Use FEA to analyze real and potential damage to a ship's bow thruster motor assembly.

1. Bow Thruster Assembly
Underwater Collision
Damage Stress Analysis
3/22/2019 rev A
Don Blanchet
3B Associates
dwb3298@outlook.com
Prepared for: August Marine Systems Ltd.

2. Solidworks model
Test
Accelerometer
location
18 feet
7 ft
dia

3. Cowling with 3 struts

4. Solidworks model
FEA mesh
Bottom Collision
Contact surface
Treated high
Strength steel
Bronze propeller

5. Measured accelerations
10 second collision with rocky
bottom in a tow tank

6. Maximum strut stress ,psi

7. Peak stress in strut
during collision from FEA
Yield stress
Permanent deformation

8. Resonant Frequency
analysis

9. First vibration mode: 1 hz
or 60 rpm excitation

10. vibration mode: 26 hz
or 1560 rpm excitation
Propeller
Blade
Flutter
Resonance
Possible
cavitation

11. Vibration mode: 60 hz
or 3600 rpm
Propeller
Blade
possible
collision
cowl

12. Conclusions
◼ Calculated stresses agree with the
damage caused by the underwater
collision.
◼ Recommend strengthening the
support struts.
◼ Qualification testing must include
operation over the full operation speed
range to dectect propeller blade flutter
or interference with the cowl.