Trim Affected by Water Type?

[Professor Nemo]

I am pretty sure that this is either a ridiculous or stupid question, (perhaps both), but if I don’t ask it will continue to vex me. I know that a diver must adjust his weight when transitioning from freshwater to salt (sea) water and vice versa, but could transitioning form one water type to the other also affect a person’s trim and necessitate a rearrangement of weight placement? In my head the physics seems to say no, that one part of your body would not suddenly become less/more buoyant because you entered a more/less dense body of water but there may be variables that I am not accounting for. I ask because I received some new equipment for Christmas and I am probably going to try and figure out my buoyancy in a freshwater pool and then use a simple 0.025 multiplication calculation to measure how much I “should” need when doing open water ocean dives. I can then use the end of the ocean dive, when the tank is empty and I am conducting a safety stop, to gauge my buoyancy and see if adjustments are needed. I am also adding a Spare Air system which will affect my trim so I am going to have to adjust for this new equipment and master a new weight distribution order. I wanted to account for as many factors as possible before I’m in the ocean on a paid charter trying to complete a dive while dealing with poor trim control.

Thanks,

Josh

 

[northernone]

Hi.

Your question had me questioning myself a little! Dug down to what I know of physics and decided trim is unaffected because we are in homologous fluid even though it is of differing densities it is our density (the distribution of which) controls trim and remains a constant.

Regards,
Cameron

 

[Dish]

Cameron,
For the sake of getting my brain around this, let's say Prof Nemo used a weight belt, but perhaps he needed a tank weight on the upper cam band to trim out in fresh water. If he adjusted his weight for salt water (factor of 0.025) on both the belt and the tank weight, would he still be in trim in salt water?

It's kind of a nit, since he would obviously adjust as required (either by simply diving the rig or manipulating his weight through trial and error), but I'm curious about the relationship.

 

[Got2Go]

Cameron,
If he adjusted his weight for salt water (factor of 0.025) on both the belt and the tank weight, would he still be in trim in salt water?

He would need to factor for salt water on his combined dry weight (all the gear + himself).
The factor is not done "just" on the lead.

Once you have the amount of extra weight calculated, you would then need to split that between cam band and weight belt in the same ratio as they are for fresh water.

Then he would be in the same in salt water as in fresh water, with the same setup (plus calculated additional lead).

 

[northernone]

Well, what gets tricky is that it isn't dry weight. It's displacement volume and density. (Archimedes and his bathtub)

I will bow out and wait for another guy to wrestle an explanation.

What I do know is in the caves I drop into and out of fresh water and patches of high salinity water without it affecting my trim in an observable way.

Enjoyable question.
Cameron

 

[Professor Nemo]

Thanks, I appreciate the responses.
Josh

 

[Steve_C]

Got2Go has a good answer. Assuming you are trimmed out right in freshwater and move to salt water in the same gear you will have to add some weight. To stay theoretically the same you need the moments of the new weight about your old center of mass to add to zero. That could be at the center of the mass or if not then distance x mass needs to be equal. However that is theory. All the weight is usually not too far from your center of mass anyway so a few extra pounds can be easily compensated for by, for example, how straight your legs are when not moving. I do not notice any noticable difference going from fresh to salt and all my weight goes into two weight pockets.

 

[DevonDiver]

You make a change in weight (add weight) when moving from fresh to salt water. Changing the amount of weight carried can obviously influence trim. You'd need to ensure that the extra weight was added to the right location/s to sustain proper trim.

Beyond the change in weighting, there'd be no effect on trim. Salt water weights more that fresh... so the effect would be no different than changing depth. Changing depth doesn't effect trim.

 

[Got2Go]

Well, what gets tricky is that it isn't dry weight. It's displacement volume and density. (Archimedes and his bathtub)

It is dry weight in this case.
You have a known configuration, and you want to figure out the needed extra weight for that same configuration, but in salt water.
So you just need to calculate using the difference in water density between fresh and salt.
(Generally accepted average = salt water is 2.5% heavier than fresh)

If you are neutral in fresh water, and you+gear weigh 300 pounds, then in salt water you need to add 7.5 pounds of lead to that same configuration to be equally neutral. Your total weight would be 307.5 pounds.

And for trim, if you have lead in cam band and weight belt, then the question is how you do you distribute those extra 7.5 pounds on you to maintain the same trim as in fresh water.

Say you have 20% of your lead on cam band, and 80% on weight belt, then you will add 20% of the 7.5 pounds to the cam band, and 80% to the weight belt. And that will give you the same trim in salt water as in fresh water.

All this assumes the same configuration between fresh and salt, with only the added lead for salt water.


Or something like that :/

 

[northernone]

It is dry weight in this case.
You have a known configuration, and you want to figure out the needed extra weight for that same configuration, but in salt water.
So you just need to calculate using the difference in water density between fresh and salt.
(Generally accepted average = salt water is 2.5% heavier than fresh)

If you are neutral in fresh water, and you+gear weigh 300 pounds, then in salt water you need to add 7.5 pounds of lead to that same configuration to be equally neutral. Your total weight would be 307.5 pounds.

And for trim, if you have lead in cam band and weight belt, then the question is how you do you distribute those extra 7.5 pounds on you to maintain the same trim as in fresh water.

Say you have 20% of your lead on cam band, and 80% on weight belt, then you will add 20% of the 7.5 pounds to the cam band, and 80% to the weight belt. And that will give you the same trim in salt water as in fresh water.

All this assumes the same configuration between fresh and salt, with only the added lead for salt water.


Or something like that :/

What you say makes sense to me!