Pontoon effect
The pontoon effect is a loose term used by some to refer to the bouyancy of air-filled sealed containers or chambers at the bottom of structures or vessels immersed in water, and by others to refer to the tendency of a flat-bottomed pontoon to capsize when a heavy load with a high centre of gravity is placed On It.
A pontoon floats in a level position when the centre of gravity of load and pontoon is above the centre of buoyancy of the pontoon. When it starts to tip it will be stable as long as the centre of gravity does not move tranversely (to the side) more than the centre of buoyancy (which will be determined by the exact dimensions and masses involved), because under these conditions a "righting force" (a turning moment) acts on the pontoon to bring it back to the level position.
However if the centre of gravity is particularly high and the pontoon is relatively narrow, even a slight tipping to the side will move the centre of gravity further to the side than the centre of buoyancy. Now there will be no righting force, but a turning moment in the oppostie direction instead, which will capsize the pontoon.
A more stable pontoon design is achieved by using twin pontoons in a catamaran design, and the load is then placed in between the pontoons. However each one should have enough buoyancy to keep the load and weight of the pontoon afloat on its own. If the load is so great that both pontoons are nearly submerged in the level position, it must be remembered that when the pontoon tips and one pontoon becomes submerged, its maximum buoyancy is achieved, the centre of buoyancy cannot moved any further to that side; that pontoon continues sinking, increasing the tipping angle until the whole structure capsizes. If, on the other hand, each pontoon can support the entire weight, the centre of gravity then can only move transversely more than the centre of buoyancy at the most extreme tipping angle.
The principles at work are the same as those governing the stability of boats and ships, see angle of loll and metacentric height.
References
- "What is the Pontoon Effect?" see end of page.