Do you need ditchable weight?

[James79]

If you are 8 lb negative when you get in and you hit 100' with a 7 mil, you are going to be more than 8 lb negative.

Fair enough... I only use a 3 mil, and my numbers are less than that. Others have addressed more accurate numbers since my post.

I'll bow out in shame now, lol.

 

[johndiver999]

@johndiver999, I had hoped the "on the bottom" (not sinking) and "with air/time" would be sufficient to suggest something less urgent than you seem to have in mind. One person's emergency may be another's minor irritation.

I'm not sure what I've done to earn your ire, but all I said was that it is an option. The reader should assess their own skill/knowledge when selecting an approach, preferably practiced/simulated in advance. I believe they still teach gear removal in OW classes, don't they?

It sounds like you're saying one would be ripped away the instant you doff it. Remember, your suit has very little lift due to compression... You know, the root cause of the unbalanced rig and inability to swim up.

Hardly, the scenario I had in mind is separating the elbow or inflator hose. Invert it and it's a functional BC (orally inflated), easily offsetting the lead and trivial to ascend at a leisurely pace. Of course hook your arms through the straps, but my kid's school bag weighs more than that rig under water. Again, options (maybe you forgot your DSMB that day), and yes *I* would invert the BC in that situation before ditching weight. I'll just end with YMMV.

You've not upset me, but your idea of scuba unit removal (when all ballast is on the tank BC) inversion of the BC and then manually filling and then riding it up sounds way too complicated as a means to handle a broken OP spring, for example.

 

[RobPNW]

Came across this thread just as I was typing a new thread to ask how people like to distribute between trim and ditchable weight. I was hoping to be able to put most in or around a plate or cam bands and leave about 2x2lbs in QR pockets on my waist webbing. Sounds like that is not unreasonable. I like the idea of always carrying an SMB as additional buoyancy at the surface. Thanks.

 

[Nemrod]

All the calculations aside, can you swim up without ditching weight and can you maintain on the surface easily without ditching weight? Considering a total buoyancy failure or oddity, if such were to happen, can you roll back on your dead wing and get your head out of the water while waiting for pickup, or at least stay on top with a snorkel (if you have a roll up)? For a drysuit or heavy exposure neoprene it might be such that the rig cannot be swam up. For 3mm to 5mm suits, I can swim that up without ditching any weight, however, staying on top without ditching is another thing.

A DSMB is not a complete solution. If you transfer the air in your lungs to the dsmb your bouyancy has not changed. As near impossible as a total failure of your regulator might be or having jumped in with the valve closed or other improbability, how are you going to establish positive buoyancy (sufficient to ascend)? The DSMB will assist on the surface, if you can blow it once there.

James

 

[Steelyeyes]

You have to consider that you may be able to get to the surface but have limited physical capacity. Ditching weights is the fastest, least complicated way to increase your odds of staying on the surface until you can be rescued. An SMB is a great signaling device.

The only deep water rescue I have participated in was such a scenario. The injured person was a lot easier to deal with when he had more buoyancy. Once in a while he reminds me of that day. I'm pretty sure he appreciated the effort since he was probably going to be a fatality without our help.

 

[johndiver999]

Came across this thread just as I was typing a new thread to ask how people like to distribute between trim and ditchable weight. I was hoping to be able to put most in or around a plate or cam bands and leave about 2x2lbs in QR pockets on my waist webbing. Sounds like that is not unreasonable. I like the idea of always carrying an SMB as additional buoyancy at the surface. Thanks.

Perhaps having 4 lbs of ditchable lead is reasonable, but it depends on a lot of factors - primarily the exposure suit. How have YOU determined that dropping just 4 lbs is going to work?

 

[inquis]

how people like to distribute between trim and ditchable weight.

Much depends on how much air I'm carrying. Should my wing fail completely with an AL80 tank, I would drop 5 lbs (if this happened at the beginning of the dive) or 2-3 lbs (if mid-dive). This leaves me about neutral at 15 ft, and I prefer to make a normal ascent with a safety stop. This is not a coincidence, as I've weighted myself with this in mind.

Why 5 lbs? Because that is the weight of the air I haven't yet breathed from that AL80 tank (full -> 500 psi).

Everything else I consider to be trim/non-ditchable. In my "recreational vacation" setup (salt, 3mm, AL80 tank, AL backplate), I need 12 lbs total to be neutral at the stop on a normal dive (at reserve pressure with empty wing). This, less the 5 lb I want ditchable, gives 7 lbs available for trim. I've determined that putting it all on the upper tank band removes the tendency to rotate when horizontal.

