Welcome to the

White Paper: Propeller Considerations

About:

Contributor:	Tim Garton

Last update:	19 Jul 1999

Synopsis:	These are propeller design considerations that Tim put together
		in an e-mail to the listserver alias. Ray cleaned it up 
	  	the math a bit and be aware that these calculations are 
		ball park figures for a very complicated subject.

Disclaimer:	You are resposible for your own safety. Even though
		this information may work or seem resonable in some 
		cases you need to approach the subject carefully and 
		even aquire the help of a Marine Architect before applying
		into one of your designs.

Body:
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 "PM  (Inches Per Minute):
	= Pitch     x RPM 
        = 9"/Rev    x 1500 RPM
	= 13500"PM

Propeller slip stream speed in MPH (Miles Per Hour):
	= "PM x 1ft/12" x 1Mile/5280ft x 60min/Hour
	= 13500"PM x 1ft/12" x 1Mile/5280ft x 60min/Hour
	= 12.78MPH
 



Apparent Slip (in %):
	  slip stream speed - vessel speed
	= -------------------------------- x 100%
                 slip stream speed

	  12.78MPH - 7 MPH
	= ---------------- x 100%
	  12.78MPH

	= 45.23%


Nominclature:

36 x 36 3-blade Bronze
 |    |  |        |
 |    |  |         --->  Material it was made out of.
 |    |  |
 |    |	  ---> 3 blade
 |    |
 |     ---> Pitch (Inches per revolution)
 |
  ---> 36inch diameter sweeped by the propeller tips


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.