Input Data Here   How To      
Number of Motors (2 max) Max displacement in lbs
BHP per Motor
LWL in feet
Max continuous RPM Beam waterline in feet
Hull Draft in feet exc keel or deadwood
# of gearboxes or vee drives reqd speed in Knots
# of bearings
"C" for hull (150 for runabout, 190 for fast, 210 for race.)
g/box reduction ratio
Max prop dia in inches
propellers, each diameter inch pitch, with DAR
material propshaft diameter ft propshaft bearing spacing
will develop  pounds of bollard pull.
  Propeller Specification (long) How To  
Propeller Diameter (inch) Number of Propellers
Propeller Pitch (inch)
3 Number of blades
Disk Area Ratio
Maximum RPM
Weight (lbs (bronze) 3 blade)
Shaft Diameter (inch)
Maximum Static Thrust (lbs)
  Displacement Speed How To      
Formula for Speed : Length ratio
True S/L ratio = Knots / square root ( LWL ) (B)
Calculated S/L ratio = 10.665 / cube root ( max DISP / SHP ) (A)
motor HP # motors
Max "hull speed" (knots)
% transmission losses
Reqd speed
SHP at prop
Speed Length Ratio (B)
Speed Length Ratio (A)
Average of (A) & (B)
Alternative estimate of SHP reqd based on average of (A) & (B) (guide only!)
SHP reqd
SHP available SHP for ancilliaries
Engine Horsepower
# motors
Engine R.P.M. (max) 
Engine Torque ft/lb
# bearings between gearbox output and propeller.
Gearbox reduction ratio.
Percentage power loss in transmission.
 Shaft Horsepower at propeller. Total SHP
Propeller RPM
Propeller Torque ft/lb
Total prop torque ft/lbs 
NB  Max engine RPM should not be more than 85% of stated max RPM unless a continuous-duty heavy marine diesel is used!
NB This excludes power required by ancilliaries driven by the engine, such as hydraulic pumps or generators.
  Propeller Diameter (ideal) How To  
D= ( 632.7 x ( shaft HP exp 0.2 ) ) / ( RPM exp 0.6 )
 Ideal Minimum prop diameter for hull 
 Maximum prop diameter permissible.
 Theoretical ideal prop diameter (inches).
This is for a "standard" 3 blade prop with 33% Disc Area Ratio,
This "standard" configuration is the ideal form of propeller. Use of propellers with a greater Disc Area Ratio
or a greater number of blades is recommended only for special applications such as fishery. The use of props
with lower DAR and/or two blades is recommended only for special applications such as racing sailboats.
These special applications should consider the use of alternatives to the rigid propeller, such as variable
pitch or folding or ducted designs.
The more a prop deviates from "standard" configuration, the greater the trade off in lost performance at one end 
of the curve to boost performance in the other. A low drag sailing prop will not have sufficient area to generate large
thrust. A large area towing prop will have a high drag when sailing. A high thrust prop is not a high speed prop.
  Minimum Propeller Diameter. How To  
D = 4.07 x ( square root ( beam WL feet x Hull draft (exc. keel) in feet ) )
max prop dia input
# motors
Adjustment factor for # motors
Minimum Prop Diameter to efficiently drive hull in all conditions
If you see the "too small max prop dia" warning above, it means that the maximum prop diameter input by you
on the "Input Data Here" sheet is too small. If this is the maximum size that will fit the hull then you
need to carefully examine the hull, as there is apparently insufficient diameter available for a propeller
of appropriate size. Or you may need to reduce gearbox ratio to increase prop RPM.
Speed in knots required
Max prop shaft rpm 
desired speed expressed as feet per minute.
desired speed divided by max prop shaft rpm to give prop feet per minute.
Theoretical required prop pitch in inches.
Estimated prop slip at required top speed.
Wake Factor
Required prop pitch for top speed.
  Bollard Thrust (approximate) How To    
 Maximum Static or Bollard thrust in pounds.
  Displacement Length Ratio. How To  
D/L = DispT / ( 0.01x LWL ) cubed
Displacement in pounds
Displacement in long tons
D/L Ratio
  Speed / Length vs. Displacement Length  
S/L = 8.26 / ( D/L exp 0.311)
S/L ratio from max displacement and SHP
True S/L ratio from LWL
These three S/L figures should be of "comparable magnitude".
average of all three S/L figures
average deviation of all three S/L figures
If figures are out of limits some input data, i.e. number of motors, BHP or LWL is wrong or mismatched.
Look at the S/L that is mis-matched in magnitude, and what it is calculated from to determine the error.
  Planing Speed (from Crouch's Formula)        
Kts = c / square root ( max disp / SHP )
"C" for hull
max Disp lb
Knots (max planing)
  Projected Blade Area & Developed Blade Area      
Ap/Ad = 1.0125 - ( 0.1 x Pitch ratio ) - ( 0.0625 x ( Pitch ratio squared )
Developed blade area required
Pitch ratio (P/D)
PBA : DBA ratio.
