Boat Lift Explained

Brewerton, NY
44 14.33 N 076 08.50 N

I've been getting questions about our error with the bridge and the mast.  I find it hard to explain without pictures.  Indeed, it makes me feel like an engineer again.  I should grab some chalk and go up to the board to explain for my students.  We'll do it digitally.

Figure 1

Consider first what I should have done as shown in figure 1.  The gray mast sits on a blue cradle that in turn sits on the blue boat.   The red rope stretches over purple pulley P and windlass W.  The Pulley is attached to the black bridge rail.  The windlass is attached to the blue boat.

Now  pull on the tail of the rope with 500 pounds of force.  Both the mast and the boat will be lifted with 500 pounds of force.  However the mast only weighs 500 while the boat weighs 20000.  The mast will begin to lift.

Figure 2

In Figure 2, we removed the pulley P and draped the rope over the bridge rail.  If the rail is frictionless, everything behaves the same as if the pulley was still there.  When lifting, the rope slides over the rail.

Figure 3

In Figure 3, now suppose that the rope binds on the bridge rail.  If it gets entirely stuck, we would have infinite friction.  It would be as if the two sections of the rope were each attached to separate bridge parts.   As I pull harder on the rope tail, the force lifting the boat increases, but none of that force transfers to the mast.  Instead of lifting the mast we are trying to pull the bridge down on the boat (as my friend Dean said) or lifting the boat up to the bridge.  I show that in Figure 3 as 500 pounds lifting force on the mast with 5000 pounds of force lifting the boat.

Of course if we lifted the boat as in Figure 3, we would also lift the mast with the boat. The force on the mast side of the rope would drop to zero.  But if that happened, it would relieve the friction on the rope and the rope would start sliding over the bridge rail again.  So the total reality was that we had some combination of Figures 2 and 3.   Both halves of the rope had some tension, but there was much more force lifting the boat than the mast.  The distribution of forces would have been irregular as the rope alternately slipped and caught on the bridge rail.

Such big complexities arising from such a simple change.  It makes one appreciate better that "rigger" is a skilled profession.  Riggers do heavy lifting.  It reminds me of the infamous failure of a walkway in 1981 at the Hyatt Regency Hotel in Kansas city.  114 people were killed. That too was caused by a seemingly trivial change in rigging that had enormous consequences.  See the article here.  Luckily for us the consequences were minor.  By the way, we did have the good sense to not stand under the mast.  If the rope snapped I think both Libby and I would not have been hurt.