Do you need ditchable weight?

[lairdb]

I like your setup. What size weights will go in those pockets and who makes them?

Those appear to be DGX pockets, which say 2, 3, or 4; hard or soft. I have two of the similar made by XS on my upper cam bad; those are happy to take up to 5, but they only take straps in one direction, so they'd hang sideways on vertical straps.

 

[johndiver999]

It might be instructional to see how much you can output in a pool without a wetsuit (assuming roughly neutral buoyancy without a suit). Have a buddy hand you weight while you fin hard enough to allow you to breath. Drop the weight when you can't keep your mouth above water.

Then reflect upon that is the amount of weight where you make ZERO progress, so you'll want to back off that threshold a bit.

So you think that a diver might be able to swim up possibly 15 lbs pf excess ballast in an emergency from the bottom to the top? And how much excess buoyancy do you think a diver can swim DOWN for a short duration of maybe 60 seconds for the last 30 ft of ascent - in an emergency?

It seems to me that it is very common for people to assume they can swim up an awful lot of lead, but at the same time (and for some unknown reason) assume that if a diver is 4 or 7 lbs bouyant, that this is going to necessarily cause an uncontrolled ascent. Have these divers ever practiced flairing out and spreading eagle (facing up) on an ascent? Have these people ever carefully considered the physics of the situation and understood that dropping a modest amount of weight at depth is going to keep the diver negative? Do they understand that they may still have to SWIM up until such time as they reach a neutral buoyancy depth? Have they considered that the diver can then rest and relax at this neutral depth (possibly around 40 or 20 feet) and then prepare mentally and physically for the final portion of the ascent with a decreased lung volume for just 30 or 60 seconds and flaring out or kicking gently downward? Have they considered that the diver does not even have to counter-act all the excess buoyancy, but rather a modest amount of it so that their velocity (ascent rate) is moderate?

Dropping more than the "air weight" is not a rocket trip to the surface.

All these ideas are predicated on the OP's scenario of wearing a buoyant 7 mm suit.

 

[inquis]

Have they considered that the diver can then rest and relax at this neutral depth (possibly around 40 or 20 feet)

[EDIT: Please disregard the remainder of this paragraph, totally incorrect. Not sure what I was thinking when I wrote that -- sorry! The following paragraphs still apply.]
You're making zero sense. They should be neutral at 15 ft or less in a normal situation. Dropping weight doesn't make the neutral point deeper!

Look, dropping 1 lb more than the air carried clearly isn't a rocket ship. However, 25 lbs is what I'd need in a 7mm suit. Ditching all my weight -- as "training materials" advise & new divers are apt to do -- certainly will be. I'd be roughly 35 lbs buoyant at the surface if normally neutral at 15 ft.

Honestly, just carry a suitable DSMB and you don't have to drop any weight at all.

 

[lairdb]

To add some reference material, helpfully gathered by others:

Do you care about ditchable weights?
Number one cause of diving fatalities?
The case against ditchable weight
Let's discuss dumpable vs non-dumpable weights
Buoyant ascents
How much negative buoyancy can you swim up with?

As a large number of long threads have documented (Have you ever had to dump your weights? , weights advice please , Formula for estimating wing size? ), there is great interest in the fine points of buoyancy. The issue of beginning divers being significantly overweighted has occupied many discussions. The notion of diving a “balanced rig” has engendered long discussion (Question about “balanced rigs” and having all ballast unditchable ), with definitive comments made on both sides of an argument for and against releasable weight.

“Neutral buoyancy” is also discussed at length. Add Additional Weight For Shallow Dive? , Perfecting my buoyancy

 

[-JD-]

They will fit 4lb Sea Pearls coated weights. In the configuration I have, 3lb Sea Pearls is the max due to interference from the tank .

Dive Gear Express: DGX Gears Trim Weight Pocket
FWIW some folks have expressed concern about the fact that the weight is only retained by (relatively small) velcro. For my purpose (pockets are generally only up or flat, covered by the wing, and typically pinched somewhat by the tank) I'm not overly worried.

This shows more details: Freedom Plate Divers Roll Call: show us your rig!

ETA: I'd suggest checking out this thread too: Adding weights to the back of backplate setup

 

[lowwall]

I had used +8 for the 7mm above, and noted that it was an important factor. Found some sources I trust more that suggest as much as +24 for a new full 7mm, and another 5 for booties, hood, etc.

