Struggling to understand buoyancy and trim wearing my gear

[inquis]

If I "relaxed", I would nose dive completely vertical within seconds.

Being relaxed could contribute to rotating head-down. You need a little tension, especially neck/shoulders, and glutes.

 

[seaway2121]

Everyone has a different body composition, so it's certainly possible you do need that much. What water temperature are you targeting? My 8/7mm is for colder water, up to 65F / 18C, and even for 55F/12C I only wear 5mm boots. So switching to a 5mm suit and boots might stabilize you some. And of course checking for the aluminum/steel tank difference. An aluminum 80 is by far the most common tank you'll see, except in cold water regions, and it looks very similar to a steel high-pressure 100, which is only two inches shorter. A steel 80 is 6 inches shorter than an aluminum 80, and looks notably squat and small, to my eye. I think it would be hard to confuse them. (All have the same diameter.) But in any case they have very different buoyancy characteristics.

About 75 F. We were in a pool for class, but I will be practicing mostly in the ocean for now.

All, I am sorry for some confusion - The steel tank must have been HP100, as it looked very similar to the AL80 I own. It was not noticeably shorter to me.

I also forgot to add, I think we tried adding 4 lbs to a weight belt on the last day..

My instructor was very confident that my head and leg position was the source of the problem.

So I had:
2- piece 7mm suit,
7mm boots
no hood or gloves.
Steel HP100
18 lbs in pockets
4 lbs on belt.
AL neutral W/BP
Neutral fins

 

[seaway2121]

Being relaxed could contribute to rotating head-down. You need a little tension, especially neck/shoulders, and glutes.

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.

I couldn't really even get to this position without falling forward.

 

[Gone for diving]

Move some lead out off the Pockets to the belt, will help a bit,
Get heavier fins, or make it heavier just to test

 

[MichaelMc]

Get a 7 mm hood.

Well, if you're in Florida maybe not. Hood free is much more fun.

 

[Bob DBF]

Just to make the physics a bit more concrete, here is a notional force diagram of a slightly head-heavy diver. Assuming the body itself, and the uninflated plate/wing are neutral, I plotted the various sources of positive and negative buoyancy in red and blue, respectively. The length of the lines is the distance from the (again, very approximate) fulcrum. So the area is roughly proportional to the rotational torque. I can see this setup being head-heavy, with very little body fat in the spot where the weights are, and the steel 80 riding high so you can reach the valve.

Those feet and fins are waaay out there, too. One pound on the fins is worth ten in a waist pocket, torquewise.

View attachment 700080

Nice picture. If the OP has an integrated Jacket, the lead weight is probably towards the head further, rather than where it shows what looks like where a belt should be. From the sounds, moving 6 to 10# onto a weight belt could solve the issue.

 

[MichaelMc]

Best is to play around in the pool for a really detailed and slow look at your weight distribution, and posture.

If you need to shift more lower:
- Heavier fins are likely the most impactful.
- You can put lead on the crotch strap. Sidemount divers often have a rear butt plate and may add a weight to the rear crotch strap over the butt. But be careful that you can still clear and bump a weight belt or your integrated weights.
- A weight belt on the tank bottom.
- You could add gloves, though I'd cut off the finger tips of the main three for dexterity and it is a bit hackish.

Needing that much low is unusual. Mostly people are leg heavy.

You might look at where the lift of your wing is placed. I'm not an expert at that but wings differ in how well they work for different people, or just in general.

It is also possible your whole rig is higher than it needs to be, but if it was a Fundies class likely not.

 

[TravisD]

I had a student last year who had problems initially with feet floating and killing their trim. Switching them to a slightly heaver fin made a huge difference in their trim. It didn't take much.

 

[seaway2121]

UPDATE:

Summary: Slowly chipping off lead. Gaining more balance and control. 14 lbs felt good, next dive will try 12 lbs. Nose-dive issue reducing, feeling better sense of fulcrum with the less weight.

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?)

So I went to my dive shop and he seemed pretty surprised. I then went on three solo dives the past month to practice.

Dive One: 2-piece 7mm and boots, AL80, 16 lbs in pockets, 5 lbs on upper cam-band, and 2 lbs clip to BP.

