Center of gravity?

[Capnsnott]

I know this get's into DIR dive theory, but I need to understand it.

Just so we are on the same page, here is what i'm using for visual referance.

If a person is standing with all of their gear on, single tank.

Where should the center of gravity be in reguards the imaginary linear plane made up between the backplate and body of the diver? Foreward would be toward the diver, back would be toward the tank.

I see a lot of posting where the weights are in a pouch under the backplate. Is there any weights to be in front of the diver, or is the equipment being carried in front part of the center of gravity equation? Thus no extra weight is needed to maintain the CG location.
Thanks
Greg

 

[Bob3]

You need to be concerned with the metacentric height, a function of the center of gravity & the center of bouyancy.
Here's a little reading on the topic for ya: http://www.warships1.com/index_tech/tech-009.htm

 

[Capnsnott]

Ok, I understand the theories behind the different centers and how they change.

I have to deal with CG, Roll axis, squat/anti squat axis as well as other problems in building rock buggy suspensions, so I have a decent working knowledge of the principals involved.

Maybe I am reading too much into this, I assume that weight placement is pretty much standard using the Halcyon BC setups.
thanks for trying to help me.
Greg

 

[Diver0001]

Ok, I understand the theories behind the different centers and how they change.

I have to deal with CG, Roll axis, squat/anti squat axis as well as other problems in building rock buggy suspensions, so I have a decent working knowledge of the principals involved.

Maybe I am reading too much into this, I assume that weight placement is pretty much standard using the Halcyon BC setups.
thanks for trying to help me.
Greg

You're making it too complex. There are only two things you really need to consider.

1) Do you have the right amount of ballast?
2) is it positioned correctly?

On point 1 you learn how to do this in your OW course and on point 2 is a question of trial and error. You know what your goal is (horizontal trim) and there are only a 1/2 dozen places where it makes sense to add/remove weight......

R..

 

[Doc Intrepid]

Maybe I am reading too much into this, I assume that weight placement is pretty much standard using the Halcyon BC setups.

Greg,

IMO, depends on what you mean by 'reading too much into this'. But the short answer is that 'weight placement' is not at all standard using the various Halcyon BC setups.

Much depends on the body of the diver(s) in question, as well as the environmental conditions where they dive. Some divers require an aluminum backplate and a weightbelt. Some use a V-weight or a P-weight (channel weight). Some use a keel weight. Some use heavier - 9, 12, or even 15 lb - SS backplates. The amount and placement/location of weight required to achieve 'optimal trim' varies depending on a number of different circumstances, which can be different for each diver.

[Caveat: I am not a DIR expert.] Given that "Center of Gravity" is a theoretical point corresponding to the intersection of three axes: longitudinal, lateral, vertical. Oriented such that 'longitudinal' refers to diver's head to feet; 'lateral' refers to axis corresponding approximately to diver's hips; 'vertical' refers to vertical axis running through water column in which diver is oriented, intersecting with both of the previous two axes.

For maintenance of appropriate trim under constantly varying conditions, the diver rotates around only one center of gravity. There is only one theoretical point = COG. DIR-F introduces exercises designed to familiarize diver with varying body positions allowing proper trim to be achieved.

In reality there will be (some) variance between the same diver using a set of steel doubles with (a) a 6 lb SS backplate and 10 lb V-weight; (b) a 15 lb SS backplate; not to mention (c) an aluminum backplate and an 14 lb weightbelt.

This is because there are two longitudinal axes, the diver's, and then the 'axis' of the tanks and trim weight.

In each case the diver is wearing 15/16 lbs. But there is a difference between the longitudinal axis of the diver's body, and the logitudinal axis of the set of tanks/weight. With a 10 lb V-weight, the "axis of the tanks" is as much as 8" or so higher off the axis running down the center of the diver's (horizontal) body. With a heavy backplate that same 10 lbs is flat against the diver's back - separated from the longitudinal axis of the diver's body by only some 4" or so. In one sense, thats a 50% difference in 'relative stability'.