Thus, I carry 2&3 lb bricks on the waist in velcro pouches ("quick enough" to release) and 3&4 lb on the upper cam band (achieving stable trim when horizontal). Putting 6 lbs on a side keeps me from rolling as well.

I've done the analysis to figure out I am 9.5 lbs negatively buoyant worst case at depth (full tank, compressed wetsuit). After dropping 5 lbs, I only have to swim up a max of 4.5 lbs, which is within my lung capacity as well. I'm also only about 2 lbs positive at the surface, so I know I won't "cork" when ascending from the stop.

As an aside, I have done similar analysis for a 7mm suit I recently dove. (I took the time to measure its buoyancy before the dive, because doing so allowed me to calculate the extra weight I would need.) In this suit, I would still only drop 5 lbs to be neutral at the SS if my wing failed. The main differences are in the initial swim up from depth, which would be about 8 lbs (negative), and the need to swim downward during the final ascent to counteract the positive buoyancy at the surface (coincidentally also 5 lbs). This analysis also showed me that I would be basically neutral at the beginning of the dive on the surface, so starting the dive would mean flooding my suit (always fun in 7mm temps!), fully exhaling, and possibly duck-diving to get started down. All these consequences I deemed acceptable and therefore made the dive in this suit.

A warning, though: some wetsuits may have so much buoyancy loss due to compression at depth that you cannot swim them up, even after dropping weight to make you neutral at the stop. In such cases, my first suggestions would be to use a drysuit instead or don't dive that deep. However, if those are not options, I suggest you carry redundant buoyancy or work out how much additional weight you'll need to make ditchable to get off the bottom. In the latter case, also work out whether you can kick downward hard enough to avoid an unsafe ascent rate once the suit regains buoyancy. Not drowning is a good thing, but not drowning AND not bent is better.

Ease of initial descent, ease of leaving the bottom, ease of holding a safety stop, and ease of avoiding an unsafe ascent rate are all affected by the weight carried and portion you've elected to ditch should your wing fail. Find your sweet spot. You can explore such tradeoffs using @rsingler's weight analysis tool mentioned upthread. (I highly recommend you measure your suit buoyancy, as that's the largest uncertainty in my view.)

 

[inquis]

FYI, a current link to the Optimal Buoyancy tool in the Knowledge Base section.

 

[johndiver999]

....
As an aside, I have done similar analysis for a 7mm suit I recently dove. (I took the time to measure its buoyancy before the dive, because doing so allowed me to calculate the extra weight I would need.) In this suit, I would still only drop 5 lbs to be neutral at the SS if my wing failed. The main differences are in the initial swim up from depth, which would be about 8 lbs (negative), and the need to swim downward during the final ascent to counteract the positive buoyancy at the surface (coincidentally also 5 lbs). ....

I've played around with this stuff in the water on my own quite a bit. One thing you should remember is that if you are wearing a thick suit and accidentally (or purposely) drop lead, you still may be perfectly capable of making a safe relaxing safety stop. You just might have to do it at 25 or 28 feet rather than 15 or 20.

Doing the stop that much deeper will keep your suit pretty compressed and you may well be able to do a comfortable and relaxed stop. If you have the air and circumstances permit, once you complete the (somewhat deeper ) safety stop, then you should be relaxed and you can ascend with low lung volume and minimize your excess buoyancy and ascent rate by simply using lung control.

Plus, upon completion of a nice long safety stop, I would not be so worried about a slightly faster final ascent rate. It might be better and easier to just flair out, lay on your back and spread eagle, and look up and exhale for the final 10-12 feet rather than exerting yourself and trying to kick down. That has been my experience anyway.

 

[Steve Lawson]

When I first started, I had a traditional waist belt and ditched it twice due to dumb new diver errors. One time my buckle caught on the anchor line during deco and I lost my belt. I had to wrap my leg around the line to stay in place during the rest of the deco. Now, my weights are fixed and I can't ditch them, however, I typically dive on deeper sites (100-200') with a drysuit. So even if my BC failed, I could use my drysuit as positive buoyancy.

I always wondered about ditching my rig or taking it off until I lost a friend that tried it. He was a hell of good and strong diver diving solo at 130'. No one knows exactly what happened, but it's believed his tank or rig got caught on some line (he was recovering lost anchors) and he took off his rig. His BC and tanks got away from him. His tanks were found on the surface and he was found on the bottom with his weight belt still on.