Modeling Outboard-Style Propellers and Drives

Products: NavCad, PropExpert

Outboard-style propellers are different than conventional shaft-driven propellers in that they are small and typically operate at high RPM. They also use drives that are more integrated with the propeller, so the hull-propeller interactions are also different.

Propeller Parameters

Diameter, pitch, and blade count would be given by the manufacturer for the particular product model. The other important parameters are recommended as shown below.

• Series = GawnAEW
• Expanded area ratio (EAR) = 0.55 (3 blades) or 0.73 (4 blades) are typical
• T factor (PropExpert) or KT mult (NavCad) = 0.93
• P factor (PropExpert) or KQ mult (NavCad)  = 0.95
• Cup = 1.0 mm (corresponds to “very light cup” for a 15″ prop)
• Additionally for NavCad: Cavitation = ON, Scale corr = OFF
  
  

Note on Contra-rotating Propellers (CRP)

Modeling a CRP comes down to selecting a representative optimized “common” pair of propellers, and then adding a factor to account for the improved efficiency. Efficiency improvement in a CRP is subject to the particular application at hand – speed, thrust loading, propeller size, rpm – but is typically bracketed by the range of 8% (typical) to 16% (in rare cases). Solid research tends to suggest the lower figure when comparing an optimized propeller to an optimized CRP propeller. Since the improvement in efficiency with a CRP is principally due to a recovery of rotational energy, it is reasonable to model this improved efficiency by increasing the relative-rotative efficiency from 1.00 to something like 1.08 (to represent an improvement of 8%). 

So, in summary, set up your analysis as two propellers per unit (e.g., four propellers for a twin-screw installation), so the total thrust loading is distributed as it would be for the paired propellers. This also means that if you are using an engine file, you will want to use only half of the engine’s power in the file (to properly have the total input power shared by each of the unit’s two propellers).

Note: These propellers typically have a variable pitch distribution, with a cambered blade form, and typically some cupping. Also, each of the two propellers in a CRP set will have different pitch (as well as slightly different diameter, and even number of blades at times). Therefore, think of the numerical pitch value as an “effective average pitch” for the performance analysis, and not the pitch that might be stamped on the propeller.

Note: NavCad formalizes much of this definition, so refer to the "Simple CRP" section in the Help.

Hull-Propulsor Coefficients

The following hull-propulsor coefficients are typical values for different drive systems that can be used with outboard-style propellers.

Stern-Drives

• Wake fraction = 0.03
• Thrust deduction = 0.00
• Relative-rotative efficiency = 1.00
• Shaft efficiency = 0.97

Under-hull Drives (Tractor)

• Wake fraction = 0.00
• Thrust deduction = 0.03
• Relative-rotative efficiency = 1.00
• Shaft efficiency = 0.97

Under-hull Drives (Pushed)

• Wake fraction = 0.06
• Thrust deduction = 0.05
• Relative-rotative efficiency = 1.00
• Shaft efficiency = 0.97