Building a Better BC

[danvolker]

Yeah, it was a few posts prior to the one you quoted where I was talking about the zipper expansion panel and I said something like maybe I'll call Tobin and ask him about it. He actually cut me off while I was starting to say zipper. As in, he started his interjection as soon as I said "Have you ever thought about...".

As I ponder it, I suspect the thing with bungees (or A thing) is that if you're using it to keep the wing collapsed/compressed when it's not fully inflated (to reduce drag), then what you are doing is requiring that more air be put in to achieve the same amount of internal volume - i.e. more air to get the desired lift. That is because the internal pressure is working against external water pressure PLUS the constricting pressure of the bungee.

All of which means that you would increase your air usage when using a wing that has bungees/elastic to compress it, compared to a wing that doesn't have bungees.

That said, it seems like the actual difference in air usage would be so small as to be negligible, if it's even measurable.

Or is there some other reason the DIR folks don't like bungees? It seems like they would like a wing that is held in tight to the back plate when it's not fully inflated.

I believe most of our DIR rants against bungee wings originated from a couple of different tech accidents where the overpressure release valves kept dumping air due to tight bungees, and not allowing anything close to rated lift capacity ....in the DIVers Supply Triple Death Tragedy, this was one of the key issues..
On a few others, the issue was also that the diver could not orally inflate the wing to the degree needed....

These being the big ranting issues....the tiny issue of drag gets argued to death, so I dont care to use that as a DIR reason

 

[Akimbo]

…I am not an expert in hydrodynamics but maybe you are. What has more drag a floppy wing or a rigid one?

There have been a number for rigid cowlings over the years. Most were for doubles and triples and some like the At Pac covered the majority of the cylinder and the BC. There were also a few rigid BCs… from Dacor and Scubapro if I recall. The universal consensus was that the drag was not worth it, which helps explain why they didn’t survive in the market place. Attached is an ad for one that ran for months but never caught on.

That is not to say that there is nothing to learn from these market failures, but I am personally hard pressed to find a useful feature. I agree with you that a wing flopping around isn’t too great. The elastic cover is definitely better than a bunch of bungees from a drag and entanglement perspective. I never noticed much difference orally inflating it compared to one without the elastic cover, but the difference is probably too small for a casual perception.

I suspect one of the biggest problems with the cowlings and rigid BCs was that they can’t take the abuse that tanks get. They also can make changing cylinders difficult. Too bad really, they do “look” cool.

 

[TSandM]

That sounds like Tobin . . . . He's got a much easier answer to the question of how to have the right sized wing for various situations. It's called having the right sized wing for various situations! Honestly, you will eventually have a basement full of gear, and a little tiny tropical wing will be lost in the mêlée.

There are places where precision is critical, and places where we spend WAY too much energy overthinking things. Being 2 pounds overweighted is not worth anyone's effort to fix. Having a wing that is a little bit bigger than you need is also not worth spending any energy on. I dove a 30+ pound wing in the tropics for a long time before I got my tropical wing. It's a little bit harder to vent than it needs to be, and the 17 pound wing is a dream in that respect, but I didn't have uncontrolled ascents or anything horrible because my wing was oversized.

Where people get into REAL trouble is using things like 60 pound wings on single tanks, or double tank wings on single tank setups. There, you have a HUGE bladder, and the air is a long way from the inflator or the dump valve, and big excursions from trim (and a lot of careful anticipation) are required to manage buoyancy. The solution to making a massively oversized wing work properly does not lie in kludges involving the oversized wing; it lies in obtaining a properly sized wing for the application.


I personally would seriously worry about encasing a wing in some kind of shell, for fear that the crumpled or folded interior bladder would have areas that trapped gas. I have experience with someone diving a Frog wing where the inner bladder wasn't properly installed and had folds in it. The diver kept complaining that she couldn't just couldn't manage her buoyancy well, and she wanted to go back to her standard BC. When I lent her my DSS setup, she did not have those problems at all. It left me with a grave suspicion of inner bladders that are not lying smoothly within their casings.

 

[tbone1004]

Dan, I think the biggest thing against the bungees was you don't have the lung power to orally inflate the wing all the way in an emergency, added drag was a small part of it, but the restricting bungees a la OMS don't allow the diver to fully inflate the wing orally which is extraordinarily dangerous in open water, and properly irritating in a cave.

