Do you need ditchable weight?

[VikingDives]

Ditchable weight is ultimately about being positively buoyant at the surface in an emergency. You can mitigate a lot of risk and you'll probably never need to ditch your weights, but if you ever need to, having enough ditchable weight to keep your head out of the water isn't a bad plan. If your lungs are empty, your tank is full and you've lost consciousness you want to make it as easy as possible to be rescued by a 75 pound 10 year old on his first open water dive.

That being said, I don't dive with any in doubles in fresh water, but I would if I needed it/could.

There's a UTD video or podcast that talks about the buoyancy characteristics of a 7mm wetsuit. IIRC, they don't recommend using one below 100 feet because of the loss of insulation and poor buoyancy characteristics.

 

[inquis]

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

Ah, but your intuition was spot on:

On the other hand I wouldn't want to ditch so much weight that I would induce an uncontrolled ascent.

 

[FishWatcher747]

Ditchable weight is ultimately about being positively buoyant at the surface in an emergency. You can mitigate a lot of risk and you'll probably never need to ditch your weights, but if you ever need to, having enough ditchable weight to keep your head out of the water isn't a bad plan. If your lungs are empty, your tank is full and you've lost consciousness you want to make it as easy as possible to be rescued by a 75 pound 10 year old on his first open water dive.

That being said, I don't dive with any in doubles in fresh water, but I would if I needed it/could.

There's a UTD video or podcast that talks about the buoyancy characteristics of a 7mm wetsuit. IIRC, they don't recommend using one below 100 feet because of the loss of insulation and poor buoyancy characteristics.

Vikingdives, good information. On the positive side for jacket style BCDs with weight integrated pockets it is super easy to ditch the weight pockets at the surface if needed. A rescuer of me would have to unclip my upper cam band trim pockets and pull out the weights. Not as easy.

As for the 7mm wetsuit, the wrecks in Key Largo take you down to 110 feet (109 feet is my max so far actually) unless you go to the sand which is 130 feet. I don't plan to go deeper than the 110 feet which is about MOD for the 32% nitrox I use anyway.

 

[lowwall]

As for the 7mm wetsuit, the wrecks in Key Largo take you down to 110 feet (109 feet is my max so far actually) unless you go to the sand which is 130 feet. I don't plan to go deeper than the 110 feet which is about MOD for the 32% nitrox I use anyway.

FWIW, there's only around .5lb loss of buoyancy in a 7mm when going from 110 to 130. So you don't need to worry about it if you end up going a bit deeper for whatever reason.

 

[nolatom]

I too have always dived with just a weight belt, nothing ditchable, And have typically anticipated I'd breathe away about 4 pounds or so. And then added an extra pound or two just so I don't have an awkward time trying to descend those first few feet after exhaling some. It's always worked well.

But when might it be a good idea (or a life-saving idea) to be able to drop just part of the (let's say) 10 pounds on my belt? I think it would need to be some bad unexpected event where I wouldn't be able to swim it all up with me, and ergo needed to have a bit more positive buoyancy while still at depth. What kind of event? Would have to be something where I'm fully conscious at depth, but may soon be semi-conscious or unconscious, and gradually unable to move fins?

Big loss of blood from some shark mishap, or knife attack (very common, just watch Sea Hunt--pause here for laughter), or some brain or heart problem--stroke, chest pain, heart attack. And---no buddy nearby to grab on and adjust our collective buoyancy and ascent rate.

So with my last bit of dim consciousness I ditch, what, 3 or 4 pound weight from BC pocket? And inflate BC only just enough to start an ascent. Then I lose consciousness. I'll ascend too quickly after a minute or so, but not as quick as if I'd ditched the entire weight belt and all my weight, just to get positive while down deep. And sure I'll get bent as ambient water pressure decreases, but not as bad as with no weights.

To quote my very good instructor from way back, "better bent on the surface than dead on the bottom".

