Choosing the right boat propeller can be tough, since you have pitch, diameter, cup, rake, and blades to consider. Before you can choose the best prop for your rig, of course, you need to understand boat propeller basics. Here’s a run-down:
PITCH is a theoretical measurement, describing how far the prop will move forward through the water with one full revolution.
DIAMETER is the size of your prop, expressed in inches. An easy way to picture diameter is to envision two times the distance from the tip of a blade to the center of the hub. When you look at a prop, you’ll note that most are identified by pitch and diameter, expressed in inches. Usually the measurements are stamped into the hub of the propeller.
CUP is the curvature at the trailing edge of the blades. Adding cup to a prop generally increases a propeller’s bite on the water, reducing slippage.
RAKE is the degree of slant of the blades in relation to the hub. Blades with a lot of rake may help lift the bow, while blades with negative rake, commonly used only on displacement boats, will force the bow down.
BLADES are measured in area, and number of blades. Area is the surface area of each blade in square inches, multiplied by the number of blades. Standard blade area for a common three-bladed prop is about 50- to 70-percent of the prop’s overall dimensions. If a prop doesn’t have enough blade area, thrust is lost. The number of blades may range from two to five, with three being the most common and offering the best all-around performance for most modern powerboats. Two bladed props are common to sailboats and other slow-moving vessels, while four blades are a good choice for boaters who are interested in a faster hole shot at the expense of a MPH or two of top-end speed. Five or more blades are commonly seen only on large inboard boats.
So: How do you know if your boat is in need of a propeller change? The most effective method is to look at your maximum throttle rpm, and make sure it’s in the middle of the manufacturer’s recommendation. If not, remember that increasing pitch by an inch results in a drop of 200 to 250-rpm at wide-open throttle. Decreasing pitch by an inch increases rpm by the same amount. And going to a cupped propeller or adding a blade of the same pitch and diameter will also draw down rpm by around 200. Also remember that stainless-steel props flex less than aluminum ones, so they usually offer slightly better top-end speeds.