True blade area sq/in
Projected blade area is the "apparent" area as seen from end on.
Developed blade are is the true blade area.
  Mean Width Ratio & Disc Area Ratio          
MWR = average blade width / diameter
DAR = ( Pi x (diameter squared)) / 4
Disc area ratio.
Ideal prop
Dia to fit prop
"standard" DAR
max input prop dia
sq/in blade area reqd
 sq/in blade area
DAR reqd for dia
Mean width ratio
# blades
# blades
ideal prop av blade width "to fit" prop av blade width
  Block Coefficient How to      
Cb = disp / ( LWL x BWL x Hd x 64)
Max Displacement (pounds)
LWL (feet)
Beam waterline (feet)
Hull draft (excluding keel or deadwood)(feet)
Block Coefficient
  Wake Factor How to      
Wf = Q1 - ( Q2 x Block Coefficient )
Block Coefficient
# motors
  Prop Shaft Material  
Yield / TS (psi) Mod Elas (psi) Density (lb/
1 Aquamet 22
2 Aquamet 18
3 Aquamet 17
4 Monel 400
5 Monel K500
6 Tobin Bronze
7 Inox 304 (Stainless)
Enter material no 1-7
 Selected material Yield strength PSI
 Selected material density
 Selected material elasticity
Tobin Bronze has become an unfashionable material for propshafts lately, and preference given to "stainless"
steels. This is unfortunate, since these steels are far more brittle and prone to shear, though these properties 
are useful in long shafts driven by powerful motors. However a stainless shaft carrying a bronze prop
is a source of galvanic corrosion. NEVER under-specify propshaft or thrust bearing equipment. At best
you may shear the shaft and lose all power, at worst you have a hole below waterline of propshaft diameter.
  Prop Shaft Diameter. How To  
A reasonably accurate and reliable rule of thumb states that propshaft diameter should be 
one fourteenth of propeller diameter.
D = cube root ( ( 321,000 x SHP x SF ) / ( St x RPM ) )
Shaft Horsepower
INPUT > > > Safety Factor (3 for yachts, 5 - 8 for commercial / racing)
Torsional Shear
Shaft RPM
prop diameter
Shaft dia in inches + eighths one fourteenth
  Prop Shaft Bearing Spacing How To  
Ft = square root ( ( 3.21 x D ) / RPM ) x 4th root ( E / density )
Shaft Dia
E (modulus elasticity)
Bearing Spacing in feet
  Propeller Weight (estimated) How To  
Weights given in pounds. Answer must be treated as spproximate + or - 8%
Prop diameter in inches
Weight of three bladed prop
Weight of four bladed prop
Based on standard bronze prop 0.33 DAR
This is an automatic calculation for 3 BLADE prop from shaft horsepower and rpm at prop on Torque sheet.
Propeller Diameter in inches.
   The alternatives in light blue squares
Propeller Pitch in inches.
max input dia
0.33 disc area ratio blades. (This means 33% of the "disc" area of prop dia is blades)
Rules of thumb.
One inch diameter = 2.5 inches of pitch. 
Two inches extra pitch will cut engine rpm by 450.
If you can't fit the indicated diameter due to clearance, or have plenty room left, the rules
of thumb above will be a useful guide. 
If you find yourself way off, you have either entered bad data or have a badly configured vessel!
Two blade propeller. 33% DAR
diameter in inches.
pitch in inches.
Four blade propeller. 33% DAR
diameter in inches.
pitch in inches.
max input dia
  Propeller HP How To  
PHP = C x (RPM exp N)
C = sum matching constant
N = 3.0 for heavy/slow, 2.7 normal, 2.2 ducted props.
max RPM
RPM exp N
Prop HP
Note that this is of use only in producing charts for easy visualisation of
engine / propeller power curves. As can be seen from the formula it is based on the
relationships between shaft RPM, type of propeller installation and a theoretical
It takes no account whatsoever of hull type etc.
It should only be used for creating charts
  Analysis Pitch   How To  
P (feet) = (101.33 x Va) / Na
Va  = speed in knots through wake at zero thrust
Na = shaft RPM at zero thrust
zero thrust means knots and RPM at which thrust = zero
Almost NEVER quoted by manufacturers as blade thickness, pattern and width all have
a marked effect, so two props that appear identical but have different blade thicknesses
actually have different pitch.
Face pitch is measured 70% of the radius out from the axis of rotation.
This will calculate the Displacement Speed Formula for the hull.
Speed:Length Ratio up to 1.6=displacement, from 1.6 to 2.8=semi-displacement, over 2.8=planing.
Maximum Displacement of vessel in pounds.
Waterline Length of vessel in feet.
Required maximum speed in knots.
Speed:Length Ratio.
Suggested max practical displacement hull speed for LWL input ---> Knots
Shaft Horsepower available at propeller from "Torque & Shaft Horsepower" sheet
Pounds per Shaft Horsepower available (power/weight ratio)
Shaft Horsepower required at propeller
Pounds per shaft horsepower required.