Engineering problem

[Hayden Watson]

My thoughts are, No. This is a simple redirection. If the spreader put more tension on the cable above, it would do so below, as well. The tension above would be greater than at the bottom only because of the weight of the cable.
If the angle were 90 deg. You're suggesting the force above the spreader would be nearly infinite. Torque, may come into play, but that has more to do with the angle of the spreader, as a lever arm, to the direction of force.

-Will (Dragonfly)

Will, you are still thinking of a mast. This is a crane problems. The connection from the load up to the spreader bar and from the spreader bar up the the hook are two different cables, each connected to the spreader bar with a shackle. The two cables connect from the crane hook down to each end of the spreader bar. Full stop, End of story.
The load is applied to the bottom of the spreader bar with separate shackles. The connection of the load to the spreader bar is vertical and equal to 1/2 of the total to each end of bar.
The cable will be greater than 1/2 load due to the angle.

 

[Hayden Watson]

My thoughts are, No. This is a simple redirection. If the spreader put more tension on the cable above, it would do so below, as well. The tension above would be greater than at the bottom only because of the weight of the cable.
If the angle were 90 deg. You're suggesting the force above the spreader would be nearly infinite. Torque, may come into play, but that has more to do with the angle of the spreader, as a lever arm, to the direction of force.

-Will (Dragonfly)

Will, FWIW, As owner/principal structural engineer at my prestressed / precast concrete engineering firm, this is what I do for a living so have some basic understanding of the problem.

Here is a good photo of a crane spreader bar. note that the cables above and the cables below the spreader are two separate things.

 

[Davidasailor26]

I’m with Hayden.

Will - here’s another way to think of it that might help you visualize what’s going on - The spreader bar is being “squeezed” down because of the angle between the upper cable and the lower cable. That squeeze makes the spreader want to slide down the lower cable. The only way to hold the spreader up is to attach it to the upper part of the cable, and have the upper part of the cable fight against the “squeezing” force. That extra pull means the upper cable has more tension than the lower.

 

[Will Gilmore]

Thanks for the help with the visualization. I'm not there yet, but it may be that I don't understand what problem we are actually talking about. Are we talking about the tension on upper stays and compression on spreaders in a mast system or the compression and tension forces in a crane?
I know the spreaders on a sailboat, especially larger boats, are fixed in place on the shrouds and for the crane with spreader bar, I think I can see what is going on, but I'm not convinced they are the same dynamics. The spreaders on a sailboat should act more like the spoke on a pulley wheel with the axis of rotation at the mast.
I'll have to take some time to think about this more deeply. I too, have chickens, 14 of them and two turkeys, to take care of right now.

-Will (Dragonfly)

 

[joker460]

Do you want to be more confused. See attached

 

[Hayden Watson]

Thanks for the help with the visualization. I'm not there yet, but it may be that I don't understand what problem we are actually talking about. Are we talking about the tension on upper stays and compression on spreaders in a mast system or the compression and tension forces in a crane?
I know the spreaders on a sailboat, especially larger boats, are fixed in place on the shrouds and for the crane with spreader bar, I think I can see what is going on, but I'm not convinced they are the same dynamics. The spreaders on a sailboat should act more like the spoke on a pulley wheel with the axis of rotation at the mast.
I'll have to take some time to think about this more deeply. I too, have chickens, 14 of them and two turkeys, to take care of right now.

-Will (Dragonfly)

First off, this question has nothing to do with a mast, mast spreaders or mast rigging. The op used that illustration to give us a visual.
This is a question about the use of a spreader bar for slinging a sailboat into the water with a crane. He is asking about the forces on a spreader bar that is hanging from cables up to a crane hook to spread the load out wider so that the lifting straps below the spreader bar will be vertical and not crush the topsides of the boat. He is assuming a max boat weight of 15,000 lbs with an OSHA required 5:1 factor of safety for rigging of overhead loads.
If the cable was continuous and allowed to slide on the ends of the spreader like with upper shrouds that use the mast spreaders as a turning point then the tension in the continuous cable would be equal at all points (ignoring self-weight)

 

[Apex]

Hayden,
I fear you may go right over the edge if another posts up about a mast with spreaders....(all in jest my friend)

So sailboat lifted by a crane with spreader bar it is. I concur with Haydens calculations. Thanks for the math distraction.
Spreader Bar: 15,150lbs (per side), 30,300lbs compression
Upper Cable Tension: 40,445lbs each.

..ignoring rig mass, or assuming rig mass is rounded up into the total lifting weight given of 37500 * 2
(or 15,000lbs boat and rig)

 

[Crazy Dave Condon]

I is Tryin to figure out this darn question as I is no good in arithmetic. No specific boat named, so why asked to begin with. I am pulling my hair out trying to compute this thread

 

[Will Gilmore]

He is asking about the forces on a spreader bar that is hanging from cables up to a crane hook to spread the load out wider so that the lifting straps below the spreader bar will be vertical...

AhhHHH !!!!!!!!!!!!!!
Now I get it. Thanks for your patience with me.

-Will (Dragonfly)

 

[Apex]

EDIT: That was all assuming only ONE SLING!......half the answers given assuming equal distribution across the two slings...or double the weight of the boat, you can choose.

 

[Hayden Watson]

EDIT: That was all assuming only ONE SLING!......half the answers given assuming equal distribution across the two slings...or double the weight of the boat, you can choose.

Not quite. two slings, one attachment point. Before I got too deep into the discussion, I opened a PM with the OP to get the full idea of what he was trying to accomplish. It is a small club with smallish boats that need to be craned into the water and it past years they just hooked both slings (fore & aft) to the hook but some of the boats were showing some distress from the pinching force on the sides at deck level. The spreader is to widen the lift to remove the force toward the boats center-line but both slings will attach to the strong-back. Bigger boats would always have a 4-point lift but on these small boats they should see improvement by going from 1-point to 2-point.