Hi chaps
I'm new to model ships building but I am a naval architect so I might be able to shed a bit of light on the contra verses counter rotating prop query. There is no set rule that states one is better than the other. At the scales that we are discussing here there will be a very marginal difference in propulsive efficiency between the two set ups. Running a model on a pond with some waves will have a great effect.
When designing a new ship we tend to carry out model tests on the hull using a towing tank. We tow the hull up a calm tank at a range of set speeds and record the average force exerted on the towing post. These are primarily used to estimate the full scale powering requirements for the vessel.
In addition to these resistance experiments we assess the fluid flow across the aft end of the vessel. Now without this turning in to a long diatribe, if you imagine a prop running on its own, independent of the hull,l it is clear that the flow will be moving in only one plane along the line of the shaft (for argument sake lets say the x plane). Now when we introduce the hull up stream we find that the propeller seas an altered flow resulting in a flow component across the propeller (the y z-plane)
It is this "tangential" flow across the propeller disk that changes the efficiency of the props. It Is the variability in this based upon hull geometry that results in some hull forms having a greater efficiency with counta rotating and visa versa.
The % gains in efficiency achieved from this design work are small but they can have an impact on the final fuel bill for the ship operator. I hope that it's of interest. It would have made more sence with some sketches.
