Do you need ditchable weight?

[James79]

Maybe just keeping 4# total in ditchable weight pockets in harness belt is the right amount to ditch then swim the rig up and safely surface. At the surface dump the other weight from the upper cam trim pockets via ditchable pockets or remove rig to access trim pockets to dump weights if needed

Part of what was being addressed with the "balanced rig) approach, is that with proper weighting in a recreational environment you shouldn't be more than about 8 lb negative even at the beginning of the dive at depth. And you can swim up 8 lbs (which will get less as the suit decompresses and you use up air) and then ditch weight at the surface to attain positive buoyancy. If you ditch at depth, you are committed..... no getting it back and you run the risk of a rapid uncontrolled ascent.

Respectfully,

James

 

[Curious_George]

Everyone with an at all analytical thought process should spend some time with this tool: Optimal Buoyancy Computer

@James79 just saw you had already posted the same link as I did on the other buoyancy thread. I’m amazed at how many people try to re-explain something that is perfectly described on this awesome thread. Well done with the recommendation.

I say analytical because there is a ton of detail and it’s a fun tool if you like this kind of thing.

I’ve watched these free trainings probably 20 times each as well. There’s one about buoyancy that made my jaw drop first time I watched it. Free Training | 6 h 04 min split over 45 videos | sidemounting.com

 

[lairdb]

I’ve watched these free trainings probably 20 times each as well. There’s one about buoyancy that made my jaw drop first time I watched it. Free Training | 6 h 04 min split over 45 videos | sidemounting.com

Not to take anything from the rest of them, but I'm guessing this is the particular video @Curious_George means: BCD Failure : Single Cylinder | sidemounting.com

(Despite the website domain, this particular segment was shot with backmount, so no potential confusion about whether it's applicable.)

 

[Divin'Papaw]

I dive with zero ditchable weight and have for years.

 

[-JD-]

How about attacking your trim balance with some tweaking ...

Background:
I'm foot heavy in a wetsuit and have been working on this for a while myself.
I dive with HP 100/120s on a thin SS Freedom plate and mostly a 7mm or DS.
Until last year, I used a HOG 32 wing. Now I dive with a SMS100 wing which is a longer hybrid singles/sidemount wing (shifting the center of lift down) and has a redundant bladder answering the redundant buoyancy with a wetsuit, even though I have worked it out to have about 8-10 lbs on the waist/ ditchable in my configuration.
In that configuration, I am diving with either Deep 6 Eddy fins or Force Fins which are neutralish.

When diving the 7mm:
- What type of tanks?
- What fins?

Without changing the equipmentt, try adjusting your center of mass, and lift.
Raising the tank as far as you can tolerate should help to shift the center of mass closer to your head.
Lowering the wing as much as possible will shift the center of lift towards your feet.
That may help you to move weight from the upper cam band to the waist pockets.
You can also move the trim pockets to the shoulders of your BP/W from the cambands, that will increase the lever arm of the trim weights, allowing you to reduce the weights there, and further shift weight to the waist.

 

[Centrals]

Weight belt from day 1.
Adjust the weight according to the thickness of the wet suit or BP material used (Al or SS). I do not use S tank or anything larger than 11L Al nor dry suit in SE Asia.

 

[FishWatcher747]

How about attacking your trim balance with some tweaking ...

Background:
I'm foot heavy in a wetsuit and have been working on this for a while myself.
I dive with HP 100/120s on a thin SS Freedom plate and mostly a 7mm or DS.
Until last year, I used a HOG 32 wing. Now I dive with a SMS100 wing which is a longer hybrid singles/sidemount wing (shifting the center of lift down) and has a redundant bladder answering the redundant buoyancy with a wetsuit, even though I have worked it out to have about 8-10 lbs on the waist/ ditchable in my configuration.
In that configuration, I am diving with either Deep 6 Eddy fins or Force Fins which are neutralish.

When diving the 7mm:
- What type of tanks?
- What fins?

Without changing the equipmentt, try adjusting your center of mass, and lift.
Raising the tank as far as you can tolerate should help to shift the center of mass closer to your head.
Lowering the wing as much as possible will shift the center of lift towards your feet.
That may help you to move weight from the upper cam band to the waist pockets.
You can also move the trim pockets to the shoulders of your BP/W from the cambands, that will increase the lever arm of the trim weights, allowing you to reduce the weights there, and further shift weight to the waist.

