Estimating Propeller Astern Performance (at Bollard)

Propeller performance in NavCad and PropExpert utilize parametric methods based on model test “systematic series”. These tests are typically limited to the “ship-going-ahead” condition for the specific geometry of the series with no restriction to the inflow. In “ship-going-astern” operation, the propeller is rotating tail-first, so the blade is actually flying upside down and backwards. In other words, the shape of the foil section is not ideal for generating foil lift. This lightens the load on the foil, so the propeller thrust would be lighter in the astern condition than ahead (for the same water inflow speed and operating RPM). However, the backwards nature of the blade does not reduce foil drag to the same extent, so the blade torque is only slightly affected.

The available published data for astern performance is quite limited, but we can identify a few simple estimates for astern performance in the bollard condition. Prediction of performance at transit speeds will follow the general trends of these bollard estimates, but accurate predictions require “multi-quadrant” data. These estimates also assume modest amounts of skew. High-skew propeller running astern operate “tip-first”, so these estimates would be less representative.

Astern-to-Ahead Thrust Ratio

Very rough estimates for the astern-to-ahead thrust ratio of typical propellers (at bollard with comparable developed power) are:

• B-Series: 75%
• Kaplan 19A: 65%
• Kaplan 37: 75%

Propeller Corrections

Simple estimates of astern performance – including propeller torque – can be made using the ahead predictions with appropriate KT and KQ multipliers (called T and P factors in PropExpert). Simple estimates for the multipliers at bollard (J=0) have been identified from limited published data: foundfor  are:

B-Series (from 4-blade tests with variation of EAR and P/D) 

• KTM: 0.77 (P/D=0.8) to 0.78 (P/D=1.4)
• KQM: 0.92 (P/D=0.8) to 0.97 (P/D=1.4)

Kaplan 19A (from 4-blade 0.70 EAR tests) 

• KTM: 0.65 (P/D=0.8 to 1.4)
• KQM: 0.85 (P/D=0.8) to 0.93 (P/D=1.4)

Kaplan 37 (from 4-blade 0.70 EAR tests) 

• KTM: 0.73 (P/D=0.8) to 0.78 (P/D=1.4)
• KQM: 0.80 (P/D=0.8) to 0.91 (P/D=1.4)

The multipliers are found to be a function of P/D ratio, where lower P/D shows greater losses astern than higher P/D. Note that both KTM and KQM are substantially reduced further at P/D=0.60. Also, these figures are without any modification such as cup or added camber.

Additional Considerations

Astern performance is more than just about propeller thrust. Thrust deduction astern is substantially higher than ahead (on the order of twice the ahead value) as the propeller jet in reverse hits the hull.

There is yet no ability yet to predict the effect of cavitation breakdown astern, but it is reasonable to assume that it would occur earlier and be of greater magnitude due to the tail-first operation with the reverse camber.

Astern performance is where choosing a 37-type or contemporary high-efficiency nozzle (rather than a 19A, for example) can be beneficial. As you can see from the correction multipliers, the astern-ahead thrust ratio of a Kaplan 37 is similar to the B-series, but the astern-ahead power ratio is smaller as the nozzle makes up some of the astern thrust and the ducted propeller is under lighter power demand.