Vote on your preferred BC -

[simbrooks]

Actually there is one thing we are missing from this mechanical analysis. That is the actual resistance of the water to the rotating motion, although water cant hold a shear force, there is friction/resistance of sorts - we feel it every time we move due to the viscosity of the water. There is also the effect of creating a lever arm if you were turning over from things that were in line with the COG (fulcrum about which you are creating a moment couple), thus the weight belt worn on the waist becomes a factor and helps to form a righting action/moment as well. Obviously in RR's example his total weight is about the same in all 3 conditions, not sure if the location/lever arms differ greatly, ie if the weight is placed at the back or front of the belt. In the case of the options presented i would think the steel tank or 9# BP would give the best option above and UW, i am guessing your trim is more out with a 10# weight belt.

 

[-hh]

Nicely presented, i like your methodology. So lets take me as an example with my typical 3mm wetsuit weighting - i need my SS BP (-6#) and about 6# of additional weight with the aforementioned config. So here we have -6#@6" (to my BP) and the other 6#@10" on my tank straps, total net = -96in-lb...

Which would be "face up" in rotation.

To potentially counteract this, we would have:
AL80 empty: (+2lbs)(10" behind diver's center) = +20 inch-lb

So the net to this point would be -60 inch-lb + 20 inch-lb = -40 inch-lb (still face-up)

Now we can add buoyancy from a Wing:
(+TBD lbs)(5"+TBD" behind diver's center) = TBD total

If we say that its located 1" behind the diver and we're going to fill the wing until the torque is zero'ed out, then we would calculate this as:

0 = -40 inch-lb + (TBD lbs)(6")
---> 40/6 = 6.66 lbs of buoyancy

So if the Wing has less than ~6 lbs in it, the diver is still face up.
Between 6 and 7 lbs, he's sitting pretty much upright.
Greater than 7 lbs, the wing's lift will be acting to push him face down.

I still find floating on my back is much simpler and more relaxing

Simply another good example of a YMMV.

I generally hold my UW camera in front of me and since it weighs ~16lbs in air, I prefer to leave it in the water instead of trying to do the "otter with 16lb rock on his chest" routine.

I've also done some live boat diving where the boat pickup (exit) was next to some rocks/cliffs, where the resulting wave reflection makes for a pretty ugly short (and every 3rd wave breaking) chop in which a backfloat was hardly advisable, let alone possible.


-hh

 

[simbrooks]

Which would be "face up" in rotation.

Hence the rest of my post:

...in fact i have a couple that is pointing me face upwards without need for finning.

But thanks for reiterating that my rig does in fact have face up moment - until my body moves far enough over that i am again in balance, or add more air as your next point:

Now we can add buoyancy from a Wing:
(+TBD lbs)(5"+TBD" behind diver's center) = TBD total

If we say that its located 1" behind the diver and we're going to fill the wing until the torque is zero'ed out, then we would calculate this as:

0 = -40 inch-lb + (TBD lbs)(6")
---> 40/6 = 6.66 lbs of buoyancy

So if the Wing has less than ~6 lbs in it, the diver is still face up.
Between 6 and 7 lbs, he's sitting pretty much upright.
Greater than 7 lbs, the wing's lift will be acting to push him face down.

We already assumed 8# buoyancy in the original calculation:

First, let's assume that the BC is providing +8lbs of lift and the diver's still vertical, such as he would be at the surface, where its generally desirable to not be face-down....

which makes this a total wing buoyancy of ~14-15#, which is about a half full normal sized single tank wing - mine is 34#, others are nearer 27-30#. I rarely pump up my wing that much UW or above, imagine what would happen if i added a steel tank - i wont mention adding two of them as i know the answer there - i still dont rock backwards or forwards as i can keep it in balance or flop on my back.

I understand there are situations where you cant go on your back, around FL you pretty much can anyway, i agree YMMV.