I see that there are many more options that I have not tried so far. Without getting any more gear for now the SS backplate and 32# wing seem to have solved the problem of foot heavy. I have slightly positive Scubapro Nova Wing fins. I think I'll leave the 2# in each waist ditchable trim pocket and 8# in each upper cam band trim pocket and call it good with my 7mm wetsuit configuration. When I go to a warm water configuration I may not have any weight on the harness belt but add a ditchable trim pocket to upper cam band on left to ditch 3# if needed. I should only need 3# in each upper cam pocket with that configuration.

I don't think I will do the shoulder strap weight trim pockets as the shoulder areas are pretty busy already.

I'll be doing a lot of vacation diving renting AL 80s so I don't really want to do the steel tank thing yet. I already have 2 AL 80s of my own.

I always carry an SMB to another responder.

I also started carrying my AL 19 pony on all dives even 30 foot reef dives. Guides do treat you different if you carry a pony and have good buoyancy and trim. If your group surfaces and you want to hang under the boat till all the other people surface nobody cares and you can drop down at the beginning of the dive for 5 minutes before your group assembles.

 

[Centrals]

I also started carrying my AL 19 pony on all dives even 30 foot reef dives. Guides do treat you different if you carry a pony and have good buoyancy and trim. If your group surfaces and you want to hang under the boat till all the other people surface nobody cares and you can drop down at the beginning of the dive for 5 minutes before your group assembles.

Surface and submerge together with the group or buddy.
Too many things could had happened in those two stages! And I have seen a few.
I won't do that personally.
As for me that extra gas is for everyone who might need it.
And I also believe in dive plan among the group or buddy.

 

[inquis]

You are running into the unfortunate consequence of a 7mm wetsuit. You need whatever weight you need to be neutral at the end of the dive (reserve pressure) at the safety stop with empty wing. At all other times of the dive, the wing has to compensate -- the worst case is max depth with a full tank. You have a) 5 lb of air that you won't later (assuming an 80 cf tank) and b) have lost a substantial amount of wetsuit buoyancy at depth that you will have later (at the SS or surface).

Sure, drop enough weight to compensate for (a) -- 5 lbs for a full AL80 -- but more than that leads to a rocket ship as the suit regains buoyancy. If that a&b combination is more than you can swim off the bottom in the event of a catastrophic wing failure, then you simply must have redundant lift. Typically, a drysuit solves this problem very well. However, a lift bag or larger DSMB is also effective. A redundant bladder perhaps, though that's not without its own drawbacks. A buddy with sufficient wing capacity for both of you as a very last resort.

Yes, bent is better than drowned, but with a little planning you should avoid both.

That said, dumping weight to stay afloat after surfacing might be useful. However, whatever supplementary lift that allowed you to get off the bottom should more than handle the surface need after you have regained your suit buoyancy.

So no, my feeling is that you should not ditch more than an amount equal to the non-reserve air you have left. A 7mm wetsuit WILL mandate that you carry supplementary lift.

 

[FishWatcher747]

You are running into the unfortunate consequence of a 7mm wetsuit. You need whatever weight you need to be neutral at the end of the dive (reserve pressure) at the safety stop with empty wing. At all other times of the dive, the wing has to compensate -- the worst case is max depth with a full tank. You have a) 5 lb of air that you won't later (assuming an 80 cf tank) and b) have lost a substantial amount of wetsuit buoyancy at depth that you will have later (at the SS or surface).

Sure, drop enough weight to compensate for (a) -- 5 lbs for a full AL80 -- but more than that leads to a rocket ship as the suit regains buoyancy. If that a&b combination is more than you can swim off the bottom in the event of a catastrophic wing failure, then you simply must have redundant lift. Typically, a drysuit solves this problem very well. However, a lift bag or larger DSMB is also effective. A redundant bladder perhaps, though that's not without its own drawbacks. A buddy with sufficient wing capacity for both of you as a very last resort.

Yes, bent is better than drowned, but with a little planning you should avoid both.

That said, dumping weight to stay afloat after surfacing might be useful. However, whatever supplementary lift that allowed you to get off the bottom should more than handle the surface need after you have regained your suit buoyancy.

So no, my feeling is that you should not ditch more than an amount equal to the non-reserve air you have left. A 7mm wetsuit WILL mandate that you carry supplementary lift.

Thank you Inquisit, this is very helpful to my understanding. So if one descended to 100 ft with a full AL 80 tank and had a wing failure one could ditch up to 5 lbs without an uncontrollable ascent. One would then have to swim up the loss of buoyancy in the 7mm wetsuit which would then get progressively easier in the shallower part of the ascent.

One would not want to ditch more than 5 pounds. Very helpful and I did not know that.