A recent thread Drop the freaking weights got me thinking about proper buoyancy and trim in terms of simple physics. Ive come to the conclusion that to be properly weighted you MUST be in horizontal trim.
What physics are involved? For starters I simplified the problem by ignoring any hydrodynamic forces, the diver moves too slowly for these to play an important role. Secondly this applies to a moving diver. A diver who is not moving need not be in any particular trim to stay neutrally buoyant.
Some definitions:
Buoyancy and trim are different.
Buoyancy is the relative density between the entire diver/gear complex and the surrounding ocean. Practically it determines whether the MOTIONLESS diver will ascend, stay neutral, or descend in the water column. Divers adjust their buoyancy before the dive by adding or subtracting heavy (denser than water) objects to their gear. These include tanks (steel vs aluminum), tank harness systems (steel backplate, aluminum backplate, plastic and/or fabric Jacket/back inflate/something other BCD), lights and cameras, and of course lead. Divers adjust their buoyancy while diving by changing the volume of flexible air spaces.
Trim is the stable position the diver ends up in without moving or finning. This is an orientation in 3 axes (pitch, roll, and yaw for you pilots). For this discussion we will only deal with pitch, specifically a horizontal pitch and a head-up or head-down pitch.
Center-of-gravity (CG) and center-of-buoyancy (CB) are not the same.
CG is the point the object will rotate around due to gravitational forces. CB is the point the object will rotate around due to the buoyant forces (for lack of a better term). For our simple physics discussion the diver/gear complex can be reduced to a CG point and a CB point. These points may/may not be co-located. If the CG and CB are not co-located on the longitudinal axis then the diver will end up in either a head-up or head-down trim position. The CG can move by extension or retraction of limbs. The CB can move with a change in the volume of gas in each flexible air space (exposure suit, BC, lungs), and can also change if the same total volume is distributed between all the air spaces differently. In the former the buoyancy also changes.
Buoyancy compensator (BC): ALL DIVERS use some sort of BC while diving. The diver with no exposure suit and nothing other than a tank/regulator/harness is using their lungs as a BC. A corollary to this is that a neutrally buoyant diver that becomes unconscious and stops breathing will start to sink as soon as air escapes the lungs.
The direction a diver travels is simple vector physics and is the addition of all horizontal forces and vertical forces. At swimming speeds and with a typical scuba configuration the hydrodynamic forces (lift) are inconsequential.
Horizontal trim is when the propulsive force (fins) is in a horizontal line with the divers CG. Head-up trim is when the propulsive force is below the CG, and head-down trim is when the propulsive force is above the CG.
THEREFORE: The diver in:
1. Horizontal trim has only a horizontal vector when they kick. They move only straight forward. If they are also neutrally buoyant the diver will neither ascend or descent with kicking.
2. Head up trim has both a horizontal vector and a vertical vector when they kick. They will move both forward and upward unless they have a compensating downward force. This downward force is produced by being negatively buoyant. When they stop swimming they drift down. The greater the head-up trim the more negatively buoyant the diver needs to be to swim at a stable depth.
3. Head-down trim also has both a horizontal vector and a vertical vector (in this case a downward one) when they kick. This diver will move both forward and downward unless there is a compensating upward force. This force is produced by being positively buoyant. If the diver stops swimming they will drift upwards.
A diver in horizontal trim (CG and CB co-located) can move into any position and stay stable in that position unless an outside force moves them. A diver in a head-up or head-down trim (CG and CB in different spots) has to work at staying in any other orientation. As they change position the diver/gear complex rotates around the CG and this either raises or lowers the CB which in turn changes the air volume in the BC(s) and makes that position unstable. Unchecked this diver will either drift up or drift down. Since air will move inside an airspace (and change the CB) if not prevented by the design (the dry suit being the most dramatic) this is a dynamic condition. The closer to the surface the diver is, the more a small change in orientation will have on changing volume. So this effect is more pronounced closer to the surface.
A diver in a head-up trim MUST be negatively buoyant to fin and stay at a given depth. If they stop finning they will sink. Certainly this diver can add a little air to a BC when they stop kicking, and remove the same volume when they resume but that is a poor solution to an unnecessary problem.
