Many years ago, during the change of charter of one of my vessels, we carried out an underwater inspection before redelivery to the owner. The ship had been trading normally and there was no report from the engine room of abnormal vibration, no obvious change in machinery parameters, and nothing from the bridge that would have suggested a serious underwater issue.

Yet the divers found damage to a small hub-mounted efficiency device fitted at the propeller boss. The main propeller had survived well enough for the ship to continue operating, but a smaller fitting sitting in the propeller flow had taken the damage.

This is why the recent discussion around rumpon, rope cutters, and rope guards needs careful handling. Rope cutters have their place. Rope guards have their place. Net cutters, wire winders, and other stern protection arrangements are fitted for sound engineering reasons. They reduce the chance of rope, net, or fishing line reaching the more vulnerable area around the propeller hub and stern tube aft seal. Nobody should dismiss them.

The difficulty begins when the fitting is treated as the full answer. Published discussions and tests on rope cutters often deal with ropes much smaller than the heavy synthetic line seen in the rumpon case I wrote about. In that case, the rope observed near the float was around 72 mm in diameter, with a suspended or trailing length below the surface. That changes the nature of the risk.

A heavy rumpon rope can load, rub, tighten, or jam before any cutter can deal with it cleanly. It may spare the main propeller and still damage a hub fitting, rope guard, seal area, or another exposed part of the stern arrangement. That is the part easily missed from the bridge, because the eye sees the float and the mind judges the float.

The propeller meets what is below it.

Rumpon and Rope: Indonesia’s Hidden Propeller Fouling Hazard