I'm with Lynne though, no wing will be one size fits all, I've worked with these guys trying to improve on wing fabrics, and unfortunately if it gets stretchy it instantly becomes less durable to abrasion, puncture, and UV *materials that are stretchy don't like sun, even worse than the nylon we're using now*, and you still add the oral inflation problem. I have an older Travel wing from Dive Rite, the 30lb one without grommets and it has a bit of stretch to it, but not a whole lot, just enough to stretch a smidge before the OPV goes and it was supposed to help with dumping or something, never saw a need for it but it was free so what the heck, beggars can't be choosers. Optimal wings for me are no wing for tropical use, if I'm in an AL80 without a wetsuit I just don't use a wing, with a wetsuit a small 20-25lb one works just fine, 25lb seems to be the minimum for use with 8" bottles though, but it's more how wide the wing is vs. how long it is, so my travel wing is a bit longer but even at 30lbs it doesn't angel wing up too bad.
I really like the 45lb Rec wing for lightweight doubles, AL80's, LP72's, LP85's, HP100's etc, and then if you need it, get a 60lb big one if you're diving heavy doubles in the ocean. We are continually working on better materials for the wings, but without defying the physics of how textiles stretch we can't make an airtight bladder that stretches any more than a few points at best.

What is interesting about the oversized bladders is that most wings with an inner bladder are actually slightly oversized for the case, Halcyon is one of the more obvious ones, this was done to protect the seams on the bladders from holding any real pressure, they are just acting as an air tight container with the cover taking all of the pressure. You can get away with a little bit of this, but then can't make the cover stretch without putting pressure on the bladder, catch 22

 

[rhwestfall]

anyone notice the Dive Rite is using a bungie to control their doubles wing (for example the CCR).... called the gusset control system, doesn't wrap over the bladder...

As to a "old school" singles flat plate, check out VDH. Its an awesome plate, and I even dive doubles on it..... (& you don't have to have a DH)

 

[tbone1004]

the gusset control is a bit different since it is only one run of 1/8" bungee which you can orally inflate, far cry from the medical tubing that OMS was using which was the main offender. Gusset control is mainly to reshape the wing and keep it from flapping up than restrict the volume

 

[stuartv]

I personally would seriously worry about encasing a wing in some kind of shell, for fear that the crumpled or folded interior bladder would have areas that trapped gas.

I THINK this is kind of a non-issue for the specifics I'm talking about. Let's just say we want a donut wing that expands from 17# to 35#. To decrease the volume by half, keeping the same radius from the center of the wing to the center of the torus (i.e. the center of the cross section of the donut), we only have to decrease the radius of the cross section by the square root of 2. I.e. divide by 1.4142 (off the top of my head). Radius is directly proportional to circumference, so that means reducing the circumference in the same way.

So, if the 35 # donut (approximating a donut wing as a true torus - i.e. perfectly round doughut shape) has as cross section of one side that is, say, 4 inches across, then the circumference is 4 * Pi. And to reduce the wing's lift to 17#, you would reduce the circumference to (4 * Pi)/1.4141. I.e. if it's 4 inches across, the zippered expansion panel would just need to be a bit over 3.5" wide. That's taking 3.5" out of a donut that is (4*Pi=) 12.6" around.

In other words, imagine your wing bladder inside a tube that's 12.6" around the outside. Now imagine taking a 3.5" strip out of the side of the tube and then closing it down to eliminate the gap. The bladder inside doesn't seem like it would really have that much opportunity to fold itself up in such a way that, once you inflate it, you can't fully deflate it just as easily as when the outer shell is fully expanded.

Of course, this concern would be completely moot if somebody made a collapsible outer shell (i.e. expansion panel as I described) with a bladder that was stretchy and had an unstretched volume that gave 17 # lift. Especially if the spring coefficient (i.e. the stretchiness) was very light - in other words, if it stretched very easily. Offroad cars and motorcycles still use inner tubes. Those things are pretty darn reliable. It doesn't seem like it would be that hard to make a reliable, durable wing bladder that was stretchy and could still be inflated to full capacity orally, if needed.

Or I can just buy a warm water wing, at some point and have two...

 

[dumpsterDiver]

The air trapping issue associated with the folded bladder would have to be tested. It might be an issue- I had not thought about it.

So far is sounds like a reasonable concept, assuming people are so anal about drag and are willing to pay for a zipper to make a smaller wing on vacation. I'm using a 44 lb wing right now and it is bigger than I need.

 

[raftingtigger]

I was waiting until I had a chance to take photos of my BP/W which has several of the debated items on it, but I'll just try to post the pictures later (if there is interest).

First the disclaimer: I am not an engineer, just a talented experimentalist.

ZIPPERS are a common way of getting into the case and accessing the bladder. My Dive Rite wing has one on the inside perimeter. My DIY wing has one also on the inside perimeter, and one of my first DIY wings had velco, then zippers, on the end. My commercially bought inflatable kayaks have longitudinal zippers and my DIY cataraft tubes have circumferential lacing as the closure. In over 15 years of using at least one of these I have found no problems at all.

BLADDER SUBSTANTIALLY LARGER THAN THE CASING can lead to air trapping. On my first DIY wing I had this problem. This led me to start making my own bladders from scratch. How big is too big? I don't know. Reducing the tube diameter of a wing by an inch or even maybe two shouldn't give a lot of air trapping. I doubt that more than 2 inches is needed, probably more like only an inch. I make my bladders deliberately about 1-2 inches larger in diameter than the casing. The bladder is to hold the air. The casing the pressure. I have not found an air trapping issue with this size difference. I guess I can un-zip my DIY wing and my Dive Rite wing and see how much they expand fully inflated with the zippers loose.

