Saturday, April 5, 2008

Propeller and Propulsion Terminology

Diameter Defined as the maximum radius of one blade multiplied by 2.The diameter of the circle scribed by the blade tips as the propeller rotates.General rules: Diameter usually increases as engine power increases and vice versa. (all other variables remaining constant)Diameter increases for slower boats and decreases for faster boats.

Pitch Theoretical definition: The linear distance that the propeller would move in one complete revolution through a solid medium not allowing for slip. Under actual operating conditions, slip occurs as propellers rotate, so absolute forward movement (actual pitch) is less than theoretical pitch.

Different types of pitch are:

1. Constant (fixed) pitch - pitch is equal for each radius

2. Progressive pitch - pitch increases along the radial line from Leading Edge to Trailing Edge.

3. Regressive pitch - pitch decreases along the radial line from Leading Edge to Trailing Edge.

4. Variable pitch - pitch is different at selected radius. Controllable or Adjustable pitch - blade angle is mechanically varied.

Pitch Angle (Not to be confused with pitch) Angle of the pressure face along the pitch line with respect to the plane of rotation measured in degrees. Pitch angle decreases from the blade root to the tip in order to maintain constant pitch.

Pitch Line -A line that passes through the Leading Edge and Trailing Edge of a blade used as a reference for pitch angle.

Propeller Center Line (PCL)Linear reference line passing through hub centre on the axis of propeller rotation.

Propeller Center Axis (PCA)Linear reference line that locates the blade on the hub. Perpendicular to the Propeller Center Line (PCL).

Blade Center Axis (BCA)Linear reference line that indicates propeller rake.

Blade Center Line (BCL)Reference line that intersects each cylindrical section at the midpoint of the blade section width. Indicates propeller skew.

Rake Propeller blade will slant forward or aft from the Blade Center Axis (BCA). Positive rake - blade slants towards aft end of the hub. Negative rake - blade slants towards forward end of the hub. Can be specified in inches at the tip or in degrees.

Skew Blade Center Line is curvilinear sweeping back from the direction of rotation. Contour of the blade is not radially symmetrical about blade center axis.

Track Measurement of axial position of all blades with respect to each other.
Rotation Right hand propeller rotates clockwise when viewed from astern facing forward. Left hand propeller rotates counter clockwise when viewed astern facing forward. Twin screw applications utilize both LH (port side) and RH (starboard side) rotating propellers.

Blade Numbering - By convention the blade located at the position of the keyway is identified as Blade 1, the next blade in rotation is Blade2 and so on.

Blade Thickness - A blade is thickest at the root for structural integrity. Within each radial section, the point of maximum thickness may not necessarily coincide with the midpoint of the chord length.

Blade Thickness Fraction (BTF)Maximum blade design thickness as extended to the propeller center line / propeller diameter. Blades must have enough thickness to achieve desired sectional shape and provide sufficient strength under loading. Blades that are too thick produce less propeller efficiency.

Disc Area - Area of the circle scribed by propeller blade tips (P r²) where P=Pi (3.14159) and r = radius (1/2 diameter) of the propeller.
Projected Area Ratio (PAR)Area of projected outline of propeller divided by disc area.Smallest area ratio.

Camber - Defined as curvature in the mean thickness line of the blade section.
Blade Tip Maximum reach of the blade from the center of the hub. Separates the leading and trailing edges.
Leading Edge (LE)Edge of the blade that first cuts the water.

Trailing Edge (TE)Edge from which the water exits the blade.

Blade Face (Pressure Side or Pitch Side)Side of the blade facing toward you while viewing from the vessel’s stern.

Blade Back (Suction Side) Side of the blade facing away from you while viewing from the vessel’s stern.

Blade Root (Fillet area)The area where the blade attaches to the hub.
Hub Solid cylinder located at the center of the propeller. Bored to accommodate the engine shaft.

Keyway Slender - Rectangular slot broached into the interior of the hub. Helps to secure propeller to the shaft and prevent rotational slipping on the shaft.

Cup Small radius or curvature located at the trailing edge of blade. Cupping, helps to reduce or delay cavitation. Helps to reduce slip, thus increasing actual pitch and usable thrust.

Cavitation - Cavitation is the phenomenon of water vaporizing or boiling due to the extreme decrease in pressure on the forward, or, suction side of the propeller blade. Cavitation can be caused by nicks in the leading edge, bent blades, too much cup, sharp corners at the leading edge, incorrect matching of propeller style to the vessel and engine or, simply high vessel speed.

Ventilation - Sometimes the term cavitation is used when in reality ventilation is actually occurring. Ventilation is air from the water surface or exhaust gases being drawn into the propeller blades which causes the propeller to over rev and lose thrust. This is the effect that you sometimes feel when you are running in a following sea in rough weather.

Singing - Some propellers in service produce a high-pitched noise, often referred to as Singing. This sound typically is a clear harmonic tone much like a humming or ringing wine glass.More of an annoyance than anything harmful, the causes of singing are not completely understood. Many theories have been put forward to account for the phenomenon of Singing, but it appears to be affected by critical factors for which the theories make no allowance. For example a twin-screw vessel has one propeller that sings and the noise is eliminated just by switching position of propellers. Or a singing propeller is replaced by an identical spare propellers which is found to be silent. Also the lower the number of blades on a prop the less chances of "singing".

Slip - The difference between the theoretical distance the propeller should travel in one revolution and the actual distance the vessel travels. For example if you cruise at 2000 rpm and your vessel has a 2:1 reduction gear, a wheel with 24" pitch, your theoretical speed through the water should be 19.74 knots (the distance a 24" wheel should move in one hour). In reality your vessel only does 14 knots at 2000 rpm on a calm day with no current, the difference is slip.