What water sees.

[rx7diver]

I can't comment on the traditional backplate. I went from a back inflate BCD to a freedom plate with wing. The single biggest impact on stability I have felt was the addition of a crotch strap. That was like finding religion.

undrwater,

And the biggest impact on my own "stability" occurred a couple of months ago when, for the first time, I dove in open water without a BC! (And I've been diving for ~30 years!)

Smallish tank (Al 63, OD = 7.25"), double hose regulator and VDH backplate w/ crotch strap, and tee shirt and swim suit. Local quarry. Completely changed my notion of minimal drag/resistance/control/stability. From now on, the relative performance of my various open water rec diving configurations will be measured against that singular experience! I subsequently dove my kids' sans-BC configuration (OMS LP 46, Scubapro Mk 10/D400 and plastic backpack) in open water. Same remarkably liberating experience. Even more so, in fact.

In fact, I just took my OMS LP 66 (OD = 6.9") out of mothballs with the intention of using it as a no-BC tank when wearing my O'Neill 3/2 jumpsuit. It came back last month with a fresh hydro!

Safe Diving,

rx7diver

 

[Lorenzoid]

I just came across this thread. I don't understand the title of the thread, "What water sees," which is probably why I overlooked it a week ago. This thread seems to be about modelling drag. You might get more participants if you chose a more descriptive title.

 

[nolatom]

I'm hearing what I want to hear, since it arguably favors being a cheapskate and sticking with my old rookie gear, jacket BC and all.

This is like the NYTimes Sunday magazine. Stuff you thought was cheap-ass is now "minimalism", or "industrial chic", and sells for big dollars downtown.

So, cool! I'm hydrodynamic and haven't spent any money. I also haven't read the whole thread all that carefully, since it might disabuse me of what I want to hear ;-)

"What the water sees", is a cheapskate non-gearslut occasional diver who is probably still slow, but happier now.

 

[rx7diver]

I'm hearing what I want to hear...

nolatom,

I just laughed aloud, REAL LOUD! People are staring. Thanks for that.

Safe Diving,

rx7diver

 

[rx7diver]

I found time to revise my graphic further: I corrected yesterday's Al 19. (My source code had an error. Evidently dividing by two remains something I can mess up!) And, for additional perspective, I put back in the other bailout bottles people commonly refer to on here: Al 19, old Al 30, Al 40 (same diameter as new Al 30), and Al 80.

By the way, tank diameters (for back tanks and bailout bottles) in my graphic can be found on the Huron Scuba tank specification page (for example).

Safe Diving,

rx7diver

 

[rx7diver]

And the final version of my graphic is:

... at least for now.

Safe Diving,

rx7diver

 

[MrSig]

@MrSig , i was looking into drag coefficient as it relates to cars. As a layman (No engineering background), I had certain expectations what a low drag coefficient car would look like. I was surprised. I don't know if that can be generalized to fluid dynamics or not.

undrwater - Those are fun things to research (low drag coefficient cars)! That can be directly compared to this topic as you would calculate they drag coefficient of a car moving through air the same way you would calculate the drag coefficient of a diver through water. Same equations just a different fluid density. It may have seemed straight forward to me as by the time I took fluid dynamics the drag coefficient equation had been beat into me in a few other classes...

 

[REVAN]

This thread is similar to @REVAN 's related to minimalism. I'm curious if he's looked in here.

I just noticed this thread. It's good to see that some scuba divers are starting to think about and promote the subject of diver hydrodynamics.

The papers I wrote on the subject are posted on my website here:
DOL-Fin Technology - Smith Aerospace

If you want the abbreviated outcome, the performance addendum is a short summary of the differences measured:
http://smithaerospace.us/images/Beyond_Drag_Addendum.pdf

undrwater - Those are fun things to research (low drag coefficient cars)! That can be directly compared to this topic as you would calculate they drag coefficient of a car moving through air the same way you would calculate the drag coefficient of a diver through water. Same equations just a different fluid density. It may have seemed straight forward to me as by the time I took fluid dynamics the drag coefficient equation had been beat into me in a few other classes...

Comparing to car aerodynamics is not going to be very straightforward. Divers have a lot of interference drag that cars typically do not have. Calculating the drag will go beyond just calculating the frontal area, as the area of the interference is also equivalent frontal area. Also, there is more than one frontal area to consider. A diver is more like a succession of multiple bluff bodies traveling together with some unknown amount of drafting occurring between them.

When I built my streamlined kit, one of my main design drivers was to minimize the interference drag. I closed the gap between my head and tank, and then closed the gap between my tank and back also. With the inclusion of a pressure-drag recovering tail cone, I managed to make my scuba kit a hydrodynamic ghost. I measured an 85% reduction of drag for the scuba equipment as compared to an off the shelf BP/W single tank configuration. It is very close to diving like a freediver that happens to be able to breath underwater; it's a very liberating experience.

 

[rx7diver]

Also, there is more than one frontal area to consider. A diver is more like a succession of multiple bluff bodies traveling together with some unknown amount of drafting occurring between them.

REVAN,

So, in my sketches, even though it appears that the diver's head is keeping the water from "seeing" part of the top (crown) of the back-mounted tank, the reality is, the water is still seeing—to some degree—this part of the tank. Is this what you're referring to in this quote? Is this why your top-of-tank faring works? To prevent this?

Safe Diving,

rx7diver

 

[REVAN]

REVAN,

So, in my sketches, even though it appears that the diver's head is keeping the water from "seeing" part of the top (crown) of the back-mounted tank, the reality is, the water is still seeing—to some degree—this part of the tank. Is this what you're referring to in this quote? Is this why your top-of-tank faring works? To prevent this?

Yes. The wake zone behind the diver's head will exchange momentum with (diffuse into) the external freestream. The larger the gap, the more diffusion will happen. If they were very far apart, there would be no drafting effect. My fairing basically closes the gap for maximum drafting effect. By minimizing wake diffusion, it also keeps the boundary layer along the length of the tank thin and highly energized so that the tailcone can recover most of the energy that goes into displacing the flow around the tank in the first place. The result is that the effective hydrodynamic frontal area of my tank is actually smaller than the tank's physical frontal area. That is why the drag for this kit is so low.