PROPELLER MODIFICATION. (Some FAQs)

Transcription

1 PROPELLER MODIFICATION (Some FAQs)

2 WHAT IS IT FOR? For various reasons explained below, Torque Overload (sometimes described as Overtorque or Heavy Running) may occur in the main engine of a ship. If service operating points (when plotted on a Power vs Revs graph) are to the left of the Propeller Curve, the engine is Torque Overloaded and the light running margin (LRM) has become negative. The original design point for the propeller may or may not have had a suitable LRM. However, for propellers designed nowadays, this original margin would be somewhere between +4% and +10%. For an unchanged engine SMCR point, the LRM will be increased by reducing the propeller pitch. The term Torque Overload means that the power absorption by the propeller no longer matches the performance of the engine. Because of this, the engine operates in an overload condition at inadequate RPM which leads to; Higher exhaust gas temperatures. Increased wear of pistons, liners and valves (if any). High maintenance costs. Increased fuel consumption. At low RPM, the power demand is larger than the engine can deliver which may make it impossible to maintain the vessel s operational speed. BEFORE AFTER

3 WHAT CAUSES OVERLOAD? There are numerous reasons for an engine to experience a Torque Overload; Incorrect propeller design. Old/Worn engine. Change in normal operating profile and/or loading condition. Fouled hull and/or propeller. Corrugated hull plating. Modification of the ship (lengthening, draft increase etc). The engine torque can be reduced by reducing the propeller pitch. This improves the power/rpm relationship between the engine and propeller. WHAT ARE THE BENEFITS? Reduced engine maintenance costs. Increased Light Running Margin (LRM). Propeller restored to optimal condition. Improved fuel consumption. Improved ship acceleration. No practical effect upon Propeller Efficiency, Cavitation Performance or Torsional Vibrations. Very short Pay-Back time.

4 HOW DOES IT WORK? To reduce the Overload, it is necessary to increase the propeller RPM for any given power level (i.e. move the operating points to the right of the Power Curve). This is achieved by reducing the propeller pitch. Years ago, this was done by reducing the propeller diameter but this was found to be inefficient. Nowadays, the industry standard is to only change the trailing edge geometry. Fig.1. shows a typical aerofoil section with two important lines (i.e. Nose-Tail Line and Camber Line). The Camber Line is the locus of all the mid-thickness points. The Nose-Tail line is self-explanatory and exits at Point A on the sketch. Fig.2. shows a cut back and a reshaped pressure face at the trailing edge. It can be seen how the Camber Line is locally kinked at the extreme edge and it is analogous to dropping the flaps on an aircraft. It is the local kinking of this line that is the important aspect. Fig.3. shows the new Nose-Tail Line which now exits at Point B. You can see that by dropping from Point A to Point B, the Nose-Tail Angle reduces and it is this reduction in Pitch that designers base their calculations on.

5 CAN IT BE DONE AFLOAT? There is no need to wait until drydock. If the vessel can be trimmed, the modification can be performed alongside or offshore if the weather conditions permit. We have Teams of highly experienced Propeller Technicians, ready to fly to any destination. As most modifications can be carried out within 2 days, this keeps the vessel operating and without delays. WHAT INFO IS NEEDED? A power measurement with a torsion meter on the tailshaft is a more accurate method and this can be carried out by sub-contractors with temporary portable equipment during some 1 day trials. So, the more accurate the power measurement, the more reliable will be the Before and After comparison. Information needed: Current Power, RPM and Speed. Original Trials data. Propeller drawing with section aerofoil details / offsets and pitch, width and thickness distributions. Target LRM desired.

6 SHIPCARE SERVICES SERVICES NAMIBIA SERVICES S.A. SEALS PROPULSION PROPULSION NGC BRUNTONS THOMAS REID STONE MARINE SHIPCARE LIMITED Dock Road, Birkenhead, Merseyside CH41 1DT, UK T: +44 (0) E: Part of the Langham Industries Group of companies