Ergo, IMHO to be properly weighted you MUST be in proper (horizontal) trim. Let the discussion begin
What physics are involved? For starters I simplified the problem by ignoring any hydrodynamic forces, the diver moves too slowly for these to play an important role. Secondly this applies to a moving diver. A diver who is not moving need not be in any particular trim to stay neutrally buoyant.
Some definitions:
Buoyancy and trim are different.
Buoyancy is the relative density between the entire diver/gear complex and the surrounding ocean. Practically it determines whether the MOTIONLESS diver will ascend, stay neutral, or descend in the water column. Divers adjust their buoyancy before the dive by adding or subtracting heavy (denser than water) objects to their gear. These include tanks (steel vs aluminum), tank harness systems (steel backplate, aluminum backplate, plastic and/or fabric Jacket/back inflate/something other BCD), lights and cameras, and of course lead. Divers adjust their buoyancy while diving by changing the volume of flexible air spaces.
Trim is the stable position the diver ends up in without moving or finning. This is an orientation in 3 axes (pitch, roll, and yaw for you pilots). For this discussion we will only deal with pitch, specifically a horizontal pitch and a head-up or head-down pitch.
Center-of-gravity (CG) and center-of-buoyancy (CB) are not the same.
CG is the point the object will rotate around due to gravitational forces. CB is the point the object will rotate around due to the buoyant forces (for lack of a better term). For our simple physics discussion the diver/gear complex can be reduced to a CG point and a CB point. These points may/may not be co-located. If the CG and CB are not co-located on the longitudinal axis then the diver will end up in either a head-up or head-down trim position. The CG can move by extension or retraction of limbs. The CB can move with a change in the volume of gas in each flexible air space (exposure suit, BC, lungs), and can also change if the same total volume is distributed between all the air spaces differently. In the former the buoyancy also changes.
Buoyancy compensator (BC): ALL DIVERS use some sort of BC while diving. The diver with no exposure suit and nothing other than a tank/regulator/harness is using their lungs as a BC. A corollary to this is that a neutrally buoyant diver that becomes unconscious and stops breathing will start to sink as soon as air escapes the lungs.
The direction a diver travels is simple vector physics and is the addition of all horizontal forces and vertical forces. At swimming speeds and with a typical scuba configuration the hydrodynamic forces (lift) are inconsequential.
Horizontal trim is when the propulsive force (fins) is in a horizontal line with the divers CG. Head-up trim is when the propulsive force is below the CG, and head-down trim is when the propulsive force is above the CG.
THEREFORE: The diver in:
1. Horizontal trim has only a horizontal vector when they kick. They move only straight forward. If they are also neutrally buoyant the diver will neither ascend or descent with kicking.
2. Head up trim has both a horizontal vector and a vertical vector when they kick. They will move both forward and upward unless they have a compensating downward force. This downward force is produced by being negatively buoyant. When they stop swimming they drift down. The greater the head-up trim the more negatively buoyant the diver needs to be to swim at a stable depth.
3. Head-down trim also has both a horizontal vector and a vertical vector (in this case a downward one) when they kick. This diver will move both forward and downward unless there is a compensating upward force. This force is produced by being positively buoyant. If the diver stops swimming they will drift upwards.
A diver in horizontal trim (CG and CB co-located) can move into any position and stay stable in that position unless an outside force moves them. A diver in a head-up or head-down trim (CG and CB in different spots) has to work at staying in any other orientation. As they change position the diver/gear complex rotates around the CG and this either raises or lowers the CB which in turn changes the air volume in the BC(s) and makes that position unstable. Unchecked this diver will either drift up or drift down. Since air will move inside an airspace (and change the CB) if not prevented by the design (the dry suit being the most dramatic) this is a dynamic condition. The closer to the surface the diver is, the more a small change in orientation will have on changing volume. So this effect is more pronounced closer to the surface.
A diver in a head-up trim MUST be negatively buoyant to fin and stay at a given depth. If they stop finning they will sink. Certainly this diver can add a little air to a BC when they stop kicking, and remove the same volume when they resume but that is a poor solution to an unnecessary problem.
Ergo, IMHO to be properly weighted you MUST be in proper (horizontal) trim. Let the discussion begin
