Principle of Bouyancy The principle of bouyancy explains why a sailboat stays above the surface. The force that water exerts on any immersed object is called a buoyant force. This force counteracts the downward gravitational force that acts on the object. This downward gravitational force can also be called weight and is equal to (“mass” x “acceleration of gravity”)or “mg”. The force diagram for this is shown below. The hand represents the downward push of the “mg” force and the arrows represent the upward force that water exerts on the boat. When these forces are equal, the object will float in static equilibrium. (Picture from "The Annapolis Book of Seamanship", Rousmaniere, 1999.) So when a boat is floating in static equilibrium, the following equation holds true: B = Fg = Mg Where B is the buoyant force, Fg is the force of gravity, and Mg is the weight of the object. More importantly, the total weight of the water displaced is equal to the total weight of the object displacing the water (Archimedes's Principle). Therefore, “displacement” is what we call the volume of water that is moved aside by a floating boat. Displacement is recorded as the weight of the volume of water moved. (Picture from "The Annapolis Book of Seamanship", Rousmaniere, 1999.) We can take this a step further and see that if B = Mg and M =pV, then B = pVg. Where V is volume, p is density, M is mass, and g is the acceleration of gravity. So the buoyant force is a function of density and we can conclude that the density of an object determines whether or not it will float. So if a sailboat had an average density greater than saltwater (64 pounds per cubic foot) it would not remain on the surface. However, if the sailboat has an average density of 32 pounds per cubic foot, it will float on the surface of the water and be halfway submerged. This is why ships that are constructed of materials possessing much greater densities than that of water can still float. The air inside the hull ( air is 1000 times less dense than water) brings the average density of the entire ship below the density of water. Even a cement boat can float and they have! Home / Basic Parts of a Sailboat / Propulsion from Sails / Balance / Bibliography