Trim Affected by Water Type?

[oszillodrom]

Changing weights between fresh and salt water will obviously influence trim.

Apart from that I think that technically, the amount by which you are e.g. foot heavy would change. As a hypothetical example, if you are normally slightly foot heavy and you were to dive in some extremely dense liquid, the difference in buoyancy would push you to be basically vertically upright in the water. In real life, this effect will be too small to be noticeable.

 

[DevonDiver]

Changing weights between fresh and salt water will obviously influence trim.

Apart from that I think that technically, the amount by which you are e.g. foot heavy would change. As a hypothetical example, if you are normally slightly foot heavy and you were to dive in some extremely dense liquid, the difference in buoyancy would push you to be basically vertically upright in the water. In real life, this effect will be too small to be noticeable.

So a 0.3 difference in water weight doesn't equate to, perhaps, descending slightly and adding more water weight as pressure increases?

Does your trim normally change if you descend 30cm?

 

[oszillodrom]

So a 0.3 difference in water weight doesn't equate to, perhaps, descending slightly and adding more water weight as pressure increases?

Does your trim normally change if you descend 30cm?

As I wrote in my comment, this is purely theoretical and will not be noticed by any diver. Still, it is fun to think about (at least for me).

Trim depends on differences in buoyancy along the body axis, while buoyancy is directly proportional to the density of the fluid. Therefore, if the density of the fluid changes (either by change in depth or change in fluid), your trim will also change - although only by a tiny amount.

 

[DevonDiver]

There's nothing theoretical about the weight of water on body changing very rapidly over minor depth variation... but only a relatively tiny weight change caused by the addition/removal of salt in the water.

If you were in proper trim, the compressible (pressure effected) components on your rig would all be subjected to equal pressure... or at least, so near to equal as to be impossible to perceive.

However, adding/subtracting lbs of weight to you rig, to compensate for buoyancy changes between sale and fresh water, will have an immediate and apparent effect on your trimming.

 

[Steve_C]

Lift proportional to volume of water displaced. So for fixed volume the change in depth does not affect lift. But if you have say a 7mm wetsuit and you go to 100ft there is compression of the wet suit which reduces your volume and thus decreases the lift. Also depth will drive out little air pockets in BCD or wetsuit which also reduces volume.

 

[Charles2]

You are "in trim" when your center of gravity coincides with your center of buoyancy (or at least on a vertical line which goes through both). When going from fresh water to salt water, there will be approximately 2.5% more buoyant force, but it will stay in the same place. Therefore, in order to maintain trim, you must distribute the additional weight such that the net effect of that added weight will also coincide with your center of buoyancy/center of gravity.

 

[CT-Rich]

I think in theory, it would make a difference. The center of gravity will change in a extremity like you legs becoming more buoyant in salt water while you are adding lead to your BCD or belt. It is great in theory, but in practice, you will only be moving the center of gravity a few inches forward or backward on your body. The net effect is you might notice a slight change at first, but buoyancy control is aided by slight movements in the real world (versus a pool) that you are always making to keep oriented in the correct trim. In submersible ROV it would be bigger issue only because machines lack the fine motor skills of a living human.

 

[MichaelMc]

My best concept is that you want the center of mass of you and the water you displace to be the same, at least horizontally.

On depth changes, its extreme, but if you wore a heavy top, but no suit legs, descending changes the displaced water volume by your head but not your legs. So the center of mass of the water changes. Very contrived, but just to keep my concept straight.

As already said, in the regular world, you want to add ballast with the same center as your trimmed center, the water's, not the same center as the lead. Which is hard to calculate with out trying it in water.

Michael

 

[MichaelMc]

Changing depth doesn't effect trim.

In real diving, likely not. When your wet suit shrinks, your volume and that of your water/displaced body change the same. But your water 'body' loses mass and you do not. Say you wore neoprene pants. The displaced water loses mass in the leg area. You do not, it just gets more compact around your legs. It seems to me as if the center of mass of your water 'body' shifts more toward the head, but not that of your real body, making you leg heavy. Thanks for raising the issue, I've been trying to think this through lately. the impact of this is likely usually small. Ignoring that you just got negative as well.

 

[Professor Nemo]

Much of this conversation has become far too technical for my limited comprehension abilities but it has helped me understand the phenomenon better and answered my concerns. Thanks, everyone.