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Skew The transverse sweeping of a blade such that when viewing the blades from fore or aft shows

an asymmetrical shape. Aft Skew: Positive skew. Blade sweep in the direction opposite of rotation. Forward Skew: Negative skew. Blade sweep in the same direction as rotation. Slip Slip is the difference between actual and theoretical travel of the propeller blades through water. A properly matched propeller will actually move forward 80 to 90 percent of the theoretical pitch. Trailing Edge The edge of the propeller adjacent to the aft end of the hub. When viewing the propeller from astern, this edge is closest. The trailing edge retreats from the flow when providing forward thrust. Track The absolute difference of the actual individual blade rake distributions to the other blade rake distributions. Always a positive value and represents the spread between individual blade rake distributions. Ventilation Ventilation is a situation where surface air or exhaust gasses are drawn into the propeller blades. When this situation occurs, boat speed is lost and engine RPM climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine. Propeller Basics
Most marine propellers have markings which indicated it's dimensions. There are a number of different combinations of markings, but they should be in order of diameter, followed by pitch. Quite often on inboard propellers the diameter and pitch will be separated by an X or the direction of rotation. Some outboard propellers use part numbers which have to be cross referenced to a catalogue to determine the dimensions. When discussing or ordering propellers, always refer to the diameter first, then the pitch e.g.. 20 by 18, which indicates a 20 inch diameter propeller with 18 inches of pitch. Propellers are specified in millimeters around the world, except for North America, where inches prevail.

Propellers are made up of various elements.

The Hub, is the center of the propeller. It functions to provide a method of

attaching the propeller shaft to the blades. The blade root is where the blade is attached to the hub. It is typically the thickest part of the blade. The blade is the helical formed section of the propeller which transmits the

rotational torque of the propeller shaft into thrust to propel your boat. Blades are designed in all kinds of profiles and outlines, each offering various benefits in converting torque to thrust. The blade pressure face is the high pressure side of the propeller blade facing away from the bow of the boat. The blade suction face is the low pressure side of the propeller blade facing toward the bow of the boat. The leading edge runs along the blade outline from the root to the tip separating the pressure and suction face of the blade. A sharp leading edge reduces the load on the shaft, but increases the chance of damage. Most leading edge profiles are a trade off between strength and durability. The trailing edge runs along the blade outline from the root to the tip. It is where the water exits the blade. The profile of the trailing edge is critical in reducing noise and harmonics. The blade tip is formed between the leading and trailing edges on the blade outline. The distance from the center of the hub to the blade tip times 2 describes the propeller diameter.

Propellers are made of various materials, including everything from plastic to titanium. The most common materials are:

Composite. This material is very light weight and moderately strong. It is

expensive and almost corrosion proof, but is non-repairable. Aluminum. This material is light weight and moderately strong. It is inexpensive and repairable, but it will corrode readily. Stainless Steel. This material is heavy and very strong. It is repairable, but very expensive. It is corrosion resistant, but will corrode rapidly without the presence of oxygen. Manganese Bronze. This material is moderately heavy and moderately strong. It is the least expensive of the bronze materials and is quite repairable. It's main ingredients are copper and zinc. With a lack of protection, the zinc content in the material will act as an anode to protect other underwater metals on your boat leaving oxidized copper behind. This is indicated by a pink coloring to the propeller. After enough corrosion takes place, the propeller will be brittle and need to be replaced. Nibral (nickel aluminum bronze). This material is lighter and stronger than manganese bronze. It is more expensive than manganese bronze, and is quite repairable. It's main ingredients are copper, aluminum, and a small amount of nickel. Due to it's higher copper content, it's more corrosion resistant than manganese bronze, but left unprotected the aluminum can act as an anode to protect other underwater metals on your boat. This is indicated by dark craters on the surface of the material ringed with a green color. Left long enough, these craters will go right through the propeller, and it will have to be replaced.

Propeller rotation. For some reason, this is often a confusing subject.

Right hand. This refers to the direction of rotation of the propeller to provide
forward thrust to your boat. A right hand propeller rotates clockwise when viewed from astern. In other words, standing behind your boat and looking towards the bow, the propeller turns in a clockwise rotation. Left hand. This is the opposite of right hand, and viewed from astern, rotates