Forces in a Rolling Boat :: Physics Boating

Every boat has a degree of roll from which she can no longer recover. Two forces are locked in combat for a ship about to capsize: the downward push of gravity and the upward lift of buoyancy. Gravity is combined with the weight of the vessel and everything in board-crew, cargo, fishing gear- seeking the center of the earth. Buoyancy is the force of all the enclosed air in the hull trying to rise above water level. On a evenly balanced and stable ship, these two forces are equal and cancel out each other, but all of this changes when a boat gets shoved over her side. Instead of being equal, the two forces are offset. The center of gravity stays the same but the center of buoyancy migrates to the submerged side, where more air has been forced below the waterline. With gravity pushing down to the center and buoyancy pushing up from the submerged side, the ship pivots on its center and returns to an even keel. The more the ship heels, the farther apart the two forces act and the more leverage the center of buoyancy has. To simplify, the lateral distance between the two forces is called the righting arm and the torque generate is called the righting momentum. Boats want a big righting momentum so they will right from extreme angles of keel. The righting momentum has three main implications. First of all, the wider the ship the more stable she is. The next thing is how tall the ship is. The taller the ship, the more likely she is to capsize. Finally, there come the point of no return, were the boat can no longer right itself. To most it seems like this would happen when the boat’s deck has reached a 90 degree angle to the water, but in truth, trouble comes to the boat a lot sooner.