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July 2, 2015

Contra-rotating stern-drive propellers

Can you provide any guidance on how to model a stern-drive with contra-rotating propellers in NavCad?


We have never had much success in our efforts to obtain reliable performance and efficiency figures on stern-drive contra-rotating propeller (CRP) units, such as the Volvo Duo-Prop and the Mercruiser Bravo Three. It seems that these companies are not interested in releasing this information – which is their commercial right to do, and which I will not pass judgment on – so we are left with trying to derive reasonable figures from trial results, experience, common sense, and outright assumptions.


Therefore, about all you can do to model these units is first represent a stern-drive with an appropriate propeller, then account for the higher efficiency that would be expected with the CRP. If we can omit considerations of propeller “sizing” for the moment, I would first refer you to Report 116 from the Knowledge Library on our web site for more details about modeling stern-drive propellers.


Stern-drive propellers in general

The following hull-propulsor coefficients are typical values that can be used for a stern-drive unit:

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


You can model an “outboard-style” propeller as follows:

   Diameter = 14″-15″ (typical of an outboard or stern-drive)
   Blades = 3 or 4 (whatever is being used)
   Exp area ratio = 0.55 (3 blades) or 0.73 (4 blades)
   GawnAEW type
   Scale corr = OFF
   KT mult = 0.93
   KQ mult = 0.95
   Cup = 1.0 mm (corresponds to “very light cup” for a 15″ prop)
   Cavitation = ON


CRP propellers in particular

Modeling a CRP really 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.

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Article details

Article ID: 36
Category: NavCad
Date added: 2015-07-02 15:12:38
Views: 184
Rating (Votes): Article rated 3.0/5.0 (12)

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