Lift Bag size

[pkdiver]

I'm not sure this is in the right forum but I would like to know how to size up a lift bag for objects like odd shaped objects like outboard motors. I know from my DM class how to figure displacement, volume, and bouyancy but most of the problems involed ojects that were easy to determine the displacement. How does one figure the displacement of something odd shaped like a O/B motor so as to dtermine what size lift bag to use?

 

[ClayJar]

For something like an outboard motor, if I knew anything about it, I'd just go with the dry weight. Naturally, that'll be an overestimate of the lift required, but if it's just a big hunk of metal, it's pretty close. If I didn't know anything about it, I'd basically just guess. I've got 50-, 100-, and 200-pound lift bags. I'd guess that it'd be too big for the 50 (unless it was really small), so I'd probably start at 100 and go from there.

The nice thing is that a full open-bottom lift bag will not increase in lift during ascent. As the air in it expands, it'll just spill out the bottom. So, if you've got, say, two 100-pound bags on something that's only 150 pounds, you could fill one bag completely and control it with the other alone. Of course, if it's much larger than an anchor or a small outboard, I'm going to use lines. If it's big and unwieldy, trying to control it mid-water can be a bad idea.

Of course, there's also the "stepladder" style of lifting large things. You have a line with loops every 10 feet (or whatever) from the item all the way to the surface. You attach to the first submerged loop and lift that to the surface. (No need to worry about runaway, as you're just lifting 10 feet.) You leave that bag in place while you attach to the next loop (which will now be 10 feet from the surface), and you lift *that*. Then you detach the first bag and lift the *next* loop, and so on, until the item is all the way up. By doing the lift a segment at a time, you minimize the "rocket to the surface, plummet back to the bottom" potential.

 

[Mark Michaud SELAUSAR]

The weight of the object should determine the amount of lift.* Ultimately it has to come out of the water and it gets heavier as it get closer to the surface.* I prefer closed bags for objects over 100 lbs.* 2 / 200 lbs closed bags were used, by me, to raise a Yamaha Enduro 48 outboard (Maybe 200 lbs) from 12 feet in a local river a few months back.* It was a 20 minute ride from the launch and of course back.* I located the motor and marked it with a small liftbag.* I also established a line from the bottom (heavy weight) to a big surface float.* This is place near the object and used to ascend and descend.* It can also be used as a guide to life an object if needed.I took the first closed bag and set it next to the motor.* I raised the motor onto one side and slid the bag under it and layed the motor back on its side.* There are d rings on the bags and I usually put the side of the bag, that has the rings, away from the object to be lifted.* That keeps everything tight.I went back and brought the other bag down and laid it over the top.* I lashed the two together by shackling the d rings on each bag together.* I made a rope wrap from long end to long end which sandwiched the motor inside of the bags.I have a manifold that hooks to dive bottles, mine uses two.* It has a 50' hose from each side of the manifold.*** You can use compressor hoses and make them any length you want.* I attached each hose to a bag.* The current was slow at his spot.* I had a line to the boat that a used pull signals for, though sometimes I use comms.* I tugged the line and topside started adding air to both bags slowly.* There are valves on the bags and at the nmanifold.* On this one I opened the bag valves because the object was in zero vis.* When it surfaced I closed the bag valves to keep the air in.* When they started to rise, I tugged again for topside to stop.* I pushed the bag at arms length and moved away and surfaced.* The bags rose slowly to the surface, and we added air to make the motor positive at the surface, and of course, closed the valves.We knew we couldn't get the motor into the boat without help which was part of the reason for rigging the bags the way we did.* There are also over-inflation reliefs on the bags I use.* We towed the motor behind us at a nice easy pace and had help at the launch.* Always make sure you have a safe plan and take as much time as you need to get it right and safe the first time.* Not rocket science, but also not something everyone can do.* The bags I use come from Defuniak Spriings Fla (proliftbags.com).* Best bags I have ever used including subsalve.* Hope this helps.* Mark

 

[ClayJar]

Ultimately it has to come out of the water and it gets heavier as it get closer to the surface.

For all diving intents and purposes, water is an incompressible fluid. An object of fixed volume does not get heavier on its way to the surface. If it is not of fixed volume (i.e. if it has a compressible component, such as the air in a half-full lift bag), it will become *more buoyant* on its way to the surface.

Only when you're trying to lift it out of the water does it "get heavier", as at that point, you are reducing the buoyant force as you reduce the displacement.

 

[Mark Michaud SELAUSAR]

In the real world, that is true.* I keep this mindset (getting heavier as it rises) because this safety factor was taught to me.* The moment it breaks the water it regains its real weight and that is where accidents happen.* If you keep the mindset, it gives you a mental, and real safety factor.* But you are correct that the item weighs the same on the bottom as it does till it breaks the surface.

 

[pkdiver]

Thanks all for your input. Very informative and interesting.