 

[RiverRat]

There is also the effect of creating a lever arm if you were turning over from things that were in line with the COG (fulcrum about which you are creating a moment couple), thus the weight belt worn on the waist becomes a factor and helps to form a righting action/moment as well. Obviously in RR's example his total weight is about the same in all 3 conditions, not sure if the location/lever arms differ greatly, ie if the weight is placed at the back or front of the belt. In the case of the options presented i would think the steel tank or 9# BP would give the best option above and UW, i am guessing your trim is more out with a 10# weight belt.

Yeah, I was worried about messing up my trim taking the AL plate on a trip and using the 10# belt. But I was having so much fun I never noticed any major trim problems. It would be nice if I could get some pics of me diving that rig to compare. I did notice stability issues however when my belt slipped to the front/side. You could feel the opposing forces. Keeping the weight in the small of my back has been key.

 

[-hh]

Actually there is one thing we are missing from this mechanical analysis.

There's actually a lot missing. I simplified this down to Sophomore Year "Statics".

...resistance of the water to the rotating motion, although water cant hold a shear force, there is friction/resistance of sorts - we feel it every time we move due to the viscosity of the water.

True, but that mostly just determines how long it takes for the 'face down' rotation to occur, not "if" it will occur. The buoyancy and gravity forces are acting constantly, so they require an equally constant force to counteract them for the simpler "Steady State" situation.

There is also the effect of creating a lever arm if you were turning over from things that were in line with the COG (fulcrum about which you are creating a moment couple), thus the weight belt worn on the waist becomes a factor and helps to form a righting action/moment as well.

Don't use a more complex model when a simpler one will do

Cranking up the model's refinement, we can ignore the location of the weightbelt if we assume that the diver and the rest of his negatively buoyant widgets is a rigid system: he can be reduced to a point mass. However, you are on track to the very important recognition that our "weight" and "buoyancy" values are acting along the local gravity vector, which means that our moment arms lengths are the value of the perpendicular (horizontal) distances between them and that this will change as the system is rotated. This has three implications:

First, at "some rotation", the CB (Center of Buoyancy) will be directly above the CG, so the Moment Arm magnitude is zero, so there will be no rotating torque: this is as stable as a hot air balloon above the whicker basket. Coincidentally, this also happens to be the general configuration of a Back Inflate BC, which means that arguements that it is an easier configuration to dive are generally true...at least for that portion of your dive where you're horizontal. While this may be more comfortable during a dive, it does not necessarily mean that its more comfortable --or safer-- when vertical on the surface after the dive.

Second, since the "diver horizontal" and the "diver vertical" orientations are perpendicular to each other, the theoretically ideal system is the one with the greatest insensitivity to orientation change, which means that the distance between the CB and CG has been minimized for _all_ possible rotations. This infers that the Jacket is the more desirable design solution, although we do also have the option of moving around ballast, so we can also shift around the CG too, some. This helps to define our trade-off's.

Third, astute engineers will point out that there's a "second rotation", 180 degrees from the first, where there is also no rotating torque because of CB-CG vertical allignment. While technically true, this is also a point of dynamic stability because the mass is located directly above the buoyancy! Try diving with a horsecollar to get a feel for the effect, preferably a deep coldwater dive in a wetsuit so as to get enough compression to need to pump up the HC.

Obviously in RR's example his total weight is about the same in all 3 conditions, not sure if the location/lever arms differ greatly, ie if the weight is placed at the back or front of the belt. In the case of the options presented i would think the steel tank or 9# BP would give the best option above and UW, i am guessing your trim is more out with a 10# weight belt.

Yes, but if this is a diver on vacation somehwhere, the Hotel Resort is going to have AL80's, not Steels, so that option's not available. Similarly, a weighted BP may "have to be" be left at home in order to be able to carry UW photo equipment within the airline's allowed weight budget, although replacing it with 10# in the small of the back will result in a relatively small net change. Both are YMMV's in the trade-space.


-hh

 

[simbrooks]

There's actually a lot missing. I simplified this down to Sophomore Year "Statics".

Glad you are keeping this simple - helps people to keep up with the long posts I wont mention my occupation, years of studying and working within such parameters, but apart from the length of time i would say we are "similarly" educated, at least on terms of types and disciplines of degrees.

