Some thoughts on buoyancy and stability

Consider a "particle" immersed in water...

It is acted upon by a force equal to the weight of the water it displaces.

Consider a cluster of such particles gathered together in the shape of say for instance a diver.

Since a diver is a solid object, this force on those particles sums together into a single force towards the surface.

That force acts along a line through the body at the point where the volume is equal on both sides of the line, both fore and aft, and port and starboard.

If that body is turned and spun, the line through which that force always acts describes a point in the body.

That point is known as the centre of buoyancy.

Every particle also has mass. Every particle feels a force towards the centre of the earth, proportional to its mass.

Again that cluster of particles can be gathered together into the shape of a diver.

Since a diver is a solid object, the force on those particles sums together into a single force towards the centre of the earth.

That force acts along a line through the body at the point where the mass is equal on both sides of the line, both fore and aft, and port and starboard.

If that body is turned and spun, the line through which that force always acts describes a point in the body.

That point is known as the centre of gravity.


If that diver is stable and stationary (neutrally buoyant and stable) in the water we can make a few other observations.

1) The force due to buoyancy is identical in size, and opposite in direction, to the force due to gravity.

2) The centre of buoyancy is

a) directly above the centre of gravity.

b) the centre of gravity is directly above the centre of buoyancy.
This is only a metastable situation, any minor perturbation of the body, and the metastability is lost.

c) The centre of gravity and the centre of buoyancy are the same point, in which case any orientation would be stable.

If there is a difference between the location of the centre of buoyancy and the centre of gravity and they are not on a line that links the centre of the earth to the surface, there will be a net turning force on the body. This is known in ship hull design as the "righting moment" and that seems a good term to use here.


It follows from the above that there are a large number of states of neutral buoyancy, all of which are stable but which differ significantly in the righting moment that will be experienced if the body is displaced from it's stable orientation. All will meet the definition of being good trim, but some will have a stronger tenancy to restore that trim if disturbed, others will have less or none.


Note that the centre of buoyancy depends on volume, the centre of gravity on mass.

As divers we have some control of the position of both centres, and those choices affect the righting moment we experience.

How much righting moment (if any) we experience is a matter of taste to a large extent.

Assuming that we are aiming for a nicely trimmed horizontal attitude in the water, we can move the centre of gravity downwards by putting weights towards the front of the weight belt, slinging negatively buoyant tanks below us.

This is of course in addition to moving weight and buoyancy fore and aft to move the centre of gravity and centre of buoyancy fore and aft and side to side as Gareth describes in his notes on trim. What I'm trying to explain is that you can also move them up and down, closer together or further apart in the vertical plane and affect the amount of force that is holding you into the trimmed position.

We can move the centre of gravity upwards by adding weight on the spine of the tank, at the back of the weight belt, on the tank bands.

Opportunities for moving the centre of buoyancy are more restricted, choosing a wing rather than a wrap around stab jacket moves the centre upwards, conversely choosing a stab jacket moves the centre of buoyancy down somewhat.

The centre of buoyancy moves as we inflate and deflate the bc device, with a wing the buoyancy added is clearly above the divers centre of gravity, the stab jacket, towards the top of it's buoyancy range may be adding buoyancy low down so may actually be moving the centre of buoyancy down.

To get to a configuration where you can do the dive master yoga position, i.e. lotus position in the water you need to have a configuration with a low righting moment. That means weights backwards and buoyancy forward. I'm far from clear that this is the ideal configuration for real diving, but others may have a different view.

Personally I like quite a strong righting moment, and have weights low on the front of my weight belt.

Because I use a doughnut wing, I can move the centre of buoyancy around quite a bit by moving the bubble in my doughnut.

This is of course predicated on my using relativly lightweight gear, a low buoyancy suit and thus low weights and a relativity small freely moving amount of air in my wing.

If I force a horizontal position, the bubble splits into two haves either side of my tank and I'm nicely stable with the centre of buoyancy high and my centre of gravity low.

If I push myself vertical in the water the bubble runs around to the back of my neck and I have another stable position that I can hold without effort, and it's compatriot, the head down position with the bubble over my butt.

Similarly if I force myself horizontally onto either the left or right side the bubble moves to one side or the other and I can hang there fairly stably.

As soon as I roll back towards my normal trim however the air evens up and I'm back flying my blimp.

Cheers Harry

Nice dissertation, probably slightly to indepth for some on here though for something so simple it didnt arf take some reading, but well done for writing it.

You must admit though Si, it's a bit more helpful than some of the "You've got bungee? you're going to die" posts we've had lately

I agree there Kev.

Lol...it's only you and your guru who have been saying it, trolling for respons...damn.

I thought... ... you were my new guru?

Great read Harry.........do you have any pics I could colour in to make it simplier !!

What's the lotus position for again?

Deep penetration and mutual stimulation

i believe this is the lotus dive position

This was more what I had in mind




Standard show off by instructors doing PADI Peak Performance Buoyancy


BrindoSteve: I'm giving it some thought...

Cheers Harry

Yep, just fell of my chair laughing.

Nearly as funny as that strange way of emphasising the okay signal some instructors have a habit of doing.