DPV, rebreather and stages drag

[Snusmumrik]

Not sure where to post story and question, but here it is.
Hope someone share their opinion.
Last weekend i enjoy rare occasion to ride DPV in cool deep cold place with uncommon visibility of 5-7 meters at 20m depth.
And I noticed that my bailout stage, 7l neutral buoyancy steel hanging on left side D-rings cause significant drag.
I held DPV handle by my right arm and all my ride DPV tends to turn left.
I need to pay attention to compass and must instantly adjust heading to the right to keep ride straight.
Yes, i'm thinking about it and presume it's all about streamlining my setup, possibly moving stage to butt D-ring,
but hope someone share their DPV+stage(s) expirience and give some advices.
Moving stage to butt D-ring is a hard decision, due to main DIL and balout reg feeds are plugged into this stage.

 

[Dsix36]

I don't think there is any way to completely eliminate the drag from a stage. I notice it also and although keeping it as tight as possible helps, it does not eliminate the drag (and tendency to cause direction error).

 

[Garth]

You mentioned steel. If this is a stage, could you switch to aluminum for stage?
I use steel sidemount same on both left and right with opposing valves pulled back by bungee so they are inline with my body.
Any stages are clipped to the butt usually.
Drag is not reduced by having two equal cylinders on both sides but it definitely helps with unequal drag which I find is the drag that annoys me the most.

 

[ofg-1]

I held DPV handle by my right arm and all my ride DPV tends to turn left.

You have some effects from the torque of the prop as it turns. In an airplane you adjust for this by pressing the right rudder.

 

[Garth]

Do you have your stage rigged like your profile picture?

 

[broncobowsher]

Looking at your profile picture, that is low hanging. I found a bit of bungee to suck that valve up into your armpit will do wonders for drag. Typically sidemount will do that to keep the tanks under control. Just doing that I dropped 2-3 gears on my Blacktip for the same cruising speed, using others on scooters that didn't change anything else for reference.

 

[Snusmumrik]

You mentioned steel. If this is a stage, could you switch to aluminum for stage?

I mention neutral steel.

Looking at your profile picture, that is low hanging.

Thanks, but that's pretty old picture - 2010. Things changed, now there's different reb and different stage cyl.
Sorry, no underwater pics due to i always dive solo.

 

[Nick_Radov]

I mention neutral steel.

I'm not sure what you mean by "neutral steel". We use Luxfer aluminum 40 / 80 tanks as stages because they're positively buoyant when empty and thus minimize drag. If possible add some helium to the gas mix in order to minimize gas weight.

 

[lostsheep]

I'm not sure what you mean by "neutral steel". We use Luxfer aluminum 40 / 80 tanks as stages because they're positively buoyant when empty and thus minimize drag. If possible add some helium to the gas mix in order to minimize gas weight.

That doesn’t make any sense unless you’re saying the orientation of an empty cylinder has lower drag ; drag is not affected by the mass/buoyancy. Drag is only affected by geometry of the cylinder and orientation with respect to the diver and/or direction of travel.

I am skeptical a butt floaty tank, that has the long axis of a cylinder perpendicular to the direction of travel , has lower drag than a cylinder with the long axis parallel to the direction of travel.

I strongly suspect (but only have anecdotal evidence) SM style rigging with cylinders parallel to the body and tucked up high behind the arm pits, has the least drag.

Bottom line is, without actual testing we’re all taking WAGs. Starting with the assumption you want a minimal cross sectional area when viewed from the front is a good idea; butt floaty tanks aren’t doing that.

 

[Nick_Radov]

That doesn’t make any sense unless you’re saying the orientation of an empty cylinder has lower drag ; drag is not affected by the mass/buoyancy. Drag is only affected by geometry of the cylinder and orientation with respect to the diver and/or direction of travel.

I am skeptical a butt floaty tank, that has the long axis of a cylinder perpendicular to the direction of travel , has lower drag than a cylinder with the long axis parallel to the direction of travel.

I strongly suspect (but only have anecdotal evidence) SM style rigging with cylinders parallel to the body and tucked up high behind the arm pits, has the least drag.

Bottom line is, without actual testing we’re all taking WAGs. Starting with the assumption you want a minimal cross sectional area when viewed from the front is a good idea; butt floaty tanks aren’t doing that.

Fair enough, I haven't conducted a quantitative speed test. But if you pay attention while riding a scooter you can feel points of drag and subjectively the more buoyant stages seem to exert less backwards force. With standard stage rigging kits they have some freedom to move around and will naturally swing into the lowest drag position once you get moving.