Boat snaps cable during loading - GIF

[awright]

Well, of course we don't know the weight of the boat, the strength of the cable, or if possibly the boat struck or dragged on something on the deck/pier, but the roughly 45 degree angle of the two cables would increase the tension in the cables by the square root of 2 (i.e., 1.414) times the tension that would exist in vertical cables, each carrying 1/2 the weight of the boat. The clearance between the block and the crane arm above is not shown, but the riggers would have been making better use of the cable load capacity by increasing the height of the block above the boat and minimizing the clearance between the block and the crane arm.

Also, I think there is a slight tilting of the boat forward just prior to the cable break and that would have shifted load from the rear cable onto the forward cable, making a marginal situation worse. (I'm using forward here to indicate toward the bow of the boat being lifted.)

Another thing comes to mind: Notice that a barrel or something tumbles forward just prior to the break. That may be a clue that "stuff" inside the boat or even bilge water also tumbled forward, shifting more load onto the forward cable.