Design Report Guided Missile Submarine SSG(X) - AOE - Virginia ...

SSG(X) Design – VT Team 3 Page 72
For the optimization, initial estimates were made for the Expanded Area Ratio (EAR), Pitch to Diameter Ratio
and propeller diameter (DP). Using the Wageningen B Series, POP optimizes all these values. The wake fraction is
updated using the optimized DP. The optimization is redone with the new wake fraction. This iteration process is
performed until DP does not change. During the optimization process, 7-bladed propellers were found to be the most
efficient.
The optimization results were used to evaluate the snorkel and sprint conditions to ensure the efficiencies
satisfied the ORD and that the propeller does not cavitate. The POP program uses Burrill’s Simple Cavitation
Diagram, shown in Figure 65; this is a plot of the mean thrust loading against the local cavitation number. To keep
the signatures as small as possible, a strict 5% Burrill back cavitation criteria is used. If the propeller violates the
cavitation criteria, the POP program gives a warning and changes must be made. The most effective change is a
decrease of P/D.
Figure 65 - Burrill's Simple Cavitation Diagram [Principles of Naval Architecture]
The iteration process is repeated until all ORD efficiencies are satisfied and the propeller does not cavitate
during snorkel and sprint. The snorkel resistance calculation included a wave induced drag while snorkeling, which
the original model did not include. Because of this the snorkel endurance did not satisfy the ORD requirement. To
meet this requirement, 40 ltons of diesel fuel were added to the baseline design. Figure 66 and Figure 67 show the
propeller curves for AIP endurance and AIP sprint respectively. Figure 68 shows the propeller curves for snorkel
endurance.
Figure 66 - Propeller Curves for AIP Endurance (5 knt)