BP/W setup too heavy for enough droppable weights?

[ardilla]

Recently had to look for a new BCD and finally went for a BP/W setup (partially motivated also by discussions/reviews on this forum). Have done around 15 dives with BP/W so far and in principle feel very happy with decision to switch to BP/W setup, however starting to worry a bit about very little amount of ditchable weights that I now need.

The diving is in cold(ish) fresh water lakes and quarries, the depth is mostly 10 m or above, but on occasional dives could be up to 20-30 m. BP/W is Hollis gear with S38 wing, steel plate, STA and droppable weight pockets. I am mostly diving with single 15 l (or sometimes 12 l) steel cylinder and 7 mm wetsuit, the optimal droppable weights in which case is only around 2 kg. Which looked quite nice until I noticed that it starts to get difficult to swim up with an empty wing already from the depth of 10 m (or so).

Looking at some numbers I then realised that at the start of the dive I am probably 3 kg negative on the surface
(the weight of air between 230 to 50 bar), which is already more than 2 kg of ditchable weights. With BCD I have not noticed that wetsuit noticeably loses buoyancy with depth; unfortunately it does and while the exact amount is difficult to measure, some tables suggest that could be as much as 3-4 kg at 10 m and 4-8 kg at 30 m. Which leaves me up to 7 kg negative at 10 m and 11 kg at 30 m, or 5 kg and 9 kg, if I drop my weights. According to some posts 9 kg should be doable, but quite difficult, and this is from 30 m in coldish water with probably zero visibility... In other words, not the best location to encounter a total wing failure early in the dive (however unlikely that might be).

Unfortunately most posts I have seen have focused on benefits of extra weights for BP/W; happy that I have been skeptical about the extra weight option for STA (2.7 kg) and other trim weights. Still the current "minimal" setup already seems much too heavy. Have I known this, would have chosen aluminium plate (1.5 kg less), but buying it now is another $100 lost and the weight difference is not that large. STA could be another 1 kg off, wing allows not to use it, but I like the extra rigidity it provides. But, anyway, I do not see how to configure this BP/W system to be neutral or positive at 20-30 m after ditching all droppable weights...

Thanks for any opinions about this, experience or suggestions!

 

[guruboy]

But, anyway, I do not see how to configure this BP/W system to be neutral or positive at 20-30 m after ditching all droppable weights...

You don't like hearing this but switching to an AL or kydex plate will help. Or going drysuit. Maybe even going to AL tanks.

However, I don't see how being neutral at 20-30m is a reasonable goal given that you are diving wet. You would just rocket to the surface as you come up.

The goal is to avoid being so negative at depth that you cannot swim yourself up with no help from the wing. Not be neutral at depth.

To me, dropping weight would be done at the surface to let you float.

 

[ams511]

I would think doable is being able to swim up 5 kilos, sure some people can do 10 but I don't think the masses can do it. A lighter weight plate and STA will lighten your fixed weight and then you can compensate with ditchable weight.

I am not sure how you can be neutral at 20-30. Your wetsuit will compress and lose most of its buoyancy at these depths and you will be like a cork at the surface and unable to hold any type of safety stop. You want to be neutral at 5 meters with 500 PSI in your tank with an empty wing.

Remember total wing failures are rare. If a wing gets punctured it should still be able to maintain enough gas for you to reach the surface. Also your dsmb can give you extra buoyancy too.

 

[divad]

I switched from a SS BP to an AL BP so I could move weight to my waist without adding extra weight.
(with drysuit and double fleece underwear)

 

[2airishuman]

I do not see how to configure this BP/W system to be neutral or positive at 20-30 m after ditching all droppable weights...

Thanks for any opinions about this, experience or suggestions!

You do not have to be able to get to neutral at 20-30m. You have to have some sort of plan to get to the surface in the event of a wing failure. and if you're on a wall or over a deep or unsafe bottom you have to have some sort of plan to arrest your descent in the event of a wing failure. You also need to have some sort of plan to dealing with emergencies on the surface.

Part of those plans may involve ditching your kit.

Knowing how negative your rig can be and still allow you to swim it up is a good thing.

You'll have to decide what's going to work for you, and yes, you could possibly come to a conclusion that you need to switch to a lightweight backplate or a lighter STA for those particular conditions.

