Too much lift?

[RayfromTX]

That was the case. No clue where the leading decimal point got off to.

I went back and reread the entire thread, and I did get rather off-topic. Sorry, everyone. If someone wants, we can have a tech-related discussion in another thread.

John

Go for it.

 

[BDSC]

I haven't done any measurements for a 17 pound wing, but the difference in drag between a 40 pound wing and a harness without a wing is about 40% for the equipment alone, and about a 20% difference for the diver with equipment as a total.

I would be curious as to how you actually measured this. The reason I ask is that for years we have seen debates on the drag of one BC vs the drag of another and yet no one has ever seen or referenced an actual study where the drag was measured of BC's or at least I never saw an actual link to a real test method. It was always based on feelings.

 

[REVAN]

I would be curious as to how you actually measured this. The reason I ask is that for years we have seen debates on the drag of one BC vs the drag of another and yet no one has ever seen or referenced an actual study where the drag was measured of BC's or at least I never saw an actual link to a real test method. It was always based on feelings.

I made a tow system in my 20 meter pool. An operator pulls the tow line with a fish scale at a fixed force, like 20 pounds. This tow line pulls the diver through the water at some achieved speed. Another operator times the towed diver between fixed markers to calculate the achieved average speed. Note that this measurement must be done after the towed diver's speed settles to a relatively constant value. With average speed and force you can calculate a drag coefficient. This measurement gets taken multiple times and averaged to knock out the variables and noise in the measurements. It gets pretty time consuming.

 

[BDSC]

I can imagine it would be. Thanks for sharing.

 

[Chavodel8en]

To the OP, for what its worth, I use the same back-inflate for local NorCal diving (in a 8 mm wetsuit) & sometimes for tropical vacations. So it is possible, although there are drawbacks.

Its a Genesis "Tropics" BC. The name seems like a misnomer as it has 34 lbs of lift, which is fine for NorCal, but way more than what you'd need for tropical diving. Its not bungeed.

As I am a cheap bastard (bought it on clearance for $220) I tend to use it for all my diving. I like that it has two utility pockets. Its bigger than a travel BC packing wise -- that's a drawback.

However, I'd be interested in determining the difference in drag in warm water between my BC and a smaller bladder BC. If it was a significant difference, I might consider shelling out for a BC appropriate to warm water. I don't think it tacos, but I could be wrong.

I would be curious as to how you actually measured this. The reason I ask is that for years we have seen debates on the drag of one BC vs the drag of another and yet no one has ever seen or referenced an actual study where the drag was measured of BC's or at least I never saw an actual link to a real test method. It was always based on feelings.

Very much agree with the last 2 sentences. I had always heard the claim that a BP/W had less drag than a stab jacket, but I never saw any numbers to back that up. To me, it seems like whether the bubble surrounds your body, or is on your back, it wouldn't make a big difference for drag. If anything, I could see a stab jacket giving someone a seal-like profile (as opposed to I guess a Mickey Mouse head profile), and being beneficial in terms of drag, compared to a BP/W. But Im no scientist or engineer, so I'd like to see smarter folks than I come up with the actual numbers.

And Im not trying to defend stab jackets -- they are not my preference. However, I'd just like to see them discussed based on hard facts, not biased views.

The testing REVAN describes is the first time Ive heard of an actual hard measurement of drag. Good to see. I would love if such testing could tell me if my 34 lb. lift BC is significantly dragging me down compared to a smaller bladder.

 

[BDSC]

So here is my real world thoughts on "drag" when it comes to recreational diving. The entire idea is highly overrated and makes for good conversation on SB. When I am diving I'm not in a race nor am I fighting against some strong current as if my life depends on it. I'm typically going along a reef or a wreck, taking my time, and enjoying all there is to see. I have dove several different BC's since 1991 from jackets to back inflates to now I have a small travel friendly back inflate BC. I can't tell a dimes worth of difference in any of them when I am diving. One does not appear to be any easier or harder to dive than the other. I don't see any improvement in air consumption because I'm somehow working less. I think the idea of drag is important in Nascar, bike riding at the level of the guys who compete in the Tour De France, and maybe Olympic swimming, but recreational diving.....not so much.

 

[ronscuba]

Scuba gear like many things can be adequate, good, optimal. What you buy and use is mainly a matter of how picky you want to be. Now a days, everything will be adequate and most things will be good. Whether or not that is good enough or you want to take the extra effort to optimize is a personal decision.

Little changes can add up to make the overall change move from adequate to good to optimal. Drag, weight, travel size, venting.

Drag: smaller wing, better hose routing, no snorkel, no danglies, better trim, better kicking = less drag, faster, better air usage

Weight/size: smaller/lighter wing, lighter backplate, lighter suitcase = no extra weight charges or ability to bring more stuff

Venting: smaller outer shell, smaller inner bladder = easier venting

Add it up, and the 17 lb wing has advantages. The 30 lb wing will still be good. Is the 30 good enough or do you want the 17 ?

Want, not need.

 

[REVAN]

I have dove several different BC's since 1991 from jackets to back inflates to now I have a small travel friendly back inflate BC. I can't tell a dimes worth of difference in any of them when I am diving.

If all the systems you have dived have fairly similar drag, I'm not surprised that you don't feel much difference, especially when going slow. If a diver doesn't move at all, drag is completely irrelevant. The faster you go the bigger difference the drag will make. If you don't cut any drag, the power required goes up with the cube of the speed. Try to go twice as fast, expect to work 8 times harder to do it.

I definitely notice the 20% drag reduction from diving a harness without a BC, and the 40% drag reduction of my streamlined kit is a night and day difference compared to commercially available scuba systems. Streamlining can work very well. Modern divers have a lot of drag, so there's a lot of low hanging fruit to pluck if anyone cares to go after it.

The main problem is that there isn't really any commercially available gear that is truly streamlined in a meaningful way. When almost everyone insists that streamlining scuba equipment makes no difference and isn't worth using, why would manufacturers lift a finger to improve the hydrodynamics of what divers are already buying from them?

Personally, I care very much about drag. I'm forced to design and build my own streamlined gear because no one else has done it before. I wish I could have just bought it, because making my own experimental equipment is very expensive and time consuming.

This is my latest completed and functioning streamlined kit (dolphin action starts at 1:30):

 

[Streydog]

I removed my comment

 

[tdallen]

There's tons of great feedback in here, thank you. Based on some of the conversation I've been trying to match up lift needs and the buoyancy of my kit.

Follow-up question - what is the average buoyancy of a full 3mm wetsuit and the corresponding loss of buoyancy at 15ft? FYI, I'm a pretty standard body size - 5'10" 190lbs, typically wearing a large wetsuit.