BC lift question

[cast]

I just bought a new BP/W setup. The wings have around 25lb lift, I thought it was enough for the type of the diving I do.

On the first dive, I had a hard time swimming on the surface. I guess the lift can't hold my head high enough to breath. Diving underwater was great. I don't think I am overweighted.

I normally do a lot of shore dive, so keeping my head up is a serious issue. Do I have to get a bigger wing? Is it a good idea to use a bouy when swimming on the surface? Does anyone have similar experience? Any advice?

 

[RichLockyer]

It sounds like you are overweighted.
Theoretically, if you are PERFECTLY weighted, you will only need enough air in the wing to compensate for the weight of the air in the tank to be neutral at the beginning of the dive.
That means that, in a perfect world, you would never need more than 5 pounds of lift for an AL80, which would translate to nearly 10 pounds of total buoyancy at the end of the dive with the wing inflated.

Dump that tank to 500psi and get in 10ft of water. Dump ALL of the air out of your wing and gradually reduce weight until you can hover.
Move to the surface, and add JUST enough weight to allow you to slowly sink.

 

[Charlie99]

While Rich's comments are correct about lift needs on the SURFACE, you must also consider your lift needs while at DEPTH if you wear a wetsuit.

As you descend, you will need additional buoyancy to offset the loss of buoyancy in your wetsuit as it compresses. For a large diver and thick wetsuit, this may exceed 20 pounds.

 

[cast]

Thanks. But I don't think I am overweighted. I am neutral in the pool with 10lb weight on the belt. I only put on 16lb in the saltwater.
With 12lb weight in the saltwater, I couldn't sink with the full tank.

It sounds like you are overweighted.
Theoretically, if you are PERFECTLY weighted, you will only need enough air in the wing to compensate for the weight of the air in the tank to be neutral at the beginning of the dive.
That means that, in a perfect world, you would never need more than 5 pounds of lift for an AL80, which would translate to nearly 10 pounds of total buoyancy at the end of the dive with the wing inflated.

Dump that tank to 500psi and get in 10ft of water. Dump ALL of the air out of your wing and gradually reduce weight until you can hover.
Move to the surface, and add JUST enough weight to allow you to slowly sink.

 

[cool_hardware52]

Cast,

I don't disagree with any of the advise given so far.

I do have a question. Are you using the crotch strap? IMHO the crotch strap is required most at the surface. Without one people tend to over inflate the wing in a futile effort to rise out of the water.


As far as lift is concerned two values need to be considered. The first is how much lift do you need to float your rig at the surface without you in it? Nice not to have the rig disappear if you ditch it at the surface.

Second is how positive is your wetsuit? Your wetsuit is the only thing losing buoyancy as you decend. I don't mean how much weight is on your belt, I mean how much to just sink your wetsuit.

If you are neutral at the surface with no air in your wing, the only thing the "buoyancy compensator" has to "compensate" for is the loss of buoyancy of your suit. For most people even wearing thick neoprene no more than 25lbs will sink their suit, most would need far less.

If you have to blow up your wing to stay at the surface, you are overweighted.

Do remember that all BCs will lose effective lift because they will fill with water as they are used. If you are still mastering buoyancy you are likely venting and filling more than you will in the future.

Good luck, my guess is that you have plenty of wing.


Regards,



Tobin

 

[cast]

Thanks for the reply.
I don't have crotch strap. But I don't quite follow why it could make such a difference. Could you explain it a bit more? Maybe I just have to try it.

Cast,

I don't disagree with any of the advise given so far.

I do have a question. Are you using the crotch strap? IMHO the crotch strap is required most at the surface. Without one people tend to over inflate the wing in a futile effort to rise out of the water.


As far as lift is concerned two values need to be considered. The first is how much lift do you need to float your rig at the surface without you in it? Nice not to have the rig disappear if you ditch it at the surface.