 

[-JD-]

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

Just as a clarification, I was suggesting top of shoulders, back by the plate. Reachable for ditching if you use a pocket or attachment that makes that practicle, but generally out of the way.

 

[FishWatcher747]

Just as a clarification, I was suggesting top of shoulders, back by the plate. Reachable for ditching if you use a pocket or attachment that makes that practicle, but generally out of the way.

Wouldn't it be better to ziptie 2 pound weights to the top of the plate? Not sure if this is doable as it may prevent the wing from laying flat on the plate and effect hydrodynamic position.

The HOG system has padding over the 2 inch webbing at the shoulders so I don't think it would be easy to get shoulder weight pockets there.

 

[lairdb]

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. [...]

A 7mm wetsuit WILL mandate that you carry supplementary lift.

@inquisit -- can you show your math on that?

Major edit -- below italicized original left for reference, but see lower down.

This is just back-of-the-envelope, but: a reserve AL80 is +3, and some quick searching suggests a 7mm is +8*. Let's add +5 for floaty BC padding and because I don't entirely trust that +8 source.

(*This number matters -- if I got that wrong, speak up.)

Ignoring suit compression at 15fsw stop, that's gets me to +12 +3 for -15 of lead.

Now we're going to bail out from +5ATM, which we'll assume is 90% crush on the floaty bits**, with a full tank (which it wouldn't be, but <handwave>). Now the floaty bits are +1.2 and the tank is -2 and the lead is -15 -- so we're at -16 with a buoyancy failure.

(**Anybody got data on suit compression curves?)

I'm inclined (see @fdog 's thread) to think that's swimmable, though right at the limit -- and it will get lighter as it goes up. Adding compound failures (injury, confusion, lack of fitness, etc.) could make that worse. Ditching 5 would help -- though you're going to be out of control after passing through about 20ft. (Ditch the whole -15 and you'll be out of control almost immediately.)

(Rough calculations in my head while typing. I am not your instructor. 2+2=42.)

Major Revision:

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. Not as bad at shallower depths, but still a heavy load. Using that math, I'm now with @inquisit -- need some redundant buoyancy. Grab that DSMB.

 

[-JD-]

Wouldn't it be better to ziptie 2 pound weights to the top of the plate? Not sure if this is doable as it may prevent the wing from laying flat on the plate and effect hydrodynamic position.

The HOG system has padding over the 2 inch webbing at the shoulders so I don't think it would be easy to get shoulder weight pockets there.

The advantage of the pockets is ditchablity and convenience. They will make it easier when traveling, especially if boat diving. Get on the boat, pop in your desired weights. Reverse when you get off. Nice at home, too.

This is how I adapted my plate:

Because of the single point shoulder attachment, pouches attached at the very top of the straps won't work, but the upper pouches end up a bit above the camband helping with the balance.
The lower pouches are for fixed ballast with a drysuit.
The plate attachment lowers the CM of the weights towards my body and also means that I'm not playing with the weights on the band when changing tanks.

Do you have the HOG TBCS/Comfort harness? From what I understand, they are just shoulder pads with a few straps holding the pad under the 2" webbing. Should present no issue other than affecting placement by an inch or so.

 

[Wibble]

I see this as a question about redundant buoyancy.

Obviously it depends upon the failure you've had. If you're completely out of gas, you'd either bolt for the surface and a possible helicopter ride or get some more gas off of some other diver (who'd then stick with you for the ascent).

If it's some other failure such as your BCD inflator or dump falling off (I've had a dump fall off; just stay upright and use the elephant's trunk).

Firstly you'd use your drysuit if applicable.

Secondly you'd use your SMB. If not enough, then use your spare SMB as well. Some people think that the purpose of a SMB is a Surface Marker Buoy, but that's not so. It's a backup buoyancy, it's a lift bag, it's a signal to the surface (send up two bags on the same string, but do tell the skipper first!) and it's also a surface marker buoy.