The Physics of Rowing

     The sport of rowing, although not very well known, is a great example of multiple parts of physics working together. In this sport, there are eight people lined up in a narrow boat facing the direction opposite the boat’s motion. Each rower sits on a sliding seat and is strapped in by his or her feet. Each rower holds an oar, which pivots around a rigger. Rowing combines the concepts of momentum, energy, and resistance to achieve its motion. 

    First of all, the rowers use momentum to move the boat. When the boat is not moving, the total momentum of the system is zero because there is no velocity. However, the boat starts moving when the rowers begin to apply pressure on their blades, which in turn moves the water in the direction opposite of the boat’s movement. The forward momentum of the moving boat must equal the backwards momentum of the water. You can see this by looking at the water once the blades are in the air- there are swirls of water clearly moving towards the back of the boat. 

    You can achieve the same amount of momentum by either moving a small amount of water quickly or by moving a large amount of water slowly. By looking at kinetic energy, it becomes clear that it requires less energy from the rowers to move a large amount of water slowly than it does to move a large amount of water quickly. To increase boat speed most efficiently, you want to increase the momentum of the water while minimizing the change in kinetic energy of the water. You want to minimize kinetic energy because that energy comes from the rowers, so the less effort they have to put in to increase boat speed the better. The equation for kinetic energy is KE=½mv2. Because the velocity is squared, the faster the water moves the more kinetic energy it has. Therefore, if you want to minimize the energy but you have to increase either the mass or the velocity of the water, it makes more sense to increase the mass, which has a smaller effect on the KE. That’s why the blades of the oars are fairly large and why rowers always try to keep their blades deep in the water for as long as possible- to increase the mass of the water they are moving.

    Finally, if you think about the free body diagram of a boat, you have to think about friction. There are several forms of drag opposing the motion of the boat. The main three forms of drag are friction, which acts because of the hull of the boat moving through water, form drag, which is due to the turbulence created by the boat, and wave drag, which is energy lost due to creating waves. Friction accounts for 80% of the resistance and is increased when rowing into a strong wind. These forces greatly slow down the speed of the boat and mean that the rowers need to supply 8 times more power to double the speed of the boat.

References:

http://eodg.atm.ox.ac.uk/user/dudhia/rowing/physics/basics.html

http://eodg.atm.ox.ac.uk/user/dudhia/rowing/physics/rowing.pdf