Ocean Magazine : 2 July August 2005
The recent improvements in engine design and efficiencies, discussed in OCEAN May/June issue, have been a huge positive for boat builders, owners and operators alike. But it hasn't stopped there, as designers strive to improve propeller performance and efficiencies. Whilst many of us have a good understanding of what's in the engine room, what's under the hull translating that power into performance is often overlooked. In brief, propellers are of the following types: Traditional Fixed Propellers The fixed propeller is still one of the most efficient propeller configurations this is because the propeller is designed to match the exact needs of the vessel. Propeller design is a highly technical subject that requires specialised knowledge. There are many interrelated factors affecting the design of an efficient propeller solution, this is why it is imperative to seek the advice of a naval architect. Folding Propellers These are found on sailing yachts and are designed to automatically fold when the vessel is under sail, thus reducing drag. Until recently this type of propeller was excluded from the superyacht market due to inability to design and manufacture propellers of sufficient size. Recently much larger propellers have been developed to absorb the power generated by large diesel engines. Blade shape is critical to propeller efficiency however the need to create a blade capable of folding compromises the design possibilities. Hence as a general rule these propellers are not as efficient as some of the other options available. They also tend to perform poorly when operating in reverse. Feathering Propellers The term feathering propeller and folding propeller are often confused or used interchangeably. The two are quite different and utilise a different approach to minimising drag when the blades are not required. A feathering propeller twists the blades so that the leading edge faces the water flow. The folding propeller folds the blades at the propeller boss so that the blades are dragged through the water length wise. A feathering propeller is generally more efficient than a folding propeller due to the opportunity for better blade design. When operating in reverse the feathering propeller provides markedly better performance than the equivalent folding propeller. Generally speaking the drag from a feathering propeller is less than that from a folding propeller. Surface Piercing Drives Surface drives are typically installed on high speed planing vessels where there is a need to minimise or eliminate appendages such as large keels, skegs and rudders that are found in a conventional instillation. The specially designed propellers pierce the surface of the water to reduce the drag caused by the propeller and the associated appendages. The entire assembly is designed to move both vertically and horizontally to provide trim and steering. This eliminates the need for rudders and trim tabs, thus further reducing drag. This feature also dramatically improves steering characteristics. Some manufacturers suggest fuel savings of between 15 and 30 percent with the added bonus of higher speeds for the same horsepower. Installations range from 74.6 kW (100 HP) up to 7457 kW (10,000 HP). Propellers can be as large as 3.0 meters in diameter. For the same engine size the surface piercing propeller is typically larger and frequently has more blades. The propellers for this type of drive are highly specialised and can look unusual. Controllable Pitch propellers Controllable pitch propellers are not new, they have been around for a long time. If well made they will provide many years of reliable service. The engine is directly coupled to the propeller shaft. The engine is loaded up by varying propeller pitch, thus eliminating the need for a gearbox. The propeller pitch is controlled while underway by an electrical, mechanical or hydraulic 116 | ocean ENGINE ROOM Blade Running - Propulsion Technologies BY Christopher Murman Propulsion systems have come a long way. The development of superior metals and the application of advanced design and analysis techniques, have brought about some new innovations. Many of the so called "new" ideas have been borrowed from very large commercial vessels and adapted to the pleasure boat market, including superyachts.
3 September October 2005
1 May June 2005