You obviously have read some things about propellor design and it is also obvious you accept some statements as true without fully understanding them. Go read some real text books talking about propellor theory and not just the brochures at the marina before you make statements like these. As an engineer with many hours of course work and work experience involved with both air flow and liquid flow I would have to say you are out of your league when it comes to propellor design. Don't quit your day job!
Later,
Dick Morrisson
Tim Garton wrote:
Here are some notes on Props for info purposes:Diameter: The diameter of a propeller is the diameter of the circle swept by the tips of the blades. Diameter is the single most important factor in propeller calculations. A slight change in diameter has more effect on power absorbtion than a considerable change in pitch or blade area. The most common error is to install a propeller of too large a diameter for the power available.Pitch: Like a screw turning into a piece of wood, the pitch of a propeller is the distance it will advance in one revolution through the water, assuming there is no slippage.
Slip: There are two kinds of slip. Apparent slip is the difference between the advance observed and that calculated by pitch times revolutions. It is what is more important. True slip is greater than apparent slip, due to wake moving with the vessel. The wake that affects slip is a body of water surrounding the propeller and moving along with the vessel. The amount of wake is determined by the amount of friction produced by the hull moving through the water.
Example: Find apparent slip, knowing the vessel speed, revolutions, and propeller pitch. Lets say the boat makes 7 MPH with a propeller having a pitch of 9 inches, turning at 1500 rpm:
Propeller slip stream speed in MPH = 1056 So....... 1056 = 12.7 MPH Slip Stream
So....... 12.7 - 7 Apparent Slip in % = slip stream speed - vessel speed Slip Stream Speed
Cavitation: Excessive propeller tip velocity is the main cause of the creation of cavities or voids in the water, creating the phenomenon known as cavitation. Other contributing factors are air-foil section propeller blades (sections with even curvature are more desirable), insufficient tip clearance (12% of the diameter is good), and a disturbed flow of water to the propeller. Disturbances are easily created by posts not faired, struts too close. Clearance ahead of a propeller should be 20% of its diameter. In order to delay cavitation, it is desirable to use sections with even curvature rather than sections with uneven curvature (airfoil sections), and wide blades. 12.7 = 45% Apparent Slip Pitch Ratio: Pitch Ratio is the ratio of pitch to the diameter, or the pitch divided by the diameter. Practicable pitch ratios range from .5 to 1.5. Below and above these limits the efficiency is very low.
Mean Width Ratio:
Mean Width Ratio is the ratio of the average width of the blade to the diameter of the propeller. Average width is arrived at by dividing the area of the blade by its length from hub (at root) to tip.
Temperature:As water temperature drops, its density increases. So a propeller suitable for warm water operations being used in colder water has its Rpm’s drop below the designed level. Based on a 70F temp, you can reduce the diameter approximately 1% for each 10F drop in water temp.