Here is how to make a very good catapult that throws over 25 yards.

Step 1: Step 1 and 2

build 2x2 square for a base.
Then screw in 2 3 foot base arms for support of the throwing arm
once that is done drill 2 inches above the centor of the base arms with a 1 1/4 inch drill bit

Step 2: Step 3

get another piece of wood that is about 30 inches long
drill a hole directly in the center of the arm with the 1 1/4 inch drill bit
get a hollow pip that is 1 1/4 inches in diameter

Step 3: Step 4

insert the metal pipe through the bases and the arm
next drill 2 U rings on the inside of the square base at the bottom

Step 4: FINAL STEP

get a gatorade bottle cap and hot glue in or super glue it to the top of the throwing arm facing away from the U rings
finally hook up 2 springs to the U rings and the throwing arm

If built correctly the catapult should throw easily over 35 yards
ours was the best in the class and it rocketed a golf ball 46.9 yards
the acceleration of the arm was 308m/s