-- Last Updated: Jun-20-13 1:42 PM EST --
I can't see the details in that photo you posted links to, but with the arrangement you described, the two ends are free to move independently, exactly as they would if the two ends were simply supported by a single rope that was looped over the top of a single pulley. Explaining why that is so would take me an hour, but if you could actually compare models of the two methods you'd see that this is true.
If all pulley wheels had exactly the same amount of turning friction, you might be able to get the two ends to raise and lower together, but I doubt it. I think the only practical way to keep both ends at the same elevation while raising or lowering is to have a partner tilt the boat while you operate the rope, or you could stop periodically during operation and adjust it yourself (but fear not - see more details below).
Off the top of my head, I haven't thought of a way to control this issue with additional pulleys, but perhaps it can be done. The good news is that you can pull the rope and let one end go up until it stops, and THEN the other end will do the same. By the same token, if one end drops to the ground while the other rises, that's no problem either. That end will stop dropping at that point and the ropes will continue to run their route while lifting the other end. In other words, everything works out by the time you are done raising or done lowering. It's only the during the process of lifting or lowering that the boat will be uneven. It WILL become level at the top or bottom.
The differential of a car works the same way but with gears instead of pulleys (but both are WHEELS). The only thing that keeps both driving wheels turning at the same rate during straight-ahead driving is that they are on the same ground. Put the wheels on two different free-rolling treadmills and it would be completely impossible to make them turn the same speed, and when stationary, you could spin one of them forward and the other would compensate by spinning backward, just like the two ends of your boat can be made to teeter-totter in place.
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