Decreased drag and increased maneuverability are among some of the benefits of the AVP Autoprop.
If you've ever had a nautical outing go awry when maneuvering in tight quartersbacking out and prop walking your way into a neighboring boat or dock (and who hasn't?)or if you've ever had the pleasure of watching the competition sail away to leave you pondering the size and hydrodynamic deficiencies of your propeller, you might consider looking into changing to a variable-pitch prop.
The AVP Autoprop results from 10 years of research and development by Bruton's Propellers Ltd. The company's prop-manufacturing expertise stretches back to 1908 and includes making propellers for the British Admiralty as well as some of the largest propellers in the world. The advent of the high-speed diesel required a precise match between specifications of engine and hulls of differing classes of vessels, fostering a natural foray for the Bruton company into this advanced prop. Conceived by aeronautical engineer John Coxon, and developed by Brunton's Propellers, the first Autoprop was produced in 1988, and several thousand are currently serving sailors worldwide.
The prop is engineered to find optimal pitch at nearly all settings.
Among the satisfied customers are such renowned sailors as Sir Robin Knox-Johnson and Sir Chay Blyth who both discovered the advantages of the AVP Autoprop in the middle of stormy oceanic waters on their respective circumnavigations. Sir Chay Blyth had specified AVP Autoprops for the fleet of identical 67-foot yachts built for the 1993 and the 1998 BT Global Challenges, and he chose them again for all yachts in the coming Millennium Challenge. When British Steel II
was dismasted during the second leg of the British Steel Challenge in 1993, an AVP Autoprop enabled the damaged yacht to motorsail to windward for 2,000 miles through the Furious Fifties and the Roaring Forties at an average speed of four knots. The fuel consumption of the dismasted yacht was only 1.25 gallons of diesel per hour, enabling her to make port without an expensive resupply via aircraft.
The idea behind a variable-pitch prop is simple: reduce drag while sailing, and increase maneuverability while maximizing fuel consumption under power. The mechanics behind the AVP prop may prove a little more complex. Two opposing forces act on the blade pivot axesthe centrifugal force imposed by the rotation of the propeller shaft, and the hydrodynamic force caused by the boat's passage through the water. The blades balance these two forces. Centrifugal force, acting on the mass of the blade, throws the blade into an optimal pitch. The cross-section and profile shape of the blade is such that the blade assumes an angle of incidence to the water flow. From this position, as the water flow past the blade changes direction due to the forward movement of the yacht, the blade tries to maintain the same angle of incidence to the water flow by countering the centrifugal forces until it reaches a new position where the two forces are once again in balance. When this is achieved, the blades will be set at a pre-determined pitch setting. Free of external controls, the blades automatically adapt to optimum pitch settings to suit boat speed, prop rpm, and engine power for all conditions. In spite of its unique functioning, the unit is installed in the same way as a regular propeller.
Horsepower and rpm are still important factors in sizing the prop.
Unlike feathering props, the Autoprop is constantly adjusting its pitch to variation in rpm for optimal thrust. Other benefits of the Autoprop include reduced engine noise and vibration as well as less propeller-induced turbulence. When the boat is under sail with the prop shaft locked in gear, the blades automatically align themselves with the flow of the water, reducing drag by up to 85 percent when compared to conventional props. When going astern, only one revolution is required for the blades to adjust to a position where the leading edges are properly oriented.
The blades of the Autoprop rotate 360 degrees and feather when locked off.
However advanced the Autoprop may be, it can foul just like any other propeller, although a line cutter may also be fitted on the shaft. Due to the finer pitch, there is also less 'bite' at low speeds when engaging ahead or reverse from a standstill. Consequently, the Autoprop requires more revs than would normally be used at very low speeds. The mechanical nature of the device may provide an enticing home for various types of marine growth if allowed to sit for long periods of time in high-growth areas, though thoroughly cleaning the prop and then running it in gear should grind the growth into oblivion without damage. The manufacturer claims that under normal wear, the bearings should not need any adjustment for over 1,000 engine hours, a figure dependent on propeller size and on how hard the engine is used. Eventually the bearings will need replacing, which the manufacturer qualifies as a straightforward procedure.