I can't add much to what everyone has said. You need a 12-foot wide slip. The lift bolts to the dock, but the dock is not used for "flotation". The dock is only used to hold the lift in place.

A 110-volt electric blower motor is used to pump air into the pontoons. To lift the boat, you open the valve(s) and turn on the blower. You know that the boat is fully lifted when air starts to come out of the holes in the bottom of the pontoon. Once the boat has been lifted, you close the valve(s) and turn off the blower. To lower/launch the boat, you open the valve(s) but leave the blower off.

$4000 for a used 12,000 pound lift sounds like a great price. But remember to add in transportation and installation costs.

Good luck! :cool:

Lifts can be made for most widths from 10' to ??'. The one in the ad is a "side tie" lift that does not actually need a slip. It is designed to be attached on one side only, and actually has bulkheads in the tanks making 4 independent air chambers. The white parts above each tank are styrofoam floats that are attached to the tanks with 3-4' arms, and keep the lift from going any deeper. These are exclusive to the side tie type lifts. When raising a boat on this type, you have to tie the boat to the lift with a spring line on each side, inflate the front air chamber on each tank, raising the bow up. The spring lines hold the boat on the lift during this time. Then you inflate the rear chambers and top off the front afterwards. Each chamber has it's own valve, and YOU have to keep it adjusted to make the raising and lowering even. Most side-tie lifts are quite wide, often 14 or more feet, to give the lift a stable "foot print" in the water. Side-tie lifts can work in a slip in the above manner, but they lack the normal attachment arms of conventional lifts, and I'd expect it to be expensive to retro fit them.

Conventional lifts are attached to both sides of the slip with 4 or more arms that only allow the lift to raise in a flat arc. The dock simple provides stable attachment points. these usually only have one valve, and the air stabilizes itself between the two tanks because the arms and the dock prevent any twisting or racking of the lift.

You could actually raise your boat out of the water by blowing into the air lines. May take awhile and you may get light headed too. :) Actually pretty sure both of those conditions will prove to be true.

The 4 (or 6) arms attach to the dock and only serve to hold the lift in location and to keep the lift level at all times.
1 arm can be seen on my lift going backwards and attching to the dock. My neighbors lift mount is visable in the upper left corner of pic.