PSModel a look inside



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and Data

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with Data


Test Sheet


19' Ski Boat

34' Sailboat

41' Utility Boat


Blade Area

the Cavitation


Hull Speed

Kt Breakdown


Propeller Law

Wake Factors




Ae -- Expanded Area of propeller blades, ft2

Advance Coefficient --  Va/(nD), also see J below

Advance Velocity -- average flow velocity into the propeller in feet per second, see Va below

Ap -- Projected Area of propeller blades, ft2: Ap ~ Ae × (1.067 − 0.229 × P/D)

Atmospheric Pressure -- approximately 14.7 psi or 2116.8 psf; 14.69595 psi = standard atmosphere set by the 10th Conference General des poids et measures (CGPM)

BAR -- generic for DAR or EAR, see the Blade Area Ratio section

Brackish -- water that is a mixture of salt and fresh water; usually a fresh water layer is on top of the denser saltwater

Cavitating Series -- A Propeller Series that is tested at various Cavitation Numbers, some of which result in propeller cavitation.

Cavitation -- vapor filled bubbles that form in water when the local pressure drops below the vapor pressure of water:  When the pressure increases above the vapor pressure, the bubbles collapse rapidly producing extremely large pressures at the instance they disappear.  If the collapse takes place on the back or face of a propeller blade, the extremely high pressure can cause pits in the propeller material. 

Cavitation Number -- also called Sigma, (Pa + Pd − Pv) / (½ρVa2); see Calculating The Cavitation Number section

Clipboard -- a file that stores data rows for a subsequent paste

Cup\Cupping -- propeller blade trailing edge modification that has an influence on blade cavitation and effective pitch; see the NV3 Series in the Propeller Geometry section for a cupping diagram.

D -- propeller diameter in feet

DAR -- Developed Area Ratio = (Developed Area of all propeller blades) / (propeller disc area)

Developed Outline\Area -- the blade outline on a propeller drawing generated by plotting propeller blade sections on arcs corresponding to their radius on the blade:  By connecting each of the plotted section end points with a line the developed outline is formed.  The area within the developed outline is called the Developed Area of the blade.  The Developed Area of the propeller is simply the Developed Area of one blade multiplied by the number of blades.

DHP -- Delivered Horsepower or power absorbed by the propeller

EAR -- Expanded Area Ratio = (Expanded Area of all propeller blades) / (propeller disc area)

Effective Pitch -- pitch of an uncupped propeller that gives the same performance as a similar propeller with cup.

Expanded Outline\Area -- the blade outline on a propeller drawing generated by connecting end points of blade sections that are plotted with nose-tail lines on straight horizontal lines at corresponding section radii:  The area within the expanded outline is the Expanded Area of the blade.  The Expanded Area of the propeller is simply the Expanded Area of one blade multiplied by the number of blades.  Note that the difference between Expanded Area and Developed Area is due to how the expanded outline and developed outline were formed: nose-tail lines in one case are laid out on straight lines, and in the other, on arcs.

Efficiency -- propeller efficiency = (J × Kt) / (2πKq)

ERPM -- Engine Revolutions Per Minute

g -- standard acceleration due to gravity = 32.174 ft/sec2; value set at the 3rd General Conference on Weights and Measures (or 3rd Conference General des poids et measures, CGPM)

H -- Head of water in feet that produces a pressure equal to the total pressure = atmospheric pressure + water depth pressure + vapor pressure

Horsepower -- power output by an engine or absorbed by a propeller = 2πQn / 550

Hull Speed -- approximate speed of a boat, operating in the displacement mode, above which powering requirements start to rise rapidly for relatively small increases in boat speed, see the Hull Speed section 

J -- Advance Coefficient = Va/(nD)

Knots --  nautical miles per hour:  Multiply speed in Knots by 1.15078 to get speed in MPH (e.g., 10.0 Knots x 1.15078 = 11.5078 MPH).

Kt -- Thrust coefficient = T / (ρn2D4)

Kt Breakdown --  see the Kt Breakdown section for discussion

Kq -- Torque coefficient = Q / (ρn2D5)

L.E. -- propeller blade leading edge

n -- propeller revolutions per second, or RPM / 60

Noncavitating Series -- A Propeller Series that is tested at cavitation numbers great enough to eliminate propeller cavitation.

