Tuesday, April 29, 2008

Propeller Facts

Marine propellers work by converting power produced by your boat's engine to thrust. As a propeller rotates, it draws water from ahead (the suction side) and pushes it out astern (the discharge side). It is this force that propels your boat.

Any given prop's size is described by two numbers. A propeller that is sized 20" x 24". The first number in this case, 20 represents the diameter of the propeller. Diameter is a critical dimension when converting engine horsepower to thrust.

The larger the diameter of the prop, the larger the column of water in which it's working. To calculate the correct diameter of any prop, measure the distance from the center of the propeller to the tip of one of the blades, and then multiply by two.

The second number (24 represents the pitch. Pitch is the distance the propeller moves through the water with each complete revolution. There is a certain amount of slippage between the prop and the water, so the distance actually traveled will be less than the distance.

Propellers come with 2, 3, 4 or 5 blade configurations. Having enough blade area is an important factor in transferring horsepower to thrust. Too little blade area causes high blade loading, making the prop incapable of using all the available power from the engine. This leads to cavitation, vibration and even pitting of the prop. To much blade area puts excessive load on the engine and does not allow it to produce its maximum hp.

Using a four-blade prop instead of a three-blade prop has several advantages. Your boat will accelerate and come up on plane faster, and stay on plane at lower rpm's. You experience less vibration with a four-blade prop. You will also have an increase in speed at mid-range rpm's, but there will be less speed at wide open throttle.

Cupped propellers - Have an extra curve on the trailing edge of their blades, which enables the prop to cut through water better. A properly cupped propeller should give your boat a higher top speed or allow you to go faster at the same rpm's.

The four most common materials with which propellers are made are:

Composite - Very durable and some models allow you to replace a single damaged blade instead of the entire prop. they do have more flex in them than a metal prop, so they lose some performance.

Aluminum - The most common material used in outboard and I/O installations. They are relatively easy to repair and have the lowest initial cost. They also have some flex in them with a small loss in efficiency.

Stainless Steel - These props have higher performance than composite or aluminum and have less flexing, but are more expensive to purchase and repair.

Bronze - Has high performance and good durability. These props are mostly used on inboard installations, such as ski boats, cruisers and yachts. They have a higher cost but are easy to repair.