Fixing diver trim and weight placement

[inquis]

I totally get that but if I go horizontal and motionless and my head rises you say "shift weight toward the end that rises" so I should move weight toward my head. Really!?

Yes, raise some weight in that particular case. The fact your head rises when horizontal indicates you're head-light / foot-heavy.

I suspect your "Really!?" comment is sarcasm. (My apologies if I've misread.) What you're assuming/described is the intuitive situation, and the obvious solution is the correct one. OTOH, the OP described the opposite scenario, and the obvious path of "stable with feet low = raise weight" would be exactly backwards. The horizontal test conveniently resolves that ambiguity.

 

[crofrog]

The horizontal test conveniently resolves that ambiguity.

It's funny how even this is controversial...

 

[inquis]

It's funny how even this is controversial...

One of the funniest things I've ever seen was a buddy in doubles going on about strapping weights to the tank valve or wearing a freediver neck weight to correct his seahorse trim. He finally let me hold him gently in my outstretched arms and orient him horizontal while he was neutrally buoyant. When I dropped my hands, he was like a bucking bronco -- absolutely galloping around in a circle, kicking for all he was worth and flapping his arms to avoid going ass over tea kettle. Sometimes awesome pictures are just not enough to explain physics.

 

[JoeTPhilly]

These two videos from a GUE instructor may be of interest:

 

[wdeutsch]

OK. After reading through all 5 pages, I think I have the gist of this.

My situation: Steel backplate, 3mm wetsuit, 4# lead in waistbelt mounted pockets. AL80 tank at around 2800 PSI. When I get horizontal and stop moving, my feet drop. Edited to add: This is in a freshwater/pool situation, hence the small amount of lead being used.

So, based on what I understand here, I should move weight towards my head. So, tighten shoulder straps, loosen crotch strap to get plate higher on my back.

So far, so good?

However, one followup question: Do I move my entire backplate assembly, including tank? Or, backplate moves toward head and I shift tank position so it stays in the same relative position? Is it worth getting my lead off my waist and putting it up higher? (It's my only droppable weight.)

I ask that because I've read here that AL80 tanks should be thought of as bouyancy points, with the lift at the bottom of the tank, rather than weight. OTOH, when full AL80s are definitely negative. So... I'm a bit confused.

 

[inquis]

I've read here that AL80 tanks should be thought of as bouyancy points, with the lift at the bottom of the tank, rather than weight.

Its less of a buoyancy point than your wetsuit. I say don't go down that rabbit hole of thought.

I would leave the backplate in the "normal" spot (can barely reach the top) with arms loose enough to easily enter/exit (at least a fist of space from chest). Position the tank where you can reach the valve while diving but doesn't substantially impede tipping your head back. (Don't evaluate these when you're standing on dry land.)

If your feet still rotate down, hold some of your lead in your hands near your chest to evaluate. If that fixes the rotation, then move that much to trim pockets on the upper tank band. I would guess 4 lbs to shift at most.

 

[Comm]

Do I move my entire backplate assembly, including tank?

Id recommend getting some cam band weight pockets. 4#'s isnt a ton of weight and so shifting 2 of the 4#'s from the waist to the top cam band strap is the quickest way to do it. That way you dont need to fumble around with the current plate setup

 

[wdeutsch]

@Comm and @inquis - if I move my 4# to tank trim pockets, that's all my weights and then I have no droppable weight. I'm led to believe that's a bad idea?

Edit to add: This is in fresh water (pool), so I guess in an saltwater situation that may not be an issue?

 

[inquis]

Salt water will have to add ~5 lbs to freshwater values (2.4% of your weight+gear+tank weight) because of the higher density.

Also, when properly weighted, you would be neutral at the safety stop with empty wing at reserve pressure. The only difference before that is the weight of the non-reserve gas and wetsuit compression loss. In an AL80, that's about 5 lbs (and less as the dive progresses) and a 3mm suit is about 3 lbs lost (and less as you ascend). You can likely ascend without even kicking if you wanted on lungs alone.

The need for ditchable weight is much greater when a diver is severely overweighted.

 

[JoeTPhilly]

This may help: