Struggling to understand buoyancy and trim wearing my gear

[rongoodman]

He said my arms should be relaxed so my shoulder blades are relaxed. This way I can move my head and keep it up. He also said my legs should be straight and bent at the knee.

You still need to feel a little tension in your glutes in order to keep things in a straight line from shoulders to knees.

 

[Bob DBF]

. I think what I still don't understand is how much air should be in my BC at start of dive, at 15 ft depth? Even with 14 lbs

Depends.

On the first dive, I have to do a surface (duck) dive with an empty BC because of trapped air in my wetsuit, and in some cases can hover at 15' without adding air.

 

[rongoodman]

Next dive I will try 12 lbs. I think what I still don't understand is how much air should be in my BC at start of dive, at 15 ft depth? Even with 14 lbs, I felt like I was on the sand until I added about 3 small pumps to the BC.

At the beginning of the dive you're carrying about 6 pounds of gas, so you'll need enough air in your wing to compensate for that, adding however much additional you need as all that neoprene compresses on the way down. If you're too light, at the end of the dive when you've used up most of that gas, you'll have trouble holding your safety stop. When you're hanging at your safety stop you can go a little head down and reach around to feel the bottom of the wing. If there's more than a couple of fists full of air, drop a pound or so and check again on the next dive.

 

[Bob DBF]

A comment on switching tanks, check the empty buoyancy over both tanks (AL 80 +4#)(LP85 0#) and in this case you can drop 4# going to the 85 since the tank is less buoyant. Or you could just dive knowing you were only 4# overweighted.

SCUBA Cylinder Specifications – Huron Scuba, Snorkel & Adventure Travel Inc. PADI 5 star IDC in Ann Arbor, MI

 

[Gone for diving]

After this original post, I took my 2-piece 7mm suit and 20lbs of lead to the beach. I found the 20lbs was not enough to sink me at the surface wearing the 14mm, fully exhaled (why?)

What can happen, that I have found is the suit traps air either between you and the suit or in the lining, when you first go in to the water, and hasn't absorbed and removed all the air, sometimes you can feel floaty when you start the dive, but you are fine at the end of the dive,

 

[Bob DBF]

After this original post, I took my 2-piece 7mm suit and 20lbs of lead to the beach. I found the 20lbs was not enough to sink me at the surface wearing the 14mm, fully exhaled (why?)

You may need more. I need more in an XL 7mm farmer John when I freedive. It depends on the ammount of neoprene, how new it is, and your body type. Freediving is a good way to sort your buoyancy out, because you don't have any of the scuba gear giving other variables to worry about.

 

[inquis]

I still don't understand is how much air should be in my BC at start of dive, at 15 ft depth

At the beginning of the dive you shouldn't need much in your BC at 15 ft. You should be negatively buoyant by the amount of gas you haven't yet breathed (because the goal is to be neutral with an empty BC when it's gone). For your tanks: AL80/3000 psi = 5 lb, LP85/2640 psi = 5 lb, or LP85/3000 psi = 6 lb). That requires 2.3-2.7 liters, so slightly more than a large soda bottle. If you have a 30 lb wing, it would be less than 20% full. (It will probably be even less than that because of residual air trapped in your suit.)

 

[seaway2121]

You still need to feel a little tension in your glutes in order to keep things in a straight line from shoulders to knees.

Should there be any arch in the back? Are my knees and shoulders at the same level?