 

[Blasto]

If your worry is losing buoyancy, why not consider ensuring backup buoyancy first?
- Dual-bladder option (don't think one is available for your wing)
- SMB and lift bags - not real backup BCD, as it's harder to control over ascent speed, but quite viable
- Diving dry - though the added complexity might not be worth it

I take it that 20-30 m is rock bottom where you dive? If so, you don't have to worry too much about uncontrolled descents either. As long as you've got air and a no-spill bag, you can get lift. If you're solo diving (sounds like that from the post), you've got two air sources, so you've got air.

Worst-case scenario... Say -2 kg for half a tank, +1 suit, -1 to -2 the rest - dropping nonessentials that's -2 to -3 kg, easily overcome with finning. Once you're at 10 meters, your problem will rather be keeping yourself from a rapid ascent as the wetsuit expands.

If you seriously want to configure your system so as to eliminate any chance of negative buoyancy, trading big steel for 2 AL80 would do the trick, but that's going to be overkill and require a lot of weight.

 

[Colliam7]

I am a bit confused by some parts of your narrative, so I will ask some questions before offering additional suggestions to the good feedback you have already received.

You mention that you are using, and prefer, a single tank adapter. Are you using the Hollis STA? How much does it weigh?

Are you already diving with 2 kg of added (and ditch-able) weight?

When you say 'the current "minimal" setup already seems much too heavy', is that a perception based on your calculations, or is that your physical sense in the water while diving the rig?

Have you actually tried swimming your rig up to the surface from, say, 30m, with a completely empty bladder?

Have you tried handing off you 'ditch-able' weight to a dive buddy at depth, to see what your buoyancy is like?

If you truly do not have a balanced rig (i.e. enough weight to make a descent, and yet still be able to swim the rig to the surface), then you need to lighten it. As has been mentioned, you can go with a lighter plate (AL, or one of the 'lite' SS plates). Find a buyer for your current plate, so you really don't have to sink another $100 into the rig, if that money is an issue. You can also go with a lightweight STA instead of the Hollis (e.g. OxyCheq - Light Weight Single Tank Adapter). You can use and AL cylinder instead of steel.

Honestly, even diving in fresh water, if you are diving with a 7mm suit, I would be surprised that you would be unmanageably negative, even with a SS BP and a steel cylinder. So, I am curious if your concerns are based only on calculations only, or on actual in-water experience.

 

[cool_hardware52]

Recently had to look for a new BCD and finally went for a BP/W setup (partially motivated also by discussions/reviews on this forum). Have done around 15 dives with BP/W so far and in principle feel very happy with decision to switch to BP/W setup, however starting to worry a bit about very little amount of ditchable weights that I now need.

I'll leave you with two thoughts:

1) My definition of minimum ballast is the least ballast necessary to hold a shallow Safety Stop with a near empty tank. In a thick suit and normal sized single cylinder that may result in me being slightly positive at the start of the dive. Swiming down 2-4 lbs is no big deal.

2) What exact problem do you hope to solve by dropping ballast? Take a hard look at that question before you make equipment changes.

Good luck,

Tobin

 

[ardilla]

Thanks a lot for so many and so constructive suggestions!

What I am worried is not being able to swim up after dropping everything droppable, if buoyancy completely fails (even I know that chances of that are very very small, also >20 m dives are quite rare and 30 m is the deepest anybody here goes). The aim would be to have enough droppable weights to be able swim up without them and with empty wing (however dangerous, it sounds better than realizing that you are stuck on the bottom without any options...)
What I have realized (tested is that with 2 kg weight I already need air in wing to ascend from 10 m; will check with buddy what happens if I remove one of both 1 kg weights - might be ok, but (currently just estimates) likely need 3-4 kg more for 20-30 range. Would be happy to test everything up to 30 m, but all the nearby "playgrounds" happens to be 10 m at most - deeper quarries need daytrips and I still prefer to go there with guided groups, limited options for equipment tests however...

Thanks for some concrete suggestions - will consider light weight STA (this could be up to 1 kg less) and (apparently...) need to look also for AL BP (another -1.5). With some luck this even might be sufficient (with 4.5 kg of ditchables). SMB is some kind of extra protection I have already thought about - this will be useful anyway as occasionally there is a need to mark some objects.
AL cylinders are practically unseen here, however (and will not be practical to own, with renting the same price as filling). Also 15 l tanks are quite large (and heavy) with usual fill around 220 bar this looks like 116 cf - not sure about AL equivalents. 12 l (~93 cf) are the next smaller size and already ~2.5 kg more buoyant (few weeks ago did my own tests with the the gear from my usual rental shop) - this already sounds good, unless you are not the very best with the air and others in your group have 15s.

 

[guruboy]

Look up the "balanced rig" concept.