Tieg Laskowske24 July 2015

Contra-rotating Propellers (CRPs)

Objectives & Outline

A single propeller wastes energy in water’s rotational motion

Adding a second, contra-rotating propeller recovers the rotational energy

• Why CRPs?• Greater efficiency• Better handling

CRPssingle propeller

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Objectives & OutlineSpecification SI English OtherTorque/prop 11.6 Nm 8.6 lb-ft -

Angular Velocity/prop 34.9 rad/s - 333 RPMInput Power/prop 405 W - -Total Input Power 810 W - -

Total Thrust 171 N 38.5 lb -Total Output Power 689 W - -

Boat Velocity 4.0 m/s 9.0 mph 14.5 km/hrTotal Efficiency 85% - -

Total Weight 2.3 kg 5.0 lb -

• Why CRPs?• My project• Design, manufacture, and test CRPs

for the 2015 Solar Splash Endurance race, with the 2016 (or 2018) Netherlands race in mind

• Why is CRP design so difficult?• The way in which the propellers

affect each other must be analyzed in order to optimize the design

Objectives & Outline• Why CRPs?• My project• Why is CRP design so difficult?• Outline• Introduction to OpenProp• Parametric studies• Modifications to OpenProp• Final design• Manufacturing• Testing• Status

Specification SI English OtherTorque/prop 11.6 Nm 8.6 lb-ft -

Angular Velocity/prop 34.9 rad/s - 333 RPMInput Power/prop 405 W - -Total Input Power 810 W - -

Total Thrust 171 N 38.5 lb -Total Output Power 689 W - -

Boat Velocity 4.0 m/s 9.0 mph 14.5 km/hrTotal Efficiency 85% - -

Total Weight 2.3 kg 5.0 lb -

OpenProp• Under development since 2001 by MIT, Maine

Maritime Academy and Dartmouth College• Open source MATLAB code for propeller design

and analysis• Based on moderately-loaded lifting line theory • Parametric Study tool used to select diameter,

shaft speed, and number of blades• Single Design tool used for geometry generation,

off-design analysis, and more detailed on-design analysis• Used by Cedarville U. Solar Boat Team since 2009

Lifting Line Theory

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OpenProp• Latest version:• Published in 2013, but I am the first at Cedarville to

use it• Includes lifting surface corrections

0.2 0.3 0.4 0.5 0.6 0.70.5

0.550.6

0.650.7

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0.951

Diameter (m)

Effici

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New Version

Old Version

The latest version showed significant differences in predicted efficiency at higher diameters for our operating range

Lifting Surface Theory

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Parametric Studies• 2015 Solar Splash CRP propeller shaft speed: 333 RPM OK

2015 SS CRP shaft speed parametric study

600 350

Parametric Studies• 2015 Solar Splash CRP propeller shaft speed: 333 RPM OK • 2015 Solar Splash CRP hub diameter: 0.089 m (3.5 in) OK

2013 forward-facing pod design

2015 forward-facing pod design

Hub designed at D = 46 mm (1.8 in) but in

reality variable

Hub designed at D = 89 mm

(3.5 in)

Parametric Studies• 2015 Solar Splash CRP propeller shaft speed: 333 RPM OK • 2015 Solar Splash CRP hub diameter: 0.089 m (3.5 in) OK• 2016 Netherlands CRP propeller shaft speed: 1000-2000 RPM

2013 forward-facing pod design

2015 forward-facing pod design

Hub designed at D = 46 mm (1.8 in) but in

reality variable

Hub designed at D = 89 mm

(3.5 in)

Modifications to OpenProp• 2009 method – assumes:

1. Induced velocity due to front propeller is the same at the two propeller planes

2. Rear propeller does not induce velocity at the front propeller plane

• Masters Thesis of Demetrios Laskos (2010) discusses two methods of modifying OpenProp for CRP design that avoid these assumptions• The code Laskos used is both unavailable and outdated• My main project this year has been to modify the most recent version

of OpenProp to implement the easier of Laskos’s methods, his so-called ‘uncoupled’ method• I have also made some improvements to help it suit our needs better

Direction of rotation

Cavitation analysis

CRP separation distance

Aft propeller specifications

Aft propeller non-dimensional parameters

Inputs

Outputs

Open-water efficiency

Panel for Rear Propeller Outputs

Pitch-diameter ratio and slip

• Corrected off-design calculation for CRPs• Added (non-

dimensional) power to the off-design performance plots

Outputs

Validation of Modifications to OpenProp

Image credit: Sasaki et. al., “Design system for optimum contra-rotating propellers,” Journal of Marine Science and Technology (1998) 3:3-21.

• Comparison with Laskos’s results showed similar circulation distribution• Replication of an industry study produced similar geometry and predicted

performance within 10% of experimental results

Validation of Modifications to OpenProp• Comparison with Laskos’s results showed similar circulation distribution

• Replication of an industry study produced similar geometry and predicted performance within 10% of experimental results• Primary difference between 2009 CRP method and iterative 2015 CRP method due

to differences in tangential velocity predictions in the root region, with the 2015 method shape matching published results more closely

2009 method 2015 method Published results Imag

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Final Design Specification Required PredictedSI English SI English

Total Efficiency 85% - 91% -Total Weight 2.3 kg 5.0 lb 1.6 kg 3.5 lb

Learn Manufacturing Process• In-house 3-axis CNC mill used since 2005• Learned manufacturing process in parallel with design• Replicas of previous designs made from MDF and aluminum

Manufacturing• Propellers made with CNC mill• Hollow nose-cone made with CNC lathe• Bushings splined by Trojon Gear, Inc. (Dayton, OH)• Shrink-fit used for hollow component and bushing assembly• Sanded components for

optimal hydrodynamics

Testing• Recalibrated previously installed

strain gauges with a setup similar to the one shown

Testing• Recalibrated previously installed

strain gauges with a setup similar to the one shown• Ran a test to compare with the

current single propeller• The CRPs performed slightly

better than the current propeller (~3%), a good first step• Unfortunately, we did not

succeed in gathering strain data to evaluate thrust and efficiency

400500

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14004.55.05.56.06.57.07.58.0

CRPsDesign Power

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Boat

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Learn manufacturing process Learn OpenProp

Status

10090807060504030201000

First iterative CRP design

Test

Manufacture