Let us now consider ROPE CUTTERS:
There are a number of Rope Cutters or Net Cutters on the market and I am not going to mention any brands or types, but only express a general view on potential hazards for the propeller shaft.
The principal of a rope cutter or net cutter is that it is fitted onto the shaft, between the propeller and the aft stern tube seal. The idea being that if the propeller gets entangled in a rope or net the interaction between some rotating blades and some fixed blades will cut through the rope or net and prevent the propeller from becoming fouled. Some designs have blades attached to the forward side of the propeller which rotate with the shaft and interact with fixed blades fitted to the aft end of the stern tube. Other designs have a rotating sharp blade disc attached to the shaft.
The attachment to the shaft is the critical point I want to make to you all. We need to consider how the ‘Cutter’ is attached to the shaft.
If the attachment of the cutter to the shaft requires the shaft to be drilled in any way, then we have to consider that the integrity of the shaft is compromised.
If the shaft is drilled in any way then ‘we’
are introducing a ‘Stress Raiser’ point. Even if there is a small ‘Notch’ drilled to accept a grub screw or locating screw, then this must be considered as a ‘Stress Raiser’.
We must also consider the implication of dissimilar metals and electrolytic corrosion. The propeller shaft is working in Salt Water (Electrolyte). The propeller is probably made from a Bronze Alloy. The propeller Shaft is made from a Carbon Steel. Now we are introducing another material into the equation. We have a very effective electrolytic ‘cell’. The cutter may be made from a high grade Stainless Steel, which is good, but what about the securing bolts and grub screws? What material are they made from?
Now perhaps you can see where I am going with this? By adding a rope cutter or net cutter you ‘may’ be:
1. Adding a ‘Stress Raiser’ to the propeller shaft. 2. Adding to the electrolytic action around the shaft and propeller. 3. Setting up accelerated corrosion at a localised point in the shaft line.
All the above ‘could’ result in early failure of the propeller shaft. ADVICE:
If you are surveying a vessel that has a Rope Cutter or Net Cutter fitted, ask the owner to remove it so that you can inspect the shaft.
1. If there are any drillings into the shaft I would suggest that you apply either MPI or Dye Penetrant NDE methods to see if there are any cracks propagation from the holes.
2. If not already applied – any hole drilled into or through the shaft should have smooth ground edges and no sharp corners.
3. Observe if there is any active corrosion or electrolytic activity in or about the localised position of the cutter on the shaft.
4. Finally, I would suggest that you try and establish from the shaft designer / engine manufacturer, if they approve of the application of an after sales attachment to the shaft line.
(This may negate any future insurance claim on behalf of the owner if they have a shaft ‘problem’). I offer the forgoing only as advice with no preconception, with no legal implications and without prejudice.
Please take a closer look at the propeller shaft next time you conduct a Survey, if a rope or net cutter is fitted. Any feedback welcome: e-mail:
peter@broadreachmarine.com
The Report • June 2017 • Issue 80 | 23
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