L.J.Doctors

University of New South Wales, Australia

Figure 7 is interesting in that neither the conventional nor the tip-fin propeller appears to be operating at its optimum efficiency. In fact, the plot shows a design efficiency of 61% or 63% compared to an optimum value of 68% or 71%. Is this simply a problem of there not being sufficient space behind the vessel for a (larger) optimally sized propeller?

The discussor correctly points out the remarkable fact that despite optimization neither the tip-fin nor the conventional propeller operate at their highest efficiencies. They are obtained at thrust loading coefficient C_Th=0.57 (Figure 7) and advance ratio J=0.83, appr. (Figure 6), whereas the propellers were designed for C_Th=1.545 and J=0.64 (appr.).

This is typical for propeller designs. The propeller is optimized with a constraint, in this case on thrust loading coefficient, resulting in an optimum geometry and advance ratio. A decrease in thrust loading coefficient relative to the design value for the given propeller is obtained by increasing the advance ratio. The blade sections will then experience less inflow angle and lift, and also less induced drag. This will give higher efficiency of the entire propeller. For a sufficiently high-advance ration, the thrust produced by the lift of the blade sections will be annulled by the negative thrust from the frictional drag of the sections. That will also give zero efficiency. In between this and the design advance ratio, the propeller will have a maximum efficiency.

In the present case, the thrust loading coefficient C_Th=0.57, where the maximum efficiencies are obtained, would correspond to a propeller diameter of 8.4 m which is not practicable. Even if it were, these propellers would not be optimal since they were designed for a different thrust loading, and an optimization for that much lower thrust loading would give propellers of even higher efficiencies.