Second is how positive is your wetsuit? Your wetsuit is the only thing losing buoyancy as you decend. I don't mean how much weight is on your belt, I mean how much to just sink your wetsuit.

If you are neutral at the surface with no air in your wing, the only thing the "buoyancy compensator" has to "compensate" for is the loss of buoyancy of your suit. For most people even wearing thick neoprene no more than 25lbs will sink their suit, most would need far less.

If you have to blow up your wing to stay at the surface, you are overweighted.

Do remember that all BCs will lose effective lift because they will fill with water as they are used. If you are still mastering buoyancy you are likely venting and filling more than you will in the future.

Good luck, my guess is that you have plenty of wing.


Regards,



Tobin

 

[cool_hardware52]

Thanks for the reply.
I don't have crotch strap. But I don't quite follow why it could make such a difference. Could you explain it a bit more? Maybe I just have to try it.

Cast,


The reason you can't hold your head high enough is that the wing is just lifting your rig, not you!

With a crotch strap as the rig rises you will also.

It's almost classic to see a fully inflated BC standing a foot out of the water and all you can see is the top of the divers head.

IMHO this is the source of the "Back Inflate BCs will force you on your face" theory.

If you are properly weighted, and using a crotch strap, very little air is needed in your wing at the surface, Just the upper "arc" of the wing will have air in it. This won't force you face forward.

Constrast this with an overweighted diver and no crotch strap.
Blowing the wing up fully in a fruitless effort to raise the diver results in the "face in the water" position as the full lenght of the wing fills.


Hope this helps.



Tobin George

 

[nauidiver2004]

"If you have to blow up your wing to stay at the surface, you are overweighted."


I don't understand, if I am perfectly weighted and I don't add air to my wing to stay at the surface, Every time I exhale I would begin to sink wouldn't I???

 

[RichLockyer]

"If you have to blow up your wing to stay at the surface, you are overweighted."


I don't understand, if I am perfectly weighted and I don't add air to my wing to stay at the surface, Every time I exhale I would begin to sink wouldn't I???

Yes, but you don't have to FULLY inflate the wing to remain at the surface.
Theoretically, perfectly weighted with an empty tank, you should be able to float at the surface with the wing only containing one "lung volume"... that's not much... 2-3 pounds of lift?

Most wings will allow you to inflate them to well over 30% of their capacity before they'll even start to turn you face-down. I dive dry, so I can fully inflate mine, but the suit counters that with lift at the chest. I usually DON'T fully inflate, because if I do, when I release the harness to remove the rig before climbing back into the boat, the bottom will flip up and the reg will smack me in the back of the head

 

[DA Aquamaster]

"Perfectly weighted" will leave you neutral at 10-15' with 500 psi left in the tank. At the surface this normally translates to floating about eyeball level in the water with a normal lung volume and sinking fully underwater when you completely exhale.

Given that the normal sized human head weights about 15 pounds it is going to take minimum of 15 lbs to get your head above water when perectly weighted. Neck, tank(s), tank valve(s), reg(s), and anything else that may be above water start to add to this. A crotch strap certainly helps keep the tank and other hardware from getting a head start on the rest of you, but 25 lbs may not be enough for some people to be comfortable on the surface, particularly in rough water.

At depth you have to be concerned about achieving neutral boyancy at depth. A large and negatively bouyant single tank combined with a 7mm wet suit compressed at depth can create a situation where 25 lbs is again not enough lift.

There is a popular following today for the the idea that having the absolute minimum lift in a BC or wing is a great thing. The idea has merti but not when taken to extreme, and in my opinion the merits are often overstated and the limitations and potential disadvantages of this approach are often ignored.

To make it worse, the usual response to concerns about small wings "if you have a big wing you must not be a skilled or experienced diver" or "if you can't float with a small wing, you must be over weighted" (which is the same thing as calling you inexperienced and unskilled.) This creates a lot of peer pressure for divers to buy smaller wings than may be optimim for them.