[Jim727]

Originally posted by Aerkuld
I think I know what you're getting at.
The Falkirk wheel and the original Anderton boat lift demonstrate this nicely as they use two containers filled with water (caissons) that counter balance each other. I would guess that the Belgian boat lift uses counterweights which will achieve the same thing as you can calculate the weight of the water filled container and make the counter weight weigh the same. The trick to this is that regardless of whether the container is just filled with water, or it has a small boat floating in it, or a huge ship floating in it, the lifting weight will remain the same. This means that, with the counterweight or the opposing water filled container moving in the opposite direction, the device will only use enough power to overcome inertia and the friction in the bearings to move the lift up and down.
The reason behind this is that a floating ship will displace exactly its own weight in water from the container.

We have a winner!!