Cobb River



Click here for the Cobb River Project Sheet PDF 


Due to low efficiencies and ongoing maintenance issues with the existing runners in machines G3 and G4 at Cobb Power Station HydroWorks Ltd was commissioned to design new runners to work inside the existing turbine.

The runner was designed in CAD and with consultation from Professor Hermod Brekke, the geometry was enhanced with a modern, high efficiency, bucket profile.

A full mechanical analysis of the runner was completed using finite element analysis and linear elastic fracture mechanics. This ensured the runner had a 1010 cycle life providing casting defects were controlled at the manufacturing stage. 


The redesigned runner entered service in the second quarter of 2007; it has since passed its acceptance crack test and operates with less vibration than the other 4MW Cobb Pelton turbines.

The second runner is under manufacture and is expected to enter service late in 2008.

Client: TrustPower Ltd
Location: Nelson, New Zealand
Scope: Two off replacement runners for existing 4 MW single jet Pelton turbines.
Head: 600m
Discharge: 800 l/s
Nominal Output: 3,000 kW
Commissioning: 2007 


Original 4MW Cobb Runner

CAD Model of bucket and runner hub

CAD Model of bucket and runner hub

FEA of bucket to determine mean and alternating stresses

FEA of bucket to determine mean and alternating stresses

LEFM Based on results of FEA

LEFM based on results of FEA to ensure fatigue life is in excess of the 1010 continuous cycles and 2.1 x 104 start/stop cycles and determine allowable defect sizes.






As cast Pelton runner: Casting was thoroughly inspected using x-ray, magnetic particle and ultrasonic techniques to ensure the casting was sound.


Proof machined Pelton runner during Machining at Mace Engineering. The machined runner was then polished and balanced ready for service.


Completed HydroWorks / Mace Cobb 4MW Runner 


Cobb Hydro Station