Placement of the lifting lugs is determined by the location of the Center of Gravity, which is determined by the engineers (yea I know that is a scary thought) that designed the vessel. Be that as it may each crane will have X (ideally 50% each) amount of the load when it is horizontal, once the lift crane begins to rise the load from vertical to horizontal it will be accepting more and more of the load until at vertical it will have accepted 100% of the load while the tail crane will be at 0%
Sorry, but that's not what the scales will show on a tandem lift. And its not the way it really works. Or the way gravity actually works.
I've done hundreds of tandem lifts, the books can say what they want. The lower crane will gain weight as the load gets closer to vertical. My cranes have scales on them, and you can watch the weight gain on the lower crane in a tandem lift.
We regularly upright tanks. And if you have a 20,000 lbs tank, the lower crane will have about 12-13,000 on it just before the upper crane gets to vertical, and then the upper crane will "lift" the rest of the load off the lower crane.
You want to make a dangerous situation, tell the lower "tail" crane that they will never have more than 1/2 the load. If the tail crane is at max with 1/2 the load, he's going to be in trouble.
Lets throw out all your fancy drawings, and just use a little common sense. I know that's hard if you're a "project manager" but lets put the hard hat back on for a little bit.
Lets do something that everyone can understand. You're in the yard and have cut a 8' chunk of 10" maple tree off a dead tree- and its laying there.
You take the tree trunk, and leave one end on the ground, and push it to close to upright. Does it get easier or harder to lift and move around when upright or when its horizontal to the ground? That's right- Its easier with it more upright- because the weight is more on the ground. Now imagine your "tail" crane is the ground. The weight on the "ground" or the "tail crane" doesn't change whether its the "ground" or a "crane". Its more on the lower end.
Don't argue with me and start showing me drawings yet. I've got more:
Lets take a 400lbs I-beam. I can lift my end, I'm going to assume you can lift your end. But we probably both have all we want. Now I'm going to climb a ladder with my end.
Think you can handle your end? I know I'm going to get better all the way up my ladder. You're going to be in trouble before I get to the top. Now when I get close to the top of my ladder with my end, I can probably just about balance that I beam on your toe. I'm not going to have much weight at all. Of course I know that I don't have a prayer of lifting that I beam up off of your toe -and dead lifting it all by myself, because I can't lift the full 400 lbs. But I don't have to have near that much lift to just get it upright, because you've already run out of lift and set your end on your toe.
Sorry about your toe, but we are "tandem" lifting - and the book says you won't ever get more than 1/2 the weight. You can tell that to your smashed toe.
Ever carry a piano up a stairs? You want to be the top man or the bottom man?
Lets push it to cranes. Ever stand something up with only one crane? Leaving one end on the ground.
It doesn't take near as much crane once you get close to straight up and down, because a lot of the weight is on the ground. The tail crane when you are uprighting is the "ground". Doesn't matter if its hanging from a crane or setting on the ground, its still more weight on the bottom.
I'm constantly amazed by people that think gravity works differently, just because they have a fancy drawing. Don't worry, you aren't the first person to make a mistake.
And if you still think I'm wrong, I've got a 400 lbs I-beam at my shop and a ladder, I can show you how the real world works, rather than a drawing. Better bring your steel toe boots. Because it doesn't matter if its 400lbs or 400 tons, it still works the same way.
