There are two methods of prop balancing: static and dynamic. To achieve a professional result, both are required. Dynamic balancing requires special equipment that only the most dedicated mechanic would have in a home garage (or would understand how to use properly). If you don't have the specialised equipment to perform dynamic balancing, you can easily perform the static balance method using only simple tools. This method will allow you to make the necessary adjustments to get a good result.
Slip the prop hub over a shaft that fits snugly through the opening but allows free movement of the prop. Place the shaft horizontally across two braces, with the prop hanging in the middle.
Move the blades manually and stop at various locations. Watch to see if any are heavy enough to pull the prop until they are at the bottom. If a blade comes to stop at the very bottom of the arc, mark it with a pencil and repeat. See if the same blade continues to end up at the bottom of the rotation arc. A blade that is heavier than the others will cause the prop to pull down until it is on the bottom.
Remove the prop from the shaft and place it on a vice mount so it is immobile.
With the heavy blade on the backside (the side that faces the boat when mounted), grind the excess metal with a metal grinder.
Remount the prop on the shaft and place it on the horizontal stand to recheck for balance. If a blade is still heavy, continue to grind excess metal off of the backside.
Place a buffing pad on the grinder and buff the back of the ground prop blade to a shine. Place it on the boat.
If your prop requires dynamic balancing, take it to a professional prop shop. The shop will use a multi-place dynamic balancing analyzer to analyse the amount of torque in the planes of motion at top speeds. It is a very precise machine that takes a lot of skill and training to understand. A prop that is not balanced in rotation will cause severe cavitations while underway. The vibrations can damage the engine or other mechanical parts, as well as create a very uncomfortable ride.
Never grind excess metal off of the front face of the prop (the side that faces away from the boat when mounted). The pitch and arc of the blade are very important on the front. The backside does not affect the motion of the prop.