By Herb Jarrett- Treasure Coast Propellers
In the last century Marine propellers were rather simple compared to modern day props. In 1995 most boats ran standard flat faced propellers, therefore the size was a combination of three factors (diameter, pitch and the number of blades). A prop stamped 30X32X4 would have 30” diameter, 32” of pitch and 4 blades.
Along with our demands for faster more efficient boats we have created demand for more complex propellers .Today we need to define propellers with the three basic factors and several more like, blade area, camber, and the pitch span.
Diameter = distance (inches /millimeters) from blade tip to the center of the propeller bore times two. Radius = 15”X 2 = 30” Diameter
Pitch =the distance (inches /millimeters) the prop would move in one rotation if it was in a solid medium.
Let’s define these less common terms.
Blade area = Expressed as BAR, EAR or DAR all mean about the same thing. They compare the actual surface area of the propeller blades to a full Circle with the same diameter as the prop. The propeller below has a70% DAR.
BENEFIT Larger BAR will produce more thrust and requires additional power. Larger BAR will allow us use smaller diameter props.
Camber = This is curvature of the entire pressure face of the prop. An example would be a swimmer cupping his hands to gain more thrust/speed. Positive camber allows the pitch to increase from the bottom( LE Leading edge) of the blade toward the top(TE trailing edge). Starting at the bottom with 30”and moving to 34” at the top will give us an average pitch of 32”. This cambered prop will be faster than a non-cambered flat faced prop with the same pitch.
BENEFIT Cambered propellers will be faster than flat faced props but also require more power. Yes it is possible for a cambered propeller to be faster than a flat faced prop with more pitch.
Pitch span = Standard propellers have the same pitch from the midpoint of the blades to the blade tips, thus a pitch span of zero. A custom design prop will have a regression in pitch as we move out toward the blade tip. The velocity of the blades increases as we move outward therefore a design with a regression of pitch allows us to maximize the average pitch of the propeller without overloading the engines .In the example below the power required to achieve maximum RPM is the same however the prop on the right has 0.8” more pitch and will be faster.
BENEFIT Pitch regression allows us to improve the match of blade loading and blade velocity thus maximizing the propeller efficiency.
Which propeller is the fastest?
The custom designed prop on the right will be faster because it has the same BAR, Camber and -8% pitch span. We normally see an increase in performance ( speed/fuel economy) of about 8-10% over a flat faced standard propeller.
Without all of this information we can’t really compare propellers.
Most propeller shops have computerized equipment to accurately measure your propellers. All boat owners would be wise to discuss their boats performance with their propeller specialist and jointly work toward maximizing performance and the longevity of their engines. Additional information such as wide open RPM, Vibration, increased fuel consumption or sluggish performance will help your prop guy provide you with the best value when working your props.
Herb Jarrett owns and operates Treasure Coast Propeller located in Stuart, Florida. Call 772-219-0881 or visit www.tcpropscan.com to speak with Herb.
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