How sailboats stay upright


When is it easier for you to balance: when you stand on one foot, or when you crawl on your hands and knees? For example, try standing on one foot and leaning to the side. How far can you lean before you fall over? Now get down on all fours and try it again. How far can you lean before you fall this time?

You might be wondering what any of this has to do with boats. You probably noticed that it is easier for you to balance, or avoid falling over, when you are low to the ground. The same concept applies to boats — they need to stay upright and avoid capsizing, or flipping over. To do this, they need a low center of mass. In physics the center of mass of an object is a point where the entire weight of the object may be represented as being concentrated. You can shift the center of mass of an object by adding weight. When you add weight to the top, the center of mass will shift up, when you add it to the side, it will shift to that side, and when you add it to the bottom, it will shift down. The more weight you add to one side, the more the center of mass shifts.

To have a low center of mass, that means the weight must be concentrated towards the bottom of the boat, not the top. That might seem like it is a problem for sailboats, boats with very tall sails that stick way up into the air. How do they stay balanced with so much weight way up high? They do so with by using a ballast, a heavy weight held under the boat that helps lower the center of mass. In sailboats, the ballast is usually part of the keel, a large fin-like structure under the boat that serves two purposes (Figure 1). It holds the ballast, and it also prevents the boat from being blown sideways by the wind. Are you surprised that adding more weight to a boat can be a good thing? In this project, you will find out how heavy the ballast needs to be in order to keep a sailboat upright, depending on the height of the sails.

Low tide reveals a large fin attached to the bottom of a sailboat
Figure 1. This sailboat is stuck in low water, exposing the keel underneath.
Technical NoteTechnically, in physics the terms “mass” and “weight” have different meanings. An object’s mass is always the same, no matter where it is in the universe. However, an object’s weight depends on gravity—so, for example, it will be different on the Moon than on Earth. In elementary school the terms are often used interchangeably, since we’re usually referring to objects on Earth’s surface and not worried about changes in gravity. Older students may be expected to make this distinction.

Similarly, the terms “center of mass” and “center of gravity” technically have different meanings, but for objects on Earth in constant gravity, they can be used interchangeably.

Terms and Concepts

  • Balance
  • Capsize
  • Center of mass
  • Ballast
  • Keel


  • What are the purposes of a sailboat’s keel?
  • Do you think a boat with a taller sail will require a heavier ballast? Why?
  • For help creating graphs, try this website:
  • National Center for Education Statistics, (n.d.). Create a Graph. Retrieved June 25, 2020.

Materials and Equipment

  • Identical wine corks (3)
  • Wooden skewers (5)
  • Craft foam or paper milk cartons
  • Identical nails or screws that are roughly the length of your corks (about 10–20, exactly how many you need will vary depending on their size)
  • Sink, bathtub, or a large container you can fill with water. The container should be deeper than the length of your nails/screws.
  • Water
  • Ruler
  • Scissors
  • Lab notebook

Experimental Procedure

  1. If you haven’t already, watch these two videos for an overview of the project and how to build the boats. The procedure you follow to do a science fair project will be slightly more detailed.
  2. Fill your container with water. Make sure the water is deeper than the length of your longest nail/screw.
  3. Create a data table like Table 1 in your lab notebook.
Number of nails required to keep boat upright
Sail Dimensions Trial 1 Trial 2 Trial 3 Average
4 × 4 cm
6 × 6 cm
8 × 8 cm

Table 1. Example data table.

  1. Cut a 4 centimeter (cm) section from the pointy end of a wooden skewer. The skewer will be your mast (the part that supports the sail).
  2. Cut a 4×4 cm square of your sail material.
  3. Poke the pointy end of the skewer through the edges of the sail, then poke it into the cork to make your first sailboat (Figure 2).
A sheet of green foam covers the top of a toothpick stuck into the side of a cork cylinder
Figure 2. A toy sailboat with no ballast.
  1. Put your sailboat in water and watch what happens.
  2. Poke a nail into the bottom of your boat to form a ballast, directly opposite the mast (Figure 3). Note: if you are using screws, make sure you twist them into the cork instead of pushing them.
A cork cylinder has a nail inserted into its side with a toothpick and green foam inserted into the opposite side
Figure 3. Sailboat with a nail for ballast.
  1. Put your boat back in water and watch what happens.
  2. If your boat still doesn’t stay upright, add a second nail in a straight line with the first one (Figure 4) and put it in the water again.
  3. Keep adding nails, one at a time, and re-testing until your boat stays upright.
  4. Record the number of nails it took to keep your boat upright in your data table.
Three corks each have a toothpick and green foam inserted into one side and 1, 2 or 3 nails inserted into the opposite side
Figure 4. This picture shows how to add more nails to the bottom of your boat. Try to make sure the nails are approximately centered on the cork-front to back. Put the first nail in the middle of the cork. If you put the second nail behind the first nail, then you should put the third nail in front of the first nail, and so on.
  1. Repeat steps 4–11 two more times, for a total of three trials for this sail size (make a new boat for each trial). Remember to record your results in your data table.
  2. Repeat steps 4–12 for sail sizes of 6×6 cm (use a 6 cm mast) and 8×8 cm (use an 8 cm mast). If necessary, you can re-use the same three corks (remove the nails and previous sails from the corks first).
  3. Make a graph of your data, with sail dimensions on the horizontal axis and the average number of nails you needed to keep the boat upright on the vertical axis.
    1. Note: to calculate an average, add up the number of nails for your three trials, and divide the total by three. Ask an adult if you need help.