- I felt heavy on the sand. Needed considerable air in BC, which made me feel out of balance/no control. I removed the 5 lbs and 2 lbs clips, and still stayed down with air in BC at 15 ft. The nose-dive issue was still a problem. With 500 psi and no air in BC, I felt pretty neutral with the full 23 lbs of lead, rising and falling with inhale/exhale.. maybe even a little floaty with normal breathing. Odd.. (breathing rate probably increased as I prepared to test myself).

Dive Two: 2-piece 7mm suit, hood, and boots, Steel LP85, 18 lbs in pockets.

-Felt more control than previous dive, still nose-dive but not as bad. Still negative with some air in BC. Felt better balanced throughout the dive with the LP85 instead of AL80. At 500 psi with no air in BC, sank when exhaled, and sank slowly with normal breathing, but not rapidly.

Dive Three: 2-piece 7mm suit, hood, and boots, LP85, and 14 lbs in pockets.

Originally went down with 18 lbs, then took off 4 at 15 ft. Stayed negative on the bottom, put some air in BC to practice kicks. Felt much more control than previous dives. The nose dive issue gone, except if I started to float and then exhale, I would go head down.

Otherwise with normal breathing, I stayed mostly motionless in the water column, although I still relied on my legs to stay in position. Took off fins, and could stay motionless and balanced with gentle leg motions. I noticed I could feel my fulcrum point much more clearly now, by pushing the harness up or down on my body. Depending where I put it, I became slightly head or leg heavy, or neutral if just right. I could use my arm position to fine tune. At 500 psi and no air in BC I felt almost prefect neutral while normal breathing, maybe slightly negative still. I would eventually be on the sand after a minute.

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.

Thanks.

 

[Robert H. Diver]

UPDATE:

Summary: Slowly chipping off lead. Gaining more balance and control. 14 lbs felt good, next dive will try 12 lbs. Nose-dive issue reducing, feeling better sense of fulcrum with the less weight.

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?)

So I went to my dive shop and he seemed pretty surprised. I then went on three solo dives the past month to practice.

Dive One: 2-piece 7mm and boots, AL80, 16 lbs in pockets, 5 lbs on upper cam-band, and 2 lbs clip to BP.

- I felt heavy on the sand. Needed considerable air in BC, which made me feel out of balance/no control. I removed the 5 lbs and 2 lbs clips, and still stayed down with air in BC at 15 ft. The nose-dive issue was still a problem. With 500 psi and no air in BC, I felt pretty neutral with the full 23 lbs of lead, rising and falling with inhale/exhale.. maybe even a little floaty with normal breathing. Odd.. (breathing rate probably increased as I prepared to test myself).

Dive Two: 2-piece 7mm suit, hood, and boots, Steel LP85, 18 lbs in pockets.

-Felt more control than previous dive, still nose-dive but not as bad. Still negative with some air in BC. Felt better balanced throughout the dive with the LP85 instead of AL80. At 500 psi with no air in BC, sank when exhaled, and sank slowly with normal breathing, but not rapidly.

Dive Three: 2-piece 7mm suit, hood, and boots, LP85, and 14 lbs in pockets.

Originally went down with 18 lbs, then took off 4 at 15 ft. Stayed negative on the bottom, put some air in BC to practice kicks. Felt much more control than previous dives. The nose dive issue gone, except if I started to float and then exhale, I would go head down.

Otherwise with normal breathing, I stayed mostly motionless in the water column, although I still relied on my legs to stay in position. Took off fins, and could stay motionless and balanced with gentle leg motions. I noticed I could feel my fulcrum point much more clearly now, by pushing the harness up or down on my body. Depending where I put it, I became slightly head or leg heavy, or neutral if just right. I could use my arm position to fine tune. At 500 psi and no air in BC I felt almost prefect neutral while normal breathing, maybe slightly negative still. I would eventually be on the sand after a minute.

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.

Thanks.

When I’m descending and ready to level off at whatever depth, I hold my breath for a few seconds. If I keep sinking, I know to add some air to my BC until I stop sinking, then I continue to breathe normally. I do this every time i descend to a different depth. I look like Superman in the water because I stretch my arms all the way out to get into trim. So you are correct, using your arms and your leg position affects your trim. Just keep practicing!