Why Dredge?
Level 1

Overview:
This hands-on activity helps young students understand the concepts of ships' draft and the need for dredging. It takes some preparation, but the children seem to enjoy it, as it involves water play.

This activity can be done in small groups, no more than four students to a group, as all will want to try filling the "cargo holds."

Skills:

1. Observation
2. Graphing
3. Deductive reasoning

Materials: For EACH setup

1. Three plastic toy boats with keels. These are likely to be the most difficult items to find, as many toy boats have flat bottoms. The boats must also be able to be filled with bulk grain seeds, so there must be an open space on the top of each. Scouring the cheap toy sections of various variety stores, I have found suitable boats for less than $2 each.

2. A clear plastic storage tub, at least 6 inches deep.

3. Enough clean sand to put 2-3 inches in the bottom of the tub.

4. Water.

5. 2 cups each of three different weight grains ( for example: wheat germ, wheat berries, and noodles, or corn flakes, cornmeal, and popcorn kernels).

6. Marker.

7. Copies of the attached worksheet, one per student.

Preparation:

1. Before the activity, float one of the empty toy boats in water. Note how deeply it sinks and mark that point on the hull of the boat. Take the boat out, fill it with the heaviest of your grains and put it in water again. Note how deeply it sinks and mark that point on the hull. Ideally, there is at least an inch or more of distance between the two marks. Make evenly spaced lines between the two, and add at least one above and below them. Label them, starting with 10 at the bottom and ascending by 10s going up the side. Mark each of the other two boats similarly.

   

2. Put several inches of sand in the bottom of the plastic tub. Carefully pour water over the top of the sand. The depth of water over the sand should be about equal to the distance that the boat will sink when fully loaded with the heaviest grain.

3. Put the grains in sacks or containers and label them #1, #2, and #3. Make sure the heaviest grain is #3.

Procedure:

1. Beforehand, the teacher or guest speaker will discuss the importance of inland waterways as the nation's underwater highways. Speak about the types of cargo carried by the different types of ships and barges, being sure to talk about bulk cargo, such as grains.

2. Talk about how the channels are dredged to be sure ships and barges can navigate safely. Discuss the concept of ships' draft. Draft is the distance a ship sinks into the water. (Older students may be able to understand draft as a measurement of water displacement, which changes depending on the density of the cargo and whether or not the ship is in fresh water or salt water).

3. Break the students into cooperative groups of no more than four students each (one recorder and three fillers). Give each student a worksheet on which to record his or her information at the end of the exercise.

4. Explain to the students that they are to fill each ship with a different grain, float it in the water, and record the draft from the scale on the hull. Do the procedure with grain #3, the heaviest grain, last. The students can take turns filling and recording the results. (Note: this is usually a lively and noisy time as they fill and spill and record! Consider having half the class in groups and the other half doing word scrambles, then switch).

5. Have the students observe what happens when the boat is filled with the heaviest grain. Is it touching bottom? If the sand were the bottom of the navigation channel, what would happen? What could be done to remedy the situation? (Dredging is one alternative; light loading is a second, less economically attractive alternative).

6. Have the students complete the worksheet and fill in the bar graph. The bar graph is colored from the top down to illustrate draft.

7. Depending on their writing skills, write in or talk about the answers to the questions.

The draft of a ship is the depth of a ship's bottom from the surface of the water. It is usually measured in feet. If the ship is full of heavy cargo, the draft will be greater than if it is not full. Heavy cargo will make the ship sink deeper in the water than light cargo.