Re: Screw size and speed

[Ray Keefer <Ray.Keefer@eng.Sun.COM>]

Hi,

I made a white paper out of Tim's input. See the web site main page for
details.

Regards,
Ray

> 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:
> 
>             Pitch x RPM 
> Propeller slip stream speed in MPH =                     1056
> 
> 
> 
>             9" x 1500
> So.......                                                             1056  
>               = 12.7 MPH Slip Stream
> 
> 
> 
> Apparent Slip in % = slip stream speed - vessel speed
> 
>                         Slip Stream Speed
> 
> 
> 
> So.......                                                         12.7 - 7
> 
>                                 12.7                 = 45% Apparent Slip
> 
> 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.
> 
> 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.
> 
> 
>