Is the variance likely to make much of a real-world difference? Depends on who you ask.

Still, DIR authorities such as George Irvine have commented on the fact that the variance exists. (I'm still trying to find a citation noting that a heavy plate can offer more stability than a V-weight.) My opinion is that in a rather open-ended effort to achieve "optimum effectiveness', this difference is at least worth considering when trying to determine the amount and location of weight that results in optimum weighting, bouyancy, and trim for YOU.

My humble opinion. YMMV.

Doc

 

[Capnsnott]

that was very helpeful, thanks.

 

[Arnaud]

You also need to consider the type of tanks you're using. The PST 104s are considered the most balanced, while, for instance the 95s are "head-heavy". Obviously, how your rig the tank (high/low), will also have an impact.

 

[GSmith]

Recently took a bouyancy class put on by a GUE DIR-F instructor candidate. It appeared that the class closely parralled the intro to DIR-F, covering buoyancy and trim & propulsion.
To start with, remembering the concept of staying in "proper position" helps to establish a constant/ baseline of how you will sit in the water with your "rig".
Proper position as we were taught is very simular to lying on the floor in a prone position as if you are watching TV. Your arms extend the horizontal plane of your body out in front of you, overlaying your hands with your elbows bent. This brings your compass, depth guage/timer and any backups into plan view(like the dashboard of your car) and serves to break the column of water your body is passing thru. On the other end, your knees should never drop below the plane of your torso, and your lower legs are bent upwards. We would do an excercise where we would practice frog, and flutter kicks while on the bottom, to try and break our habit of dropping our knees.
At the beginning of this class, with an single HP100, 6#BP and 6#'s on the back of my hips, I kept on wanting to roll over on my back, because I was so top heavy (I think I've heard Seajay refer to this as Turtling).
Moving the 6#'s of lead on a belt to the front of my pelvis, made things work allot better. But my trim was still a little screwy. Sliding my tank down so I could still reach my valve and avoid having the 1st stage planted in the back of my head, left me knose up even with my weight belt slid up high. To correct this I reset my wing lower on the backplate. I still have to tweek my belt up and down over the course of a tank running from full to empty.
Hope that helps!

 

[donacheson]

Greg,

...

[Caveat: I am not a DIR expert.] Given that "Center of Gravity" is a theoretical point corresponding to the intersection of three axes: longitudinal, lateral, vertical. Oriented such that 'longitudinal' refers to diver's head to feet; 'lateral' refers to axis corresponding approximately to diver's hips; 'vertical' refers to vertical axis running through water column in which diver is oriented, intersecting with both of the previous two axes.

For maintenance of appropriate trim under constantly varying conditions, the diver rotates around only one center of gravity. There is only one theoretical point = COG. DIR-F introduces exercises designed to familiarize diver with varying body positions allowing proper trim to be achieved.

In reality there will be (some) variance between the same diver using a set of steel doubles with (a) a 6 lb SS backplate and 10 lb V-weight; (b) a 15 lb SS backplate; not to mention (c) an aluminum backplate and an 14 lb weightbelt.

This is because there are two longitudinal axes, the diver's, and then the 'axis' of the tanks and trim weight.

In each case the diver is wearing 15/16 lbs. But there is a difference between the longitudinal axis of the diver's body, and the logitudinal axis of the set of tanks/weight. With a 10 lb V-weight, the "axis of the tanks" is as much as 8" or so higher off the axis running down the center of the diver's (horizontal) body. With a heavy backplate that same 10 lbs is flat against the diver's back - separated from the longitudinal axis of the diver's body by only some 4" or so. In one sense, thats a 50% difference in 'relative stability'.

Is the variance likely to make much of a real-world difference? Depends on who you ask.

Still, DIR authorities such as George Irvine have commented on the fact that the variance exists. (I'm still trying to find a citation noting that a heavy plate can offer more stability than a V-weight.) My opinion is that in a rather open-ended effort to achieve "optimum effectiveness', this difference is at least worth considering when trying to determine the amount and location of weight that results in optimum weighting, bouyancy, and trim for YOU.