The formulas hold, but now our hapless diver has +24 +3 of buoyancy at the stop and needs -27 of lead. The 5ATM failure is +2.7 -2 -27 = -26. Ouch.

That's not how you calculate it. Assuming proper weighting, the only weight you are concerned about is the difference between the weight that results in neutral buoyancy at the safety stop with an empty wing and the point of maximum negative buoyancy (i.e. full tank at deepest point).

So 5lbs for the gas (Al80). Using your 24lbs surface buoyancy for the suit, we need to look at the difference in bouyancy between 5m and 35m. Assuming all the 24lbs is in trapped air bubbles, then it will provide 16lbs of lift at 5m and 5.33lbs at 35m (let's call it 5). So worst case is 5+(16-5) = 16lbs negative

With the OPs 4lbs ditchable, he's got 12 pounds to swim up at the beginning. Of course this number will get smaller as you ascend and the air in the neoprene expands. But, yes, it would be a good idea to do some combination of additional ditchable or a DSMB.

 

[inquis]

@lowwall for a 7mm, one might choose to be neutral at a shallower depth to limit the buoyancy change between there and the surface. @rsingler's weight analysis tool (stickied in the Advanced Forum) indicates this could easily be more than 10 lbs from 15 ft. Rather than ascend every dive upside down kicking to limit ascent rate, a little more weight makes that expansion a little more controllable. This could add another 5 lb or more to the buoyancy difference you mentioned that has to be overcome. This only further underscores the wisdom of redundant lift for such cases.

 

[johndiver999]

Wisdom of redundant lift... as in an smb? Do we really think a very distressed (normal recreational diver) who has had a catastrophic BC failure and now finds themselves quite heavy at depth (and we assume alone I guess, because we have never talked about having a buddy help in the scenario) is going to be able to deploy, inflate and manage the expanding air in the smb while holding it during a true emergency?

Possibly we might think that... I'm not sure it is such an obviously superior solution to simply dropping enough lead to allow themselves to swim up. Dropping 4 or 6 or 10 lbs SEEMS a lot less involved and afterwards requires the management of a lot less moving parts.. as opposed to an smb for example.

There are detailed and quantitative analyses in the previously mentioned thread that (I think) address these issues well - if someone cares to actually read the material.

 

[lowwall]

@lowwall for a 7mm, one might choose to be neutral at a shallower depth to limit the buoyancy change between there and the surface. @rsingler's weight analysis tool (stickied in the Advanced Forum) indicates this could easily be more than 10 lbs from 15 ft. Rather than ascend every dive upside down kicking to limit ascent rate, a little more weight makes that expansion a little more controllable. This could add another 5 lb or more to the buoyancy difference you mentioned that has to be overcome. This only further underscores the wisdom of redundant lift for such cases.

Agreed. My calculations showed 8lbs difference in bouyancy between the surface and 5m. I like to take that last ascent from the safety stop to the surface really slow, fighting several pounds of lift would make that tough.

So yes on the need for ditchable weight or a source of redundant buoyancy in a 7mm. Personally, I'll stick with my drysuit.

 

[lowwall]

This leads to the interesting question of how much weight a diver can comfortably swim up. According to this paper, their test divers "could generate the swimming thrust force within 64 N continuously for 8 min in a steady state." 64 Newtons = approximately 14 bs of thrust. Based on this, I'd think 10-12 pounds would be a good starting point for a swim-up limit for a reasonably fit and able swimmer. Remember you aren't going to need 8 minutes to surface and the thrust needed will drop as you ascend if you are wearing any neoprene. Adjust for your situation.

If you are curious about the upper limit. Another paper - Evaluation of fins used in underwater swimming - PubMed - comparing fin characteristics found the average maximum thrust their test divers (all young dive pros) could generate over a 20 second trial was 157 Newtons (around 35 lbs).

Edit: this matches up nicely with the results of the few people that responded as requested in this old thread: How much weight can YOU swim up? . For example:

Every winter I do a "scuba tune-up" mentoring session. One thing it includes is swimming up a variety of weight belts in the deep end of a pool. Because I've tried it in a lot of configurations, I know that I can swim up:

• 5 lbs is easy. Not even noticed. Yawn.
• 10 lbs is just a strong kick.
• 15 lbs is a strong kick and fast cadence. It's work but not difficult.
• 20 lbs is difficult with a whopping RMV. Doable but we're talking a RMV of 3 or more.
• 30 lbs, I can barely do this, but only by executing short, 12' sprints.