How to Make a Dunking Tank

Measure the diameter of the tub and add 6 inches. Cut the 2-inch-by-6 inch beam to the calculated length with a circular saw.

Disassemble a boat seat and measure the width of the backrest. Measure and mark these dimensions in the center of the 2-inch-by-6-inch plank and cut the section out of the board.

Re-attach the removed wooden section with strong gate hinges to the underside of one long section and a heavy-duty gate latch on the other side, so the seat can be secured and swing open as a door would. Use exterior screws that are resistant to moisture.

Attach the boat seat backrest to the rear of the wooden beam over the cut-out section with bolts. Mount the seat to the swinging portion of the beam with bolts. The front of the seat will extend beyond the edge of the beam.

Trace a circle 10 inches in diameter on a piece of plywood and cut it out. Paint a bullseye target with red and white paint. Allow the paint to dry and attach a bracket to the center of the target's backside with screws. Reinforce with epoxy and slide the bracket onto one end of the spring.

Connect the loose end of the spring to the release pin in the latch. The spring must be stretched to full extension or it will not release the seat when the target is hit.

Secure the beam to the pool rim with epoxy on both sides. Fill the tank with water and aim for the target with tennis balls.