Open Water -- As used in PSModel, this term refers to Propeller Curves or a Propeller Series with the following characteristics:

1) propeller tests were done in open water with uniform inflow

2) cavitation may have been present during tests, but not enough to cause Kt Breakdown

P -- propeller pitch in feet; the distance a propeller would advance after one revolution with no slip: This is equivalent to how far a screw would advance in wood after one complete turn or revolution of the screw driver.  

Pa -- atmospheric pressure = 2116.8 psf

Pd -- water pressure at the propeller hub = ρ × g × (Depth to hub in feet), psf

Polynomial -- The term polynomial, as used in PSModel, refers to mathematical equations that are solved to obtain Kt or Kt values.  These equations or polynomials are obtained through regression analysis of Series test data.

Propeller Curves -- graph of J verses Kt, Kq, and Efficiency

Propeller Law -- see the Propeller Law section

Propeller Series -- A propeller Series is a family of propellers consisting of a parent and several child propellers.  The parent propeller sets the geometry of the Series around which the child propeller geometry is systematically varied.  Variations may include changes in propeller pitch, blade surface area, number of blades, etc.  By testing this family of propellers and recording such things as thrust, torque, and cavitation patterns, propeller curves can be created.  

Projected Outline\Area -- The projected outline of a propeller blade is the outline or surface you see when looking at the propeller blade from a point perpendicular to the propeller shaft, or from a point ahead or aft of the propeller.  The Projected Area is the area enclosed by the Projected Outline. 

psi -- pounds per square inch

psf -- pounds per square foot

Pv -- water vapor pressure, psf

P/D -- propeller pitch divided by propeller diameter:  Diameter and pitch are usually stamped on the hub of small propellers.  For example, the stamp 16 x 12 indicates a 16 inch diameter by 12 inch pitch  (P/D = 12/16 = 0.75).

Q -- propeller torque in foot-pounds

Rake -- If propeller blades appears to be sloped fore or aft when the propeller is viewed from the side, the propeller is said to have rake.  Rake usually has little affect on propeller thrust or torque.  For an example of rake, see the B-Series in the Propeller Geometry section.

Reynolds Number -- a nondimensional number that applies to viscous fluid flows: Discussions of Reynolds Number can be found in any technical book dealing with Fluid Mechanics or Dynamics.

Rho -- water density, lb-sec2/ft4 (also called ρ).  In output data, Rho, the type of water, and water temperature are expressed in compact notation: e.g., Fresh water at 60 degrees Fahrenheit has a density of 1.9383 lb-sec2/ft4 and is represented as 1.9383F60; Salt water at 59 degrees Fahrenheit has a density of 1.9905 lb-sec2/ft4 and is represented as 1.9905S59.

RPM -- Revolutions Per Minute

Segmental Sections -- blade section shapes which are generated from straight lines and circular arcs

Sigma -- same as Cavitation Number above

Skew -- If propeller blade sections are progressively swept back along the pitch helix, the blade is said to have skew.  Blade skew tends to reduce vibrations and improve cavitation performance when the propeller is operating in a variable wake field.   Skew usually has little affect on propeller thrust or torque.  For an example of skew, see the NV3 and NV4 Series in the Propeller Geometry section.

Subcavitating -- As used in PSModel, this term refers to propellers that operate with less than 10% of the blade backs covered with cavitation.

Supercavitating -- This term refers to propellers whose blade backs are completely covered with a cavitation bubble or sheet.

T -- propeller thrust, pounds

Tabular Data -- tables of data containing J, Kt, and Kq values derived from propeller testing

T.E. -- propeller blade Trailing Edge

Torque -- shaft twisting moment in foot-pounds (ft-lbs)

tc -- thrust load coefficient = T/{½ρ × Ap × [Va2 + (0.7πnD)2]}

V -- boat speed or velocity, ft/sec

Va -- average flow velocity into the propeller in feet per second, sometimes called the Advance Velocity = Boat Speed × (1-w)

Wake Factor -- see 1-w below

Water Density -- see Rho above and ρ below

Z -- number of propeller blades

ρ -- water density, see Rho above

π -- pi ~ 3.14159

1-w -- wake factor = Va / V, see the Wake Factor section