My humble opinion. YMMV.

Doc

A couple of picks:

(1) The COG of a diver and his equipment is a single point and one may consider axes of rotation passing through this point. It's not necessary to consider separate COG's, and hence separate axes of rotation, for the diver and his equipment. However, the COG is not fixed throughout the dive. As a diver breathes down his tanks, the weight of the gas in the tanks decreases, shifting the COG, albeit probably slightly for a properly trimmed diver. A much larger change in COG would occur when a diver tucks his knees to his chest or even extends one arm out to the side. Of course, in the latter cases, but not the former, the center of buoyancy (COB) will move as well in the same direction as that of the COG.

(2) The 4-inch difference you refer to is not a 4-inch difference in COG; that's much, much less. In any case, IMHO fore-and-aft (head down or feet down) or pitch trim is far more important the left/right or roll trim. Humans have bilateral symmetry and a diver's equipment is usually pretty symmetrically distributed, so being a bit "top heavy" means the COB may be over the COG for a face-down diver, but a tandency to roll is easily controlled with fins and, if necessary, hands.

There's one last comment I'd like to make. While metacentric height is a critical parameter in the stability of floating objects like ships, I'm not sure (but NOT positive) that it's important for totally immersed objects such as divers, but I'll defer to the submarine designers. It seems to me the things of most importance to divers are that the diver's total weight (diver + all equipment) and buoyancy are very close to equal and that the COG and COB are close to each other, especially in the fore-and-aft direction.

 

[SeaJay]

I recently approached the difficult task of trying to express the above in very simplistic terms. The results of my efforts are here: http://www.DeepSouthDivers.org/homebc.html The article was designed to express the basic premise of "CofG" and "CofB" and how a diver would benefit from keeping the two points (which we all seem to agree on in terms of definition) as close together as possible. In an effort to express why I chose a certain gear configuration, the article is also slanted to explain (in very general terms) how the gear I chose does that, exactly.

I've had a slew of reactions to the article; most of them positive, but some of them arguing that many factors are not accounted for in the oversimplified illustrations.

Some of the variables that were not accounted for:

1. Placement of "trim weights" in "standard" BCs.
2. Changes in CofB due to emptying tanks.
3. Changes in placement of CofG due to gear such as heavy fins, ankle weights, etc.
4. Changes in placement of CofB due to wetsuits of different thicknesses.
5. Placement of "P weights" or "V weights," which skew the CofG.
6. Tanks of different buoyancy characteristics.

No doubt all of these arguments are correct... That is, ANY variable is likely to change the "overall picture" of where the CofG and CofB are, and different techniques need to be utilized to achieve close CofG and CofB.

...But that's not the point of the article. The point of the article isn't to express an exact measurement of every gear configuration that a diver might choose, it's to express the idea and enable a diver to estimate his/her own positions of the CofG and CofB so as to "tune" them close together.

No matter what the gear configuration is, even the diver's breathing will change the position of the CofB to some degree. A full breath will make the CofB move towards the diver's head, while a good exhale will move the CofB towards the diver's feet. In DIR-F (mentioned only because your question specifically pertained to the dive style), negatively buoyant fins are recommended so as to allow the diver to change the position of their CofG simply by bending their knees. Bring the fins closer to the butt, and the CofG moves towards the diver's head... Straighten the leg, and the CofG moves down the diver's torso - slightly. This is how a DIR diver can change their attitude in the water without finning or moving their hands.

That said, you'll have to figure out where your own CofB and CofG are, based on all of the above information. Keep in mind that the naked body has it's CofG much lower than it's CofB... Due to the massive buoyancy of the lungs - which changes about 9 pounds from inhale to exhale. This is why they make "standard" backplates as 6 pounds... It is a nice "medium" for offsetting the buoyancy of the lungs, thereby making the body more "balanced" in the water.

Hope that helps.