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Testing:

Test Sheet

Examples

19' Ski Boat

34' Sailboat

41' Utility Boat

Technical

Blade Area
Ratio

Calculating
the Cavitation
Number

Horsepower
Losses

Hull Speed

Kt Breakdown

Propeller
Geometry

Propeller Law

Wake Factors

Glossary

References

41' Utility Boat Example:

Companion Data File: Example 41 Foot Utility Boat.psm_data. The file is  located in the PSModel Directory

Boat Description: twin screw, 41' V bottom utility boat

References: data presented in reference 15 for the baseline tests of a 41' Utility Boat

Engines: two inboard diesels

Reduction Gear Ratio: 2.0 to 1

Maximum Horsepower: 340 @ 2600 RPM without parasitic loads, manufacturer assumes a 6.5% reduction in HP at the reduction gear output shaft due to losses by the reduction gear, alternator, and raw water pump.  Net HP at the reduction gear output shaft is then 318. 

Maximum Torque: 1285 ft-lbs at the reduction gear output shaft

Propeller:  

Manufacturer -- Columbian Bronze
Model -- Tetradyne
Number of Blades -- 4
Cup -- no
Diameter -- 26 inches
Pitch -- 27.74 inches (average measured pitch at 70% of prop diameter)
P/D -- 1.067
BAR -- 0.73

Depth to center line of propeller: 2 feet

Test Course Length:  2000 feet

Water Conditions:  fresh water at 60.0 Degrees Fahrenheit, no waves

Wind:  less than 10 MPH

RPM: RPM was read from the shaft aft of the reduction gear; therefore, the RPM recorded was the propeller RPM

Shaft Torque and Thrust: a Lebow thrust/torque meter was installed on the port propeller shaft aft of the reduction gear.

Test Data:

Values are averages of runs in opposite directions for the port shaft.
 

Col. 1 Col. 2 Col. 3 Col. 4
Boat
MPH
Torque
Meter
(ft-lbs)
 Propeller
RPM
Thrust
Meter
(lbs)
9.93 315 550 922
11.57 565 701 1560
12.92 740 801 1994
15.74 882 900 2291
18.19 985 973 2473
21.02 1077 1050 2612
24.02 1170 1150 2657
26.82 1276 1271 2754

Calculations:

Propeller Series used

The Navy 4 Bladed Propeller Series (NV4) was used to make performance predictions for the following reasons:

-- the propellers tested for NV4 have geometry that is very close to that of the 41' boat propeller

-- the NV4 Series was developed under realistic conditions, see the section on Quick Series Facts for more information

The Segmental Section Series (GBL) -- also a 4 blade Series -- was used to make corrections to NV4 output as discussed below because it covers a wider range of BAR values than NV4.

Reference point for calculations

Since the engine manufacturer rates the engine at the reduction gear output shaft, propeller torque, horsepower, and thrust will be resolved to the reduction gear output shaft as well.

Torque, Horsepower, and Thrust correction for BAR

Note that the BAR for NV4 is fixed at 0.70.  Since the current propeller has an BAR of 0.73 a correction to the NV4 output for torque, horsepower, and thrust is needed.  This correction is calculated by running the input for the NV4 Series in the Segmental Section Series (GBL) at two different BAR values, 0.70 and 0.73.  Correction factors in column 6 and 7 below were calculated from the GBL output as follows:

Col. 6 -- Torque & HP correction factor = (Torque at BAR = 0.073) / (Torque at BAR = 0.70)

Col. 7 -- Thrust correction factor = (Thrust at BAR = 0.73) / (Thrust at BAR = 0.70)
 

Col. 5 Col. 6 Col. 7
Boat
MPH
Torque
& HP
correction
factor
Thrust
correction
factor
9.93 1.00983 1.00571
11.57 1.01176 1.00756
12.92 1.01266 1.00756
15.74 1.01091 1.00624
18.19 1.00913 1.00515
21.02 1.00678 1.00392
24.02 1.00437 1.00291
26.82 1.00376 1.00159

Col. 9 --  wake factors used in PSModel calculations are from the graph for Flat and V Bottom Boats in the Wake Factor Section.

Col. 10 -- torque meter values repeated here for convenience, same as Col. 2

Col. 11 -- propeller torque predicted by PSModel x Col. 6  (see discussion above for BAR correction)

Col. 12 -- propeller horsepower predicted by PSModel x Col. 6 (see discussion above for BAR correction)  

Col. 13 -- propeller torque at reduction gear output shaft, Col. 11 x 1.02,  1.02 accounts for the packing box and bearing losses

Col. 14 -- propeller horsepower at the reduction gear output shaft, Col. 12 x 1.02, 1.02 accounts for the packing box and bearing losses

 

Col. 8 Col. 9 Col. 10 Col. 11 Col. 12 Col. 13 Col. 14
Boat
MPH
Wake
Factors
(1-w)
Torque
Meter
Col. 2
Prop
Torque
from
PSModel
x Col. 6
(ft-lbs)
Prop
HP
from
PSModel
x Col. 6
 
