Oxycheq vs Golem vs DSS

[John_B]

...so I'm thinking of moving 10 lbs to a weight belt. This weight would be difficult to ditch though, due to the crotch strap.

Personally, I like it that way. YMMV.

 

[O.M.O.H.]

Lots of stuff said... Thanks to all who participated!

I am 100% behind what Tobin is suggesting. Not only i agree fully with the wing size selection methodology he is suggesting, Tobin is a known authority on BC design and diving too, for that matter. One issue for me remains - i would still like to stay away from weight belts, although i fully understand their benefits as far as "minimalization" (is this even a word ) is concerned. Pockets i use are very similar to what Halcyon has for their harnesses, they accommodate 12lbs per side i think, but i only use 4lbs per side, and they are fully ditcheable.

So i guess it's a 35 or 40 lbs wings for me.

 

[vyvial]

Great info Tobin, thanks.

 

[rstofer]

This has been a fascinating thread. I have especially enjoyed Tobin's remarks re: wing selection criteria.

Here's where I have a problem: I look at the buoyancy spreadsheet posted elsewhere and I consider things like a steel tank and stainless steel backplate and I wind up with pretty minimal ditchable weight even with a 7/8 mm wetsuit and cold water diving.

But as I look at the buoyancy numbers over depth and tank contents, I realize that I'm not positively buoyant at depth even if I ditch my weights. If the BC fails, I have to swim for the surface. If I am able... On arrival, there are situations under which I am still not positive unless I ditch the BC.

In training, years ago, the assumption was made, and stated, that ditching the weightbelt would result in a diver ascending to the surface. There are equipment decisions that invalidate this assumption.

I'm not pointing at any particular set of equipment or weight location. I am seriously trying to get my head around buying a BP/W to replace my 20 year old BC. The BC is rated at 33# lift in fresh water (call it 34# in salt water) and with a 30# weightbelt, 7 mm wetsuit and Al 80, I thought I was ok for making stops at any depth. I had assumed, but not tested, that I would be positive at any depth if I ditched my belt even with a full tank.

What I am wondering about is whether the conventional wisdom of a stainless steel plate and steel tank is really the way to go if one is to consider BC failure and gearing up in the water. When diving from an inflatable, I ALWAYS doff and don in the water.

Richard

 

[Nemrod]

Ditching weight belts at depth was never really taught, we were taught to ditch the belt at the surface to establish positive bouyancy during the pre-BC era. Since we did not have a BC to begin with it was hard to imagine one could be so foolishly negative that they could not swim themselves up and then establish positive bouyancy in an emergency by ditching the weight. This is how we were taught and trained, establishing appropriate weighting was as well part of the intsruction and is sorely missing today. I don't ever really recall being told to dump a belt at depth nor should there really be a reason to do so. Someone might have taught that but I don't remeber it beyond removing the belt at depth as part of the doff and don which at that time being 12 years old I thought they were calling it the dolphin don.

I am of the mind that weight should be split between equipment (non ditchable) and ditchable (weight belt etc) and that one should endevor not to be so negative that dumping weight should be required to return to the surface but only to establish positive bouyancy once at the surface in an emergency.

N

 

[cool_hardware52]

This has been a fascinating thread. I have especially enjoyed Tobin's remarks re: wing selection criteria.

Here's where I have a problem: I look at the buoyancy spreadsheet posted elsewhere and I consider things like a steel tank and stainless steel backplate and I wind up with pretty minimal ditchable weight even with a 7/8 mm wetsuit and cold water diving.

But as I look at the buoyancy numbers over depth and tank contents, I realize that I'm not positively buoyant at depth even if I ditch my weights. If the BC fails, I have to swim for the surface. If I am able... On arrival, there are situations under which I am still not positive unless I ditch the BC.

In training, years ago, the assumption was made, and stated, that ditching the weightbelt would result in a diver ascending to the surface. There are equipment decisions that invalidate this assumption.

I'm not pointing at any particular set of equipment or weight location. I am seriously trying to get my head around buying a BP/W to replace my 20 year old BC. The BC is rated at 33# lift in fresh water (call it 34# in salt water) and with a 30# weightbelt, 7 mm wetsuit and Al 80, I thought I was ok for making stops at any depth. I had assumed, but not tested, that I would be positive at any depth if I ditched my belt even with a full tank.

What I am wondering about is whether the conventional wisdom of a stainless steel plate and steel tank is really the way to go if one is to consider BC failure and gearing up in the water. When diving from an inflatable, I ALWAYS doff and don in the water.

