LETTERS TO THE EDITOR
The Editor, THE REPORT, The International Institute of Marine Surveying, Murrills House, 48 East Street, PORTCHESTER, PO16 9XS
Technical Advice Propeller Shafts – Rope Cutters
Dear fellow Surveyors, I have recently come across some marine aftersales equipment which may result in a serious failure of the propulsion of a vessel. I feel that I should share my observations with you in the hope that I can focus your Surveying eyes towards the avoidance of a potential catastrophic failure of a propeller shaft.
I am a Large vessel ship surveyor (existing ships and new building) these include VLCC, LNGC etc and have been a Principal Surveyor with Lloyd’s Register and DNV so I am drawing on my knowledge and experience from this environment. The information I am advising you of is based on my experience about the design calculations that should be applied to a propeller shaft, intermediate shafting and bearings. I know that smaller craft shaft design is often carried out by the engine manufacturer and that they normally include high factors of safety.
I now find that there are aftersales devices which are fitted to the propeller shafts of yachts, pleasure craft and smaller commercial craft that could cause accelerated corrosion, high stress concentration and fatigue cracking.
I would like to start by introducing the complex design theory (in a nut shell) for the design of a propeller shaft.
First, we must consider the torque that needs to be transmitted through the shaft line, from the engine to the propeller. This is a ‘Torsional’ force acting the entire length of the shaft.
Second, we must consider the force transmitted longitudinally through the shaft line. This is ‘Longitudinal’ force. Most of the longitudinal force is taken up by a thrust bearing either in the engine, or external to the engine, in bigger vessels. The longitudinal force is that which is derived from the rotating propeller in the water and drives the vessel.
Both the Torsional and Longitudinal forces in the shaft line can be considered as applying ‘Stress’ to the shaft material.
Additionally, there is a third force which is a compounded force which is applied to a shaft line and that is cyclic loading (cyclic stress). The propeller is working in a non-homogeneous medium, water, with air bubbles entrained (Cavitation) at different depths (Density) at different speeds (Force). Therefore, most shafts are designed with high factors of safety to compensate for these variables.
The Report • June 2017 • Issue 80 | 21
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