BUNGEES can reduce the uninflated profile. They have 2 problems. First they add drag. Second, if they are too tight or too few, they can cause air trapping. Try strapping down a wing to make it into a sidemount rig and you will see the problem.

MY SOLUTION(S):

Standard single BM configuration:

I use a fairly standard non-branded SS backplate and a DIY STA with a single tank with the attach bolts permanently fixed. My harness is webbing with D-ring pivots on the arm straps at axilla level. A chest strap attaches to these pivot D-rings. Above the pivots is a single fixed D-ring on each side. Below is a velco closure inside a sleeve as a protected quick release. The sleeve acts as a knife holder (left) and back-up light holder (right) and prevents accidental release of the strap. The arm straps are continuous with the waist strap. Each side of the waist strap has a single fixed D-ring. There is a 4# capacity weight pouch mounted on the back of the plate and secured by the arm/waist strap as it threads through the bottom slots (one each side). These are usually empty. I have added a butt plate (semi-soft) to the bottom of the plate with straps/book screws. The crotch strap threads through the butt plate for additional security. The butt plate has 2 angled 2-1/2" door handles and a center 5# capacity weight pocket (also usually empty). The waist strap secures with a standard SS weight belt cam buckle. The crotch strap attaches to the left side of the waist strap with a 1-1/2" side release fastex buckle. The crotch strap shortens with a simple pull.

Sidemount or BM with pony or BM with left SM (independent doubles)

A short length of bungee and dog clip that secures on the plate behind each axilla and clips to the upper chest D-ring or pivot D-ring. This bungee hooks over the sidemount tank valve or the pony valve. When not in use the dog clip is attached to the side waist D-ring. The lower clip on the SM tank or Pony clips to one of the door handles on the butt plate.

The wing is prevented from tacoing by sewing 2" wide elastic loops (4 in all) into the wing casing seams where the shoulder straps and waist straps are. A smaller (1") elastic loop is sewn into the side seam opposite one of the side plate holes. These loops allow the wing casing to attach to the shoulder straps, side of the plate, and waist straps. Check out the SMS75 and SMS100 SM systems. They do the same thing, and no I didn't steal from them - we came up with the idea independently.

The only problem I have with this is there is too much air near my shoulders and not enough in my low back. I really like the zipper to make the casing smaller idea and will probably put one on each side on the top half of my DIY (and primary) wing.

This is a long winded and wordy description. Here are some pictures. This works well for me.




​

 

[RJP]

I THINK this is kind of a non-issue for the specifics I'm talking about. Let's just say we want a donut wing that expands from 17# to 35#. To decrease the volume by half, keeping the same radius from the center of the wing to the center of the torus (i.e. the center of the cross section of the donut), we only have to decrease the radius of the cross section by the square root of 2. I.e. divide by 1.4142 (off the top of my head). Radius is directly proportional to circumference, so that means reducing the circumference in the same way.

So, if the 35 # donut (approximating a donut wing as a true torus - i.e. perfectly round doughut shape) has as cross section of one side that is, say, 4 inches across, then the circumference is 4 * Pi. And to reduce the wing's lift to 17#, you would reduce the circumference to (4 * Pi)/1.4141. I.e. if it's 4 inches across, the zippered expansion panel would just need to be a bit over 3.5" wide. That's taking 3.5" out of a donut that is (4*Pi=) 12.6" around.

In other words, imagine your wing bladder inside a tube that's 12.6" around the outside. Now imagine taking a 3.5" strip out of the side of the tube and then closing it down to eliminate the gap. The bladder inside doesn't seem like it would really have that much opportunity to fold itself up in such a way that, once you inflate it, you can't fully deflate it just as easily as when the outer shell is fully expanded.

I think the specifics of your hypothetical math are kind of a non-issue in the real world.

I'm guessing you've never actually seen the bladder inside a scuba wing. Putting a bladder with a 35lb lift capacity inside a wing with max of 17lbs capacity requires the bladder to be twice the necessary size. (I didn't even need to use pi to calculate that.) Further to the problem is that the INSIDE diameter of the bladder would be fixed (ie unzipping the adjustment panel can only make the outer circumference/diameter of the bladder larger... as the tank would be in the way on the inner portion of the wing.)

Here's a deflated bladder next to the wing that it goes in. As you can see, it's already significantly taller and wider than the outer wing. Now imagine that you want a bladder that has the ability to DOUBLE in volume when you want to unzip the expansion panel... it would need to be significantly larger than what's pictured.

As you can imagine... a much larger bladder would need to fold significantly to fit into the "smaller" wing, with a great potential to trap air.