Skellan Dam Powerhouse Cutaway: Planning an Overlook Panel

Opening and site reading (10 min): Read the site brief on the Student Handout aloud while students follow along. Ask them to list, in two columns, what a visitor at the Skellan overlook can actually see and what is hidden inside the dam and powerhouse. Collect three or four answers from each column on the board. Point out that the panel's job is to connect the second column to the first.

How the unit works (15 min): Draw a simple side section of a dam on the board and trace the path of the water with students: the intake with its trash rack near the upstream face, the penstock that carries water down through the dam, the spiral case that wraps around the turbine and guides water inward through the wicket gates, the turbine runner, the draft tube below it, and the tailrace that returns water to the river. Show that the runner turns a vertical shaft connected to the generator above it. Ask: "Why is the turbine down here instead of at the top?" Take answers, then state that the power available grows with the height the water falls, so the equipment sits as low as possible to capture the full head between the reservoir surface and the river below. Have students complete handout item 1 before moving on.

Cutaway conventions (15 min): Use the two cardboard tubes to show that a plane through the long axis of a cylinder produces a long rectangle, while a plane across it produces a circle, and ask which cut of the penstock shows water moving along it. Then introduce four conventions and sketch each quickly on the board: a cutting plane chosen so the whole path lies in one view; a flat tone or hatch on every cut surface, so the viewer can tell a sliced wall from an exterior surface; ghosting, in which the outer wall of the powerhouse is drawn as a faint outline so visitors still recognize the building they are looking at; and a single continuous water path drawn in one saturated color with arrows, while everything else stays in grays and line. Add one orientation rule: the drawing should be viewed from the same direction and height as the overlook, so the dam in the drawing lines up with the dam in the landscape without any mental rotation.

Thumbnails (20 min): Each student draws three small thumbnails, about 3 by 4 inches, on tracing paper. Thumbnail A uses a vertical cutting plane through the penstock and the turbine shaft, seen from the side. Thumbnail B uses the same plane but is drawn in a three-quarter view from the overlook's position on the east rim, looking upstream and slightly down. Thumbnail C uses a horizontal cut at the level of the turbine, seen from above. Students circle the thumbnail they will develop and write one sentence explaining the choice.

Formative check (5 min): Partners trade thumbnails and check them against handout items 2 through 5. Circulate and count. If more than a quarter of the chosen thumbnails place the turbine near the crest or break the water path into separate pieces, stop the class and redraw the path on the board once more before students continue.

Developed sketch and callouts (20 min): On a fresh sheet of tracing paper laid over their chosen thumbnail, students draw the developed cutaway at a larger size, using the heavier fineliner for cut edges, the lighter fineliner for ghosted and background forms, and the blue pencil only for the water. They add five numbered callouts that follow the path in order, from intake to tailrace, and write one or two plain sentences for each callout in the margin. Remind them that the panel will be read standing at a rail, so each callout should be short enough to read in a few seconds.

Exit ticket (5 min): On an index card, students answer handout items 6 and 7. Collect the cards and sort them into two piles, correct and not yet correct, for item 6; if more than a quarter land in the second pile, open the next class by retracing the water path from the reservoir surface to the tailrace.