Bowthruster Tunnel and Support Tube Corrosion

Inspection of a CP Tunnel thruster was carried out during a docking period due to slight contamination of the gearbox oil by sea water. The intention was to inspect with the head of oil still on the unit to see if it was possible to ascertain if this was due to blade seal leakage, or if it was necessary to remove the output shaft assembly

It was noted that of 8 anodes fitted in this space, 7 had no remaining anodic material. The final anode was severely depleted

Anodes fitted to the output shaft flange were found absent

The ropeguard was damaged by corrosion and the effects of cavitation and turbulence

This excessive damage was attributed to lack of protection from the anodes exacerbated by cavitation and localised turbulence

The Support tube was pitted to a depth of 20 mm on the blade facing face.

Again this is attributed to corrosion damage exacerbated by localised turbulence.
This damage in particular was carefully examined to ensure that there were no structural strength or stress raiser issues

The adjacent areas to the blades on the tunnel were pitted and evidence of grooving either side of a circumferential weld.

Although this damage is typical of tunnel thrusters the degree in comparison to age of the vessel and normal thrusters usage with regard to zero pitch operation was deemed excessive


The areas were grit blasted to allow further inspection. This revealed that there was sufficient depth of material remaining to allow repairs to be carried out.

Shown is an area on the Tunnel adjacent to the blades after blasting and part coated with repair product

The selected method was build up using a two compound Metallic polymer product supplied by Belzona

After further cleaning and friable material removal Belzona 1111 was applied to the damaged support tube and tunnel areas. An overlap of around 50mm was given to ensure that any edge lifting would not affect the main repair.
The initial application concentrated on ensuring full contact with the base material by forcing small quantities of product into the pits removing any air bubbles. The main layer was then built up on top of this

After a suitable cure period the areas were overcoated with a smooth layer to reduce turbulence and improve the longevity of the repair.

The product used was Belzona 1341 applied by use of a brush to provide a very smooth tenacious outer layer which also provides extra protection against edge lifting.

Cleaning of tools was made possible by use of a release agent Belzona 9111

Notes were made to specially inspect the damage at next docking. Were edge lifting is evident it is possible to cut back the damaged areas and overlay again with Belzona 1341.

The shaft seal landing face was machined to remove a grove formed by rub of the two parts. The blade palms were inspected and no significant damage noted. All seals were replaced and new anodes and ropeguard fitted.