Simple salt/fresh wt. questions

[Doby45]

Yeah me too, I've been lucky to get beer money.


Bob

My bud and I recovered a whole boat and only had our air paid for. LOL

 

[tursiops]

@TMHeimer of important note for the motor recovery, and I apologize for the narrative. I'm an engineer, and just got off of 2 weeks of running open water training, and am not running training sessions at work, so teacher engineer brain is taking over.

bcd's and lift bags don't have "weight" of lift *kg's or lbs depending on where you are*, they have volume of displacement *liters or cubic ft*.

what is important to note with this is that the "lift" actually changes based on the kind of water you are in. The way Deep Sea Supply measures their wings is by putting the wing on a backplate, strapped to an 8" tank and worn by a diver. The diver then stands on a scale and the scale is 0'd. The wing is then filled with water and the mass of water that is able to get into the wing equates to the amount of lift that it has.
The wing is filled with fresh water from a hose.
If a wing is rated at 62.4lbs, it means it displaces 1cf of water, and will actually have 64lbs of lift when in salt water.

I do not know of any other manufacturer that measures their wings or bcd's this way. Not to say they don't, just that I don't know of any that do.

The issue here is that since the ratio of amount of lift vs. mass of the object is very lopsided so while the wing experiences an increase in lift in direct proportion to the increase in mass required to offset the change in buoyancy, since the absolute change is very different, you may need to adjust the size of your wing.

LOL. I'm an engineer too, but I try and answer the question that is asked rather than the one I want to talk about.

 

[tbone1004]

LOL. I'm an engineer too, but I try and answer the question that is asked rather than the one I want to talk about.

did I not answer the questions that were asked by the OP? long answers, but still answers
I think the only no quite direct thing was the wing lift adjustment, but that was brought up with mention of lifting engines and making sure you size the lift bag appropriately for fresh vs salt

 

[MichaelMc]

The issue here is that since the ratio of amount of lift vs. mass of the object is very lopsided so while the wing experiences an increase in lift in direct proportion to the increase in mass required to offset the change in buoyancy, since the absolute change is very different, you may need to adjust the size of your wing.

Your concern in taking a large set of gear from salt to fresh?

Say 4 cu ft of gear that is 15 lb negative in salt water. In fresh water, the gear loses 6.4 lb of lift from the water, 1.6 x 4, becoming 21.4 negative. If the diver used a wing with 20 lb lift in salt water, it would have 19.5 lb in fresh. We think of us needing less lead in fresh, but maybe not of our gear needing more lift.

 

[dberry]

If the diver used a wing with 20 lb lift in salt water, it would have 19.5 lb in fresh. We think of us needing less lead in fresh, but maybe not of our gear needing more lift.

Very true, but I'd hope no one would cut it so close in terms of wing lift.

 

[MichaelMc]

Very true, but I'd hope no one would cut it so close in terms of wing lift.

The key was not the small 0.5 lb lost in wing lift, but rather that while they had 33% more lift than they needed in salt, 5lb/15lb, the rig was so big that the change in water caused loss of more than that 5 lb. excess lift when moved to fresh. (But you may have meant 20 lb lift with that much gear is rather close, regardless of the numbers.)