The Physics of the Oars

by Michael-James L. E.

Imagine for a moment living in the eighteenth century on the Piscataqua River. Your only source of income is rowing passengers in your wherry from one side of the river to the other. Because of that you need to move as quickly as possible to maximize your profit, but your only sources of power are you and your oars. How can you maximize your speed ?

There are two ways you can maximize your speed. You can either change the design of your boat, or the design of your oars. You already have a boat; you're not going to want to change the boat design. You have to change your oar design. With physics you can determine the best design.

In traditional wherry rowing (the way most people row) an oar is simply a lever. A lever is a tool that is used to lift or move an object. Much like a crowbar lifts a box lid, an oar pushes water. The real question is, what kind of lever is it?

There are three classes of levers: first, second and third class. In order to understand how these classifications are determined, we first must learn about the characteristics of oars. There are three aspects to a lever: the fulcrum, which is the pivot point of a lever; the resistance force, which is the resistance from the water; and effort force, which is the force of the rower. Depending on where you place these three aspects the lever changes.

If you put the fulcrum in the middle, the resistance at one end, and the effort at the other, you have a first class lever. If you put the effort at one end, the resistance in the middle, and the fulcrum at the other end you have a second class lever. If you have the resistance on one end the effort in the middle and the fulcrum on the other end you have a third class lever.

The third class lever can not be used for an oar because there is no way to place the effort in the middle of an oar. It is highly unlikely to see someone rowing a boat by pulling on the middle of an oar. This leaves us with only two other possibilities, the first and second class levers. Both the first and second class levers are acceptable solutions depending on personal preference.

To find out about how much work your getting out of the oar compared to what your putting in, you have to find the theoretical mechanical advantage so you're not wasting any energy. Mechanical advantage is how much a tool helps you. The way you find the T.M.A. (Theoretical Mechanical Advantage) is by taking the length of the effort arm (the distance from where effort is placed, to the fulcrum) and divide that by the length of the resistance arm (distance from fulcrum to resistance). Effort arm divided by resistance arm equals T.M.A.. Then the efficiency is the ratio of energy output to the total energy input.

Maximum efficiency is what all rowers try to achieve. This is the point at which you get out exactly what you put in. In order to figure out how to achieve this I changed the way I thought of oars. I took the oar out of the water and put it in the air. Then I turned it 45 degrees. Now I have a wing. This makes it a little easier to understand and research because there is a lot more information on wings then there is on oars. So now back to the drawing board. The first thing you have to know is how a wing works.

There are two different aspects to a wing that make it work. First is Bernoulli's Principal. Bernoulli's Principle says that when two molecules are split apart they will both meet at the same point when they come back together. This means that if you have an object with a flat bottom and a rounded top, and you run it through the air, the top molecules have to move faster then the bottom ones to get to the same place at the same time. If the top molecules are moving faster than the bottom the pressure on the top is less then the bottom, so you get lift.

Because water has the same effect as air, it can be used in the same way. If you put an airplane straight down into the water it will go forward. If you take just the wing of a plane it has the same effect. So if you make the blade of an oar in the shape of a wing and bring it straight up and down into the water, you will get the same lift effect, causing the reaction of the boat moving.

The other aspect of wings is vectors. A vector is a visual representation of force and direction. Vectors are used in wings when a wing is tilted up in the front. The force from the air hitting the bottom of the wing is converted to upward motion. If you use this same aspect with oars by making a flat straight blade and tilt it and pull it straight down then reverse the angle on the way up you get the same forward motion.

If I were the rower of a wherry back in the eighteenth century I would probably choose a different way of rowing rather then a different oar design, keeping in mind the idea of vectors. In my own opinion, vectors are where the efficiency can be effected the most.

"Those who brag of strength are weak at heart."

Michael-James L. E 2/9/98.