Richard

In cold water with buoyant wet suits I recommend having some ditchable weight. I don't like seeing 100% of the ballast on the rig.

This ditchable weight serves a few purposes.

The first is a sure means of establishing positive buoyancy at the surface in a rescue scenario. 4-6 lbs on a weight belt is IMO enough to achieve this.

I'll assume that your "7-8" suit is 22 lbs positive, and that you are using a medium SS plate and HP100 steel tank. Your rig will be about -18 lbs with a full tank, and about -10 with an empty tank. You will need another 4-6 lbs in a belt.**

Lets assume you suffered a 100% buoyancy failure at 100 ft. Your suit has lost maybe 60-70% of it's initial buoyancy, .65 * 22 = ~14 lbs. You have 6 lbs to drop. That leaves you in theory negative by about 8 lbs. Can you swim up 8 lbs? I know I can. Remember this example assumes you have a 100% failure of your wing, very unlikely, and that it occurs while you still 100% of your gas, again, not too likely. It's much more likely that you will only a few lbs negative, and that won't last long as you swim up. Modest ditchable vs mega ditchable also reduces the resultant ascent rate.

I'm sure somebody will be along shortly with a scenario that includes 100% wing failure concurrent with a incapacitated, but conscious diver who is buddyless to justify having 100% ditchable ballast.........

Remember too that with a wetsuit it will rebound as you ascend reducing the amount you are negative, and you will be positive at the surface.

If you are concerned about your gear when donning and doffing in the water run a "tag" line off the RIB. No combination of full cylinder and backplate and reg will be positive.

**A couple notes about total weighting. If you are using a heavy wetsuit I like to start eye level at the surface with no gas in my wing and a full tank. I rely on the compression of the wetsuit from the surface to the shallow stop depth (10 - 15 ft) to offset the weight of the gas I consume. If your wetsuit is +22 at the surface it will easily be 6-7 lbs less buoyant at 15 ft. Remember from the surface to 33 ft the pressure doubles, 1 ata to 2 ata. This not only reduces the total lead required, but it also reduces the amount you might be negative in the event of a failure.

Tobin

 

[rstofer]

Tobin,

Thank you for your thoughtful reply.

I would be using the Large plate at -5.8# versus the Medium at -4.7# as I'm 6'1" & 210#. So the rig weight with a steel 100 would be -19# - not a significant change. I suspect the number would be a little higher with the regulator. Clearly this can be supported by a 30# wing.

But, if I had to carry, say, #10 of ditchable weight on a separate harness (no hips!), the 30# lift seems a little iffy.

Does the 40# wing make sense?

Thanks!

Richard

 

[cool_hardware52]

Tobin,

Thank you for your thoughtful reply.

I would be using the Large plate at -5.8# versus the Medium at -4.7# as I'm 6'1" & 210#. So the rig weight with a steel 100 would be -19# - not a significant change. I suspect the number would be a little higher with the regulator. Clearly this can be supported by a 30# wing.

But, if I had to carry, say, #10 of ditchable weight on a separate harness (no hips!), the 30# lift seems a little iffy.

Does the 40# wing make sense?

Thanks!

Richard

The rig you describe, large ss plate, HP 100 and regulator would be about -19 with a full cylinder, and provide about 11 lbs of ballast with an empty tank.

If you need another 10 lbs of ballast it implies that your exposure suit is at least 11 + 10 = 21 lbs positive, probably more like 24-26 lbs positive.

If you are wearing a 10 lbs weigh belt or weight harness, your wing does not need to support this ballast if you ditch the rig. This 10 lbs does nothing to change how negative your rig is. Your rig is still -19, and a 30 lbs wing will float this easily.

Your wetsuit is still less than 30 lbs positive. There is NO need for a 40 lbs wing.

If you are wearing a suit that is +21 (or more) and a weight harness that is is -10 lbs you are still quite positive even after you ditch the rig.

You are over thinking this a bit.

1) Float your rig with a full tank if you ditch it.

2) Compensate for the max change in buoyancy of your exposure suit.

Yes it is really that simple.

Tobin

 

[rstofer]

Tobin,

Got it! The rig never needs to support me, the wetsuit does that. If I ditch the weight belt at the surface, I have a LOT of buoyancy with, or without, the rig.

The rig only needs to support itself. At depth, the wing needs to make up for the loss of buoyancy in the wetsuit.

Thanks!
Richard