the physics you can do this with any style of BC (other than a BP/wing) and trim perfectly. The BP will put this weight there to begin with but it is likely you don't need 6lbs+ there.  You would need to put more weight on your hips to counter the extra BP weight and you may end up fighting being overweighted. Or you may not.  It all depends on your physiology.

 

I would say that until you are expert you are more likely to be correctly weighted with a BC of any style than a BP/wing. The style is somewhat irrelevant and up to personal choice based largley on comfort.  Each style works equally well despite the marketing.  Back inflate styles tend to have a number of different bladder types that adjust the center of lift in one director or another.  A donut style vs a horseshoe will move that lift location around requiring a different trim setting.  Each bc is differnt so the trimming so the trimming willb e differn tfor each one.  The better ones have trim pockets built in that put the trim in the right spot for the bladder and indtended user.  My Hollis HD100 trims me perfectly with no extra adjustments required.  Its all the same physics whether it's a BC or any type or a BP.  Try to understand the physics and the rest will come naturally. 

 

There are BCs that are meant exclusively for women but most are for men.  Good ones are designed to take the CG difference into account and put the weight where is supposed to be for your gender.  They work really well if you get the right one for your gender.  BP+wing are DIY weight management and best meant for someone like Kamala who has so much experience it really does not matter what they wear.  With experience you can wear any BC or backplate and just counter it instinctivley without thinking about it even if heavily overweighted.

 

Not sure if this helps any or it made it worse.


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#3 tickledone

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Posted 18 August 2020 - 05:15 PM

The real question isn't which is better.  The real question is why are you switching to a harness/donut?  This is not required by anyone except some tech classes and there are alot of drawbacks.  I have used all types of BCs and I can tell you that you can get excellent bouyancy out of any of them.

Unfortunately nobody can tell you where to put your trim pockets but you.  That is kind of like asking what fins are the best.  If you really want to ruin your self worth then ask that one on scubaboard but I won't advise it if you get upset easily.

 

I have been there, and while there are many fine and nice divers on said board, there are also many fine (and not so fine) divers with very strong opinions and at times poor means of communicating those opinions. They are also very heavily invested in promoting BP/W and nothing else, which is kind of silly. (The same could be said of r/scuba on reddit.) 

 

I really wasn't asking which is better because every type of BCD has it's own benefits and drawbacks, and you're right, any diver should be able to dive any BC. I do not want you to think that I am all about BP+W either. I am moving to a harness + donut because I am heading off to divemaster training, and I wanted something simple (and can accommodate some technical diving). I had hoped to get time in the water when I ordered it (and before I went off to train) but again...pandemic, and pools remain closed here (and I don't have a dry suit). 

 

I have been diving the AL Soul i3, and the i3 system isn't conducive to skills teaching, but it is awfully fun to dive for pleasure because it's so...cushy. At the time when I got my first kit - I was't planning on doing much than AOW. It's a women's specific BCD, as well, which accounts for the lower center for gravity. Also, it has tank trim pockets that are sewn in, so it wasn't like I had much choice in the matter on trim.

 

 

 

So what do you do?  Unless there is a technical reason for the harness/donut then don't consider it a panacea for stability.  It is not, weight distribution is not what they are built for nomatter waht the DM, instr, or dive shopt owner tells you and you may very well end up too heavy. 

 

Oh, goodness - yes, I very much agree with this. My impression, however, has been that backplates can help with adding weight in settings where you're donning a lot of weight (i.e., cold water diving with a dry suit, where you're wearing 20-40 lbs), which then lent itself to trying to wrap my brain around how one dives in tropical waters with a steel backplate if you don't use a lot of weight without a wetsuit. It seems to me that one could potentially overweighted by the plate itself. 

 

 

As a diver you do the same thing.  Your center of lift is under the bladder (that is what is lifting you).  Your natural CG may be near there if you are a man but is closer to your hips if you are a woman. So you have to move your CG to be under the bladder. Since your CG is not under the bladder you have to put more weight under the bladder to shift it in that direction. That is what your trim pockets do for you. Exactly where to put them near the bladder is dependent on the individual. If you understand the physics you can do this with any style of BC (other than a BP/wing) and trim perfectly. The BP will put this weight there to begin with but it is likely you don't need 6lbs+ there.  You would need to put more weight on your hips to counter the extra BP weight and you may end up fighting being overweighted. Or you may not.  It all depends on your physiology.

 

Appreciate your detailed explanation. I would also agree that in the end, it does all depend on your physiology.  

 

Thanks for your 2 psi. :) 







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