Predicted
Torque at
red. gear
output
shaft
Col. 11
 x 1.02
(ft-lbs)
Predicted
HP at
red. gear
output
shaft
Col. 12
 x 1.02
9.93 0.940 315 337.18 35.31 343.92 36.02
11.57 0.943 565 606.04 80.89 618.16 82.51
12.92 0.946 740 807.09 123.14 823.23 125.60
15.74 0.952 882 923.97 158.31 942.45 161.48
18.19 0.958 985 980.87 181.74 1000.49 185.37
21.02 0.964 1077 1010.81 202.06 1031.03 206.10
24.02 0.971 1170 1101.79 241.25 1123.83 246.08
26.82 0.977 1276 1279.79 309.66 1305.39 315.85

The torque and horsepower predicted at the reduction gear output shaft for the propeller, Cols. 13 and 14 above, are plotted over the engine performance curves below.  Note that the engine is rated at the reduction gear output shaft.  The manufacturer states that this rating includes an assumed loss of 6.5% for the alternator, raw water pump, and reduction gear. 

Engine performance curves and predicted propeller requirements 

The following table and graph compare the torque meter readings and predicted torque from Col. 13.

Boat
MPH
Torque
Meter
(ft-lbs)
Predicted
Torque
Col. 13
%
difference
9.93 315 343.92 9.2
11.57 565 618.16 9.4
12.92 740 823.23 11.3
15.74 882 942.45 6.9
18.19 985 1000.49 1.6
21.02 1077 1031.03 4.5
24.02 1170 1123.83 4.1
26.82 1276 1305.39 2.3

Predicted and measured torque

The following table and graph compare the thrust meter readings and predicted thrust values.

Boat
MPH
Thrust
Meter
(lbs)
Predicted
Thrust =
PSModel Trust
 x Col. 7
%
difference
9.93 922 922.24 0.03
11.57 1560 1669.53 7.0
12.92 1994 2205.55 10.6
15.74 2291 2496.48 9.0
18.19 2473 2598.31 5.1
21.02 2612 2604.17 0.3
24.02 2657 2757.00 3.8
26.82 2754 2993.75 8.7

Predicted and measured thrust

The differences between the measured and predicted values of torque may be due, in part, to the initial values chosen for the wake factors.  If the actual torque is known -- as is the case here -- PSModel can be used through trial and error to home in on more accurate wake factors.   This is done as follows:

1) guess at a new wake factor, keeping all other input the same as before

2) run PSModel

3) make all necessary corrections to PSModel's torque output to get the predicted torque at the torque meter location -- in this case corrections for the bearing and packing box losses,  and for BAR = 0.73

4) compare the predicted torque to the torque meter value:

-- if the torque meter reading is lower, increase the wake factor value and try again

-- if the torque meter reading is higher, decrease the wake factor value and try again

5) continue until the predicted and actual torque are nearly equal  

This was done for the 41' utility boat, and the new or adjusted wake factor values are plotted below.  The adjusted wake factors look odd but consider these facts:

1) the hump -- or transition from displacement to planing modes -- occurred  around 13 MPH for the 41' boat.   The flow into the propeller can be influenced significantly by the trim angles and wave pattern formed around the boat at the hump.  Wake factors greater than 1.0 may occur.

2) test results for the 41' boat, reference 15, show that a modified propeller -- one with varying pitch from hub to blade tip -- performed extremely well at the upper speeds of the boat.  This indicates that a significant wake may exist at speeds beyond the hump speed.  This is reflected in the plot of adjusted wake factor values below.

3) the 41' boat had an unusually wide and long -- but faired -- bearing support just ahead of the propeller that probably had a significant influence on flow to the propeller.

(1-w) adjusted so that predicted torque = torque meter reading

A comparison between the predicted shaft torque, using the adjusted wake factors, and the actual torque for the 41' boat are shown below.  The agreement is excellent as it should be.

Torque predicted with adjusted (1-w) compared to torque meter

The difference between the measured and predicted thrust, based on the adjusted wake factors, is shown in the table and graph below.  The predicted thrust conforms to the thrust meter readings much better than before, but is still slightly greater than the thrust meter readings at the top speeds.

Boat
MPH
Thrust
Meter
(lbs)
Predicted
Thrust =
PSModel Trust
 x Col. 7
%
difference
9.93 922 832.73 9.7
11.57 1560 1511.34 3.1
12.92 1994 1980.86 0.7
15.74 2291 2326.43 1.5
18.19 2473 2556.10 3.4
21.02 2612 2723.63 4.3
24.02 2657 2880.36 8.4
26.82 2754 2912.62 5.8

Thrust predicted with adjusted (1-w) compared to thrust meter

Torque meter reading are normally not available.  So what to do?  Well, the prediction of propeller torque and thrust using PSModel with the initial wake factors was really not that bad even though thrust was slightly over predicted.  Therefore, if more accurate wake factor values can not be found then use the tables in the Wake Factor section.