I'm being referred to a different SDI training facility.

[drk5036]

Any air bladder you have on your back will, when inflated, push your face forward. That's just a fact. Sort of like, a law of nature. That said, you shouldn't have to inflate any bladders very much at the surface, because you should be very close to neutrally buoyant, and only need a bit of additional air to keep your face above the water. It's when you are overweighted that you need to blow those bladders up to compensate, and that's when something on your back will pitch you forward. Lots of instructors do overweight students, because it's easier to make sure they'll go down, and then just use their BC like an elevator. But that's poor diving. On the surface you add a little air to keep your face out of the water, but you should not need to blow air in until the exhaust valve pops open.

I don't buy this explanation.

The "pitching forward" motion is due to torque. This torque develops from there being a distance between where buoyancy is pushing up and where your center of mass is. Since the wing is on your back, it will develop a slight forward torquing motion, but this torque will be small if only a small amount of air is added. Furthermore, assuming you aren't significantly overweighted, the effect will be exactly the same in a jacket BCD. You'd have to put a ton of air into the BCD for the belly area to fill with air. If you have a small amount of air in the BCD, the water pressure will make sure it goes to the top, where it will...act exactly the same as the wing. The only way to counteract this would be to fill the whole 50 lbs bladder with air, then you would have air providing buoyancy from directly below your center of mass.

As someone who has used a backplate for the last 100 dives or so, I've NEVER noticed this effect. I wonder if it could be that I usually have about 2 kg placed on the cam-bands? This would result in my center of mass being pushed behind my backplate, potentially within the wing, which would stop the torque?

 

[Damselfish]

With a little air the jacket and wing may behave similarly on the surface. The difference is, with the jacket you don't have to stop at "a little air" - you can generally just fill up on the surface with impunity and be done with it. The wing, you have to be careful and do just enough.

Other things come into play, such as tanks and exposure protection. Often the folks that don't believe getting pushed forward is a thing, are using more exposure protection that's floating them up in the front, using tanks that are helping hold down the back, and usually carrying more weight to play with. I dive warm water now - my minimal exposure protection is pretty neutral, I'm usually diving an AL tank, and maybe using like 6-8 pounds of weight so it's hard to play with the distribution. I actually have used back inflates/wings for a long time now, but in my situation it is indeed hard to make it behave on the surface. (And I have an engineering degree, so a firm grasp of physics.)

 

[pauldw]

with the jacket you don't have to stop at "a little air" - you can generally just fill up on the surface with impunity and be done with it.

This is why I like jackets (although I have a back inflate BC now). If you're diving from shore, in the ocean, it's nice to be floating high in the waves when you're at the surface looking around.

 

[Mr. Ed]

I’m very pleased with the Mares Prestige as opposed to the Hollis HD 200. Easy to make trim at depth and surface. Maneuverable, lightweight and durable, however, right side Integrated weight slipped out without my knowledge until I saw it at the bottom of the training pool. Other than that I am happy with my choice of BCD.

 

[inquis]

I wonder if it could be that I usually have about 2 kg placed on the cam-bands?

That's not enough to appreciably shift the center of mass. I'd chalk it up to slight and unconscious finning to compensate.

 

[Belzelbub]

I’m very pleased with the Mares Prestige as opposed to the Hollis HD 200. Easy to make trim at depth and surface. Maneuverable, lightweight and durable, however, right side Integrated weight slipped out without my knowledge until I saw it at the bottom of the training pool. Other than that I am happy with my choice of BCD.

Before you noticed the weight was on the bottom of the pool, did you have any trouble staying down? If not, it sounds like you may be overweighted depending on tank volume at the time. Overweighting is fairly common with new divers, especially with students. So not unheard of, just something to consider.

 

[pauldw]

Before you noticed the weight was on the bottom of the pool, did you have any trouble staying down? If not, it sounds like you may be overweighted depending on tank volume at the time. Overweighting is fairly common with new divers, especially with students. So not unheard of, just something to consider.

Yeah, losing half your weight should have taken you to the surface.

 

[tursiops]

Yeah, losing half your weight should have taken you to the surface.

If he started 6 lbs heavy, for the air in his tank, unless he had more than 6 lbs in his pocket it would not take him to the surface.

 

[Belzelbub]

If he started 6 lbs heavy, for the air in his tank, unless he had more than 6 lbs in his pocket it would not take him to the surface.

Yeah, that’s why I added the tank volume piece. If the weights dumped at the beginning of the pool session, it might not be immediately noticed. Toward the end, when getting low on air, you should definitely notice.

I once forgot to zip my weight pocket (Zeagle with Ripcord). When I got to the bottom, I saw a 2lb weight in the sand. A few feet away, I saw a 1lb weight. It was then that I realized they were probably mine. Put them in and zipped the pocket, then checked the other to be sure. Being early in the dive, I was heavy enough with air where 3 lbs wouldn’t make me floaty.

 

[pauldw]

I said “should have,” not “would have.”

Edit: OK, let me spell this out some more for the person who was confused by my post. "Should have" because if someone is properly weighted, then losing weight means going up. Not "would have" because if he's overweighted then perhaps it would not.