I recently purchased a home on the water that included what I call a
cradle-type boat lift. Simply put, it's 4 pilings with an aluminum cradle
mounted between them, and the boat is lifted from the water on this cradle
via two electric motors. There are blue plastic pulleys (sheaves) at the end
of each beam and a few other places. The cradle has 2 long, carpeted bunk
boards that run the length of the cradle. It has 5/16" cables and is rated
for 10,000 pounds.

I haven't moved my pontoon boat to the lift yet, and when I run the empty
lift up and down I noticed the following problem when the lift comes up (not
down). The two bunks appear even and level but the problem is that when
there is no pontoon boat weight on the lift and I raise it up, the sheaves
on one side nearest the dock where the controls are lose cable tension and
the cables then windup unevenly on the spool. If I push down slightly on
the nearest plastic PVC upright used as a guide bumper the cables get tight
and all is well. Said in another way, if there is even a slight amount of
weight on the lift like from my foot pressure on the inboard (dockside) bunk
board there is no problem. This is not a buoyancy problem from floating
bunks because the bunks are already out of the water. This is not a bound
or frozen sheave problem because I used a come-along to take the weight off
of each of the pulleys and they spin freely. Going down is no problem at
all.

I suppose I could just add some solid cinder blocks to the center of the
cradle I-beams, but I really want to know is the physics of why it's acting
like this? Someone please help me diagnose this.

Chuck