Why do I sink?

[NAM001]

Whether a diver sinks depends on their buoyancy not their weighting, unless they have more weight than their BC can compensate for, in which vase they will be swimming up.

It's an easy guess that a diver is overweighted because the finer points of weighting are usually not covered in an OW class, but whether a diver moves up or down in the water column is dependant on buoyancy control. Overweighting may make it more difficult to maintain buoyancy when shallow, but not impossible.

If it the idea that there is a direct relationship between weight and buoyancy is one reason divers are overweighted to start with. The "if you float you must need more weight" syndrome along with "if you hold are able to maintain depth you are properly weighted". Well I've held a safety stop holding a 15# anchor, not recomending this to anyone, so other than demonstrating risky behavior, does this mean I'm weighted 15# light normally?

In inverting, the OP could be lowering his BC in the water column and making it slightly less buoyant, either by how he inverts and/or by the trim lowering his head and that end of the BC. Tricks like this are not easy and any small factor can cause failure. I agree that the OP should be properly weighted, which may help if he is not already properly weighted.


As far as trim goes, ones weight should be adjusted so one can maintain any position, or just to maintain the position you perfer. Proper trim is more a personal preference rather than a law.

Bob

Just for clarification for others heavy overall is a buoyancy issue, heavy front or back/ side to side is trim. deal with overall heavy or light first and then move weight around to deal with trim. Like real estate trim is all about location location location

 

[wetb4igetinthewater]

Just for clarification for others heavy overall is a buoyancy issue, heavy front or back/ side to side is trim. deal with overall heavy or light first and then move weight around to deal with trim. Like real estate trim is all about location location location

I'm stealing that phrase for when teaching/helping others with weighting.

 

[NAM001]

He could also be having problems with a running/moving bubble from a too large wing. dipping head raises the but and the wing air moves to the hips. Either way he has to get neutral to eliminate that from the problem first off.

 

[NAM001]

I'm stealing that phrase for when teaching/helping others with weighting.

My apologies. the term probably is not the norm It came from the submarine community . we trim the ship the same way. We ballast to and from sea to compensate for ships over all light or heavy condition. one we can hold depth we move water around the ship to ballance the teeter totter. we use terms like heavy or light overall or heavy or light forward or aft. light overall and light aft means take in sea water and put it aft.

 

[Bob DBF]

Just for clarification for others heavy overall is a buoyancy issue...

I agree, however it may not be a weighting issue. I can have too much or too little ballast on my weight belt, and at 100' begin to sink. Because I sink I feel heavy, it is because I am negatively buoyant, not necessarily because of the ballast I carry.

With a submarine it is a straightforward weight problem because the volume of the boat is a constant. So the weight of the water is removed replaced by the airspace it was holding. Weight to volume determines buoyancy, in this case weight is changed to maintain neutral buoyancy.

With a diver the BC changes the volume of the diver so the ballast weight becomes less relevent within the adjustment range of the BC. If you pick up a 5# weight off the bottom, you adjust the BC volume to maintain neutral buoyancy. In this case a diver changes his volume to maintain neutral buoyancy.



Bob

 

[NAM001]

I agree, however it may not be a weighting issue. I can have too much or too little ballast on my weight belt, and at 100' begin to sink. Because I sink I feel heavy, it is because I am negatively buoyant, not necessarily because of the ballast I carry.

With a submarine it is a straightforward weight problem because the volume of the boat is a constant. So the weight of the water is removed replaced by the airspace it was holding. Weight to volume determines buoyancy, in this case weight is changed to maintain neutral buoyancy.

With a diver the BC changes the volume of the diver so the ballast weight becomes less relevent within the adjustment range of the BC. If you pick up a 5# weight off the bottom, you adjust the BC volume to maintain neutral buoyancy. In this case a diver changes his volume to maintain neutral buoyancy.



Bob

YES all true that Is why I sdaid do the buoyancy part at 20 ft 500 psi etc to avoid those other aspects. then when you are heavy it is not weighting from the static weighting it is the dynnamic weighting like suit or air in tank etc.

 

[IDNeon357]

That's not much for up here in cold water, he lives in NY so 14 pounds with a AL80 in a 7mm or drysuit doesn't seem like a lot to me.

I wear 36 lbs, and that's no joke. It's also not noobish, it's well tested and correct for my 7mm two-piece with AL80.

I still don't know why I need so much weight...since 10-20 seems common.

I'm 200lbs solid muscle but I thought density would make me need less weight, not more weight? My idea is that because I'm so dense, proportionally, 7mm and BCD adds a lot more volume to me than it does a more rotund person...

But that doesn't make sense because a more rotund person should always be more positively buoyant anyway, and on top of that, the weight displaced by the diving-dress would in theory be rather the same regardless. (same gear, same volume that gear displaces)

I haven't given it much thought to solve this mystery.

 

[Esprise Me]

Who dives with 10 lbs. in a 7mm? At least in the ocean? I'd be very curious to meet this person and see what they're made of.

I need about 20 lbs. with a 7mm (ETA: one-piece, with separate skirted hood) and an AL-80. With a steel 100, I can do 16 lbs, and with a steel backplate and weighted STA, that leaves just 4 lbs. of lead I need for salt water, which I can leave behind if I'm in a pool. If I'm down to 500 psi at my safety stop, my BC is empty and I'm neutral. I'm a bit fluffier than I used to be but nowhere near overweight.

 

[IDNeon357]

On-topic.

Is all buoyancy equal?

When you're horizontal you are like a wafer, spread out on a large waterbed...

When you are tucked into a buddha position you are like an iceberg.

So naturally you will sink down more, until only 10% of your mass is sticking above that neutral buoyancy line (just like 10% of a bulky iceberg sticks above the neutral buoyancy line...the surface).

10% of a horizontal pencil appears much less "deep" than 10% of a vertical pencil.

Now if you continue to sink is going to be due to how much does that 2 to 3 foot difference affect your postive/negative balance?

 

[IDNeon357]

Who dives with 10 lbs. in a 7mm? At least in the ocean? I'd be very curious to meet this person and see what they're made of.

I need about 20 lbs. with a 7mm and an AL-80. With a steel 100, I can do 16 lbs, and with a steel backplate and weighted STA, that leaves just 4 lbs. of lead I need for salt water, which I can leave behind if I'm in a pool. If I'm down to 500 psi at my safety stop, my BC is empty and I'm neutral. I'm a bit fluffier than I used to be but nowhere near overweight.

Strangely half the lobster divers I was with this last trip all had about that weight. Maybe 15 at most.

Not trying to get into that weeds though, it's too highly variable between person to person.

Going off me and you, you need about 20 lbs, but I still need about 36 lbs. I'm not sure why, I am curious, but it's a little off-topic though.

The way you explain why you need the weights you do helps, but all I can say is I have a BCD no steel backplate, an AL80, a two piece with hood 7mm with 5mm gloves/boots.

I'm 200lbs right now, and maybe 12-15% bodyfat.

So other than that info, at this time I just can't tell the physics of why I needed that weight.

I want to know more, for improvement of skill reasons. The diver experience was that the trim was good, the buoyancy was responsive, and I felt I had little issues...