True, but that mostly just determines how long it takes for the 'face down' rotation to occur, not "if" it will occur. The buoyancy and gravity forces are acting constantly, so they require an equally constant force to counteract them for the simpler "Steady State" situation.

IF the moment couple is enough to rotate you this will cause a retardation in the motion, but just like friction there is a resistive force up to that certain point.

Second, since the "diver horizontal" and the "diver vertical" orientations are perpendicular to each other, the theoretically ideal system is the one with the greatest insensitivity to orientation change, which means that the distance between the CB and CG has been minimized for _all_ possible rotations. This infers that the Jacket is the more desirable design solution, although we do also have the option of moving around ballast, so we can also shift around the CG too, some. This helps to define our trade-off's.

I agree with your first point, the second i also agree with - keeping the lever arms or forces smaller is desirable. Thus the jacket is better on paper when you are vertical - shame the same cant be said horizontal UW. I think with my weight placements i have achieved a good combination (without considerable compromise) in vertical, horizontal and lateral balance - others may not have.

Third, astute engineers will point out that there's a "second rotation", 180 degrees from the first, where there is also no rotating torque because of CB-CG vertical allignment. While technically true, this is also a point of dynamic stability because the mass is located directly above the buoyancy! Try diving with a horsecollar to get a feel for the effect, preferably a deep coldwater dive in a wetsuit so as to get enough compression to need to pump up the HC.

Whilst engineers or anyone with some common sense can see this as theoretically possible, none would design this way, as you say its a dynamic stability, one minor adjustment and it all falls apart (turns over), whereas with the CB above the CG the system will always try to return to that zero moment orientation.

Yes, but if this is a diver on vacation somehwhere, the Hotel Resort is going to have AL80's, not Steels, so that option's not available. Similarly, a weighted BP may "have to be" be left at home in order to be able to carry UW photo equipment within the airline's allowed weight budget, although replacing it with 10# in the small of the back will result in a relatively small net change. Both are YMMV's in the trade-space.

In this case it wouldnt be possible, of course if you can get some tank strap weight pockets and can rent (or seeing weight is cheap, just buy - maybe $16 for 8#, sounds like a bargain vs a week of diving costs) some weight and put it how you like it behind your back - voila, all sorted
Some of us dont have the luxury of being able to take off for tropical locales to try this out, but some of us do live in near tropical regions and can bring our own gear how we like it

 

[ChrisA]

I agree with the below but I think _useage_ statistics would be higher than _onersship_ statistics. So let's say we change the question slightly. Rather then ask what percentage of _divers_ own backplate style BCs let's ask what percent of _dives_ are done with backplace style BC's. You can't determine that ratio my looing at sales statistics. I think the large majority of divers don't dive very much, just a few times a year on vacaton or maybe even every few years. So go out on the local boats and beachs and count what is the ratio there.
I see maybe about one in ten using BP/W setup. A fairly high ratio.

I think sales figure might be misleading becasue my guess is that most gear is sold into people's closets and garages and sees very little use. In my local area (So. Cal) there were several hundred OW graduates each year. Where _are_ all those folks? If they all went diving after OW the local beaches and charter boats would be full up.

There are 38.4k registered ScubaBoard members. Out of those 38.4k, I would GUESS that less than 5% of them visit the board regularly.
(disclaimer: I said that is MY GUESS - Tech or Doc, you have any actual stats on this?)

Out of 38.4k, you have a small portion of people who are BP/W advocates, but they post often, and are passionate about the BP/W, which makes it appear that there is a HUGE BP/W faction on ScubaBoard. In reality, this my not be the actual case.

Scott

 

[-hh]

...apart from the length of time i would say we are "similarly" educated, at least on terms of types and disciplines of degrees.

I saw the 'I R Ngineer' in your profile . Since this isn't the first time this type of discussion has come up, I decided to finally update my own profile on the subject.

IF the moment couple is enough to rotate you this will cause a retardation in the motion, but just like friction there is a resistive force up to that certain point.

I agree. However, I do have to admit that I'm (surpisingly) uncomfortable with your choice of words in using classical friction between solids as your analogy. I think that the reason I'm uncomfortable is because this also classically includes a high initial resistance (static coefficient) that drops off..."high to low"...whereas resistance in liquids is classically the opposite: "low to high" .

I agree with your first point, the second i also agree with - keeping the lever arms or forces smaller is desirable. Thus the jacket is better on paper when you are vertical - shame the same cant be said horizontal UW.

Actually, it can be very good horizontal too...the devil's in the details: you just need to make sure that its bladder is not high around the shoulders. My observation of the design of many modern Jackets is that they've done well in shifting the bulk of the lifting bladder downwards, to a "Cummerbund-like" configuration and location, which does a pretty good job of overlaying the weightbelt region as well as the human body's natural centroid, and reducing that moment arm.

Jackets do have the trade-off of the oft-mentioned potential for squeeze when fully inflated, although IMO this problem is probably more often due to improper sizing than design on modern Jackets: it may have fit perfectly fine five years ago, but IIRC, the average adult gains something like 3lbs/year, so after 5 years, there's ~15lbs worth of additional (cough) "girth" that's factoring into those "squeeze problem" complaints.

Whilst engineers or anyone with some common sense can see this as theoretically possible, none would design this way, as you say its a dynamic stability, one minor adjustment and it all falls apart (turns over), whereas with the CB above the CG the system will always try to return to that zero moment orientation.

But that's exactly what the old Horsecollars were that many of us dived with for years. Personally, I didn't mind the "topheavy" feel (think of it being like this) as much as I loathed its crotch strap (BC strap webbing was a lot narrower back in those days)

Some of us dont have the luxury of being able to take off for tropical locales to try this out, but some of us do live in near tropical regions and can bring our own gear how we like it

"Travel gear" is just another design constraint for consideration.



-hh

 

[meisburger]

Thanks Guys. Although I don't understand much of your discussions, I enjoy them, and I feel the forces are strong within me, and I think I need to buy a horsecollar....

Tim

 

[simbrooks]

I saw the 'I R Ngineer' in your profile . Since this isn't the first time this type of discussion has come up, I decided to finally update my own profile on the subject.

I had a quick look at yours too... its good to know roughly where folks are coming from on this type of subject.

I agree. However, I do have to admit that I'm (surpisingly) uncomfortable with your choice of words in using classical friction between solids as your analogy. I think that the reason I'm uncomfortable is because this also classically includes a high initial resistance (static coefficient) that drops off..."high to low"...whereas resistance in liquids is classically the opposite: "low to high" .

I really dont deal with friction as much as you probably do, the finesse in its definition etc, just using a bit of artistic license in my description to give it a simple feel. There is definately a resistance to movement and therefore rotation. I do deal with liquids most days, but static mostly. It probably wasnt the best example/analogy, but it got the point across.

But that's exactly what the old Horsecollars were that many of us dived with for years. Personally, I didn't mind the "topheavy" feel (think of it being like this) as much as I loathed its crotch strap (BC strap webbing was a lot narrower back in those days)

Exactly the point you were making in the earlier post and i could see the mechanics of that, in fact a ball exactly on the top of the apex a hill was my mental image and the ball in a valley for the usual design format of UW systems - it is self-righting. If you take the girl's arms and legs off the floor she now has a very delicate balance to maintain to stay on top of the ball, which would be the same if all your buoyancy were below you. I take it that it was easier to dive supine with a HC instead of BI? Not that its that hard, just a bit more air in the wing and watching your depth a bit.

"Travel gear" is just another design constraint for consideration.

I took my BP/W, regs, wetsuit, booties, fins, mask, HID and a few other bits in a large backpack to SoCal last summer, easily fit in the overhead locker and not so heavy either - maybe 15-18# IIRC. If i had expensive photo gear too i would have put the wetsuit and such in the hold, kept the regs up with me for sure. I found it easy to pack away, no extra padding taking up space, folds nice and flat and not that heavy compared to most jacket and BI BC's out there - between 6-9# for those