The sound of the water against the hull of a sailboat moving at close to hull speed will delight any sailor. He or she will be equally delighted by the sound of the boat’s engine when it’s time to rely on mechanical propulsion. However, sailors have strongly resented the loss of speed under sail caused by the drag of an unpowered propeller from the first use of screw propellers in England in the early 1800s. (English patents for screw propellers were granted beginning in the early 1800s with contra-rotating, controllable pitch and feathering props patented prior to 1840).
When steam power was introduced, fuel capacity limitations required the use of sails for much of any substantial trip. Although both feathering and folding props had been invented, the available metallurgical and manufacturing technology prevented production of large versions of these props. Ever inventive, naval engineers eliminated the drag on some ships by lifting the prop entirely clear of the water. The prop was installed in an aperture, mounted on a shaft that was supported at both ends. When the engine was shut down, a crewman lowered over the side and removed a pin from a coupling in the forward end of prop shaft, disconnecting it from the engine driven shaft. A block and tackle was then used to lift the section of the shaft on which the prop was mounted until the blades were clear of the water. Although unquestionably effective, this method of reducing prop drag did not find wide application and has not found favor with today’s sailors who want a less radical way to reduce prop drag.
The amount of drag imposed by an unpowered prop on a modern sailboat depends on the location of the prop relative to the keel, and primarily on the type and size of the prop. For example, a narrow, two-bladed prop locked in a fixed position behind the keel may create a speed loss of a few tenths of a knot. The three-bladed prop favored by many voyagers may slow a boat capable of 7.5 knots under sail by as much as 1.5 knots, a 20 percent penalty. While a speed loss of a knot during a sail of an hour or two may be acceptable, the 24-nautical mile penalty that accumulates each day during a protracted cruise may be enough to motivate the boat owner to search for an improved version of the prop elevation technique used in the mid-1800s. For props that cannot be hidden behind the keel, the drag force does not appear to be much different whether the prop is locked in place or allowed to freewheel. However a freewheeling prop can damage marine gear where proper lubrication is assured only when the input shaft is being powered by the engine.
Into a streamlined position
Although some catamaran power systems use a retractable drive leg that allows the prop to be lifted clear of the water when the boat is under sail, most drag reduction systems rely on either folding or collapsing the prop blades into a streamline position aligned with the prop shaft or by rotating the prop blades into a feathered position that aligns them with the water flow. Both systems can be very effective. However, to be an acceptable anti-drag reduction system, it must function reliably without maintenance for a long time (preferably a couple of years) in a hostile underwater environment. While these anti-drag props are relatively expensive, many sailors find them well worth their cost.
The folding prop is the least complex way to achieve the sought after drag reduction. Specially configured prop blades are pivoted near the prop hub so that they can fold aft into alignment with the prop shaft. The blades are moved into the folded position when acted on by the force of the water as the boat proceeds under sail power with the engine off. They must move back into a fully deployed position as soon as the prop shaft begins to rotate. Whether equipped with two, three or four blades, all folding props must be equipped with a mechanism that will ensure simultaneous movement of all blades. Some early folding props allowed each blade to move independently with the result that if the prop were stopped with the blades in other than a horizontal position, the lower blade might droop downward, adding drag. More important, the blades might not simultaneously return to the open, powered position, creating a massive unbalance that could shake the entire boat.
Today’s folding props incorporate sets of meshed gears that fix the relationship between the blades to ensure that they open and close in unison. The friction in the system must be low enough to both ensure that the blades will move without delay into their power position when the prop shaft begins to rotate and move into their folded, streamline position as soon as the prop shaft stops rotating with the boat moving ahead