Two speed propeller Page 2
The Denmark Test
The point of our testing in Denmark was to see how some of these things played out in practice. We first ran the Bavaria 42 with a Gori propeller in both normal and overdrive modes, then hauled the boat, changed the Gori for a conventional fixed three-bladed propeller, and ran the same tests again. We did crash stops to see how quickly the propellers could stop the boat from full speed ahead. We also brought a second, nearly identical Bavaria 42 into the picture. After putting the Gori prop on one boat and the fixed prop on the other, we sailed the two side by side to compare their performance.
With the wind gusting over 35 knots, objective testing was difficult. We ran parallel courses back and forth with the wind on the beam, measuring boat speed against engine speed at different engine speeds, and averaged the results to negate any current effects. At any given engine speed, the boat with the Gori prop clearly moved faster in overdrive mode. Gori has performed similar tests in more favorable conditions aboard a Beneteau 40, a Bavaria 44, and a Hallberg-Rassy 37. All showed speed increases of up to 20 percent. Perhaps what is more significant is the reduction in noise and vibration achieved at any given boat speed by reducing engine rpms, typically by about 400, at normal cruising speeds.
Under sail the boat with the Gori propeller was consistently faster than the boat with the fixed propeller, which is exactly what we expected. During crash stops, the Gori did significantly better than the fixed propeller.
To use a Gori propeller, a boat operator must learn some new tricks. If the boat is stationary when put into forward, the Gori goes into normal mode. To engage overdrive the boat must be put into reverse, which opens the blades in the other direction, and moved astern until minimal speed is built up. The gear lever is then shifted into forward with the boat still moving backwards. The reverse flow of the water over the blades holds them open in the overdrive position, where they remain once the prop starts turning in the other direction. To switch back to normal mode the engine must be put into neutral, which allows the blades to fold under the pressure of the water moving past the boat, and then forward gear is re-engaged.
When maneuvering in harbor, this means the propeller goes into overdrive if the boat is backed down and then put into forward gear. If you want to avoid this, you must briefly put the transmission in neutral once the boat has forward motion and then go into forward gear again to shift the blades back to normal. At sea, if the wind drops and you decide to motorsail into overdrive mode, you have to put the engine in reverse and go backwards a bit before shifting into forward gear. You must also take care not to put the propeller in overdrive at full speed, as this will overload the engine. If such an overload is sustained for any length of time, engine damage could occur.
The Gori propeller’s unique overdrive feature sets it apart from other folding props and should make it attractive to sailors who want increased efficiency under sail, but who also spend significant amounts of time motoring at cruising speed. The Gori, however, is also relatively expensive, so sailors on a budget may want to carefully weigh the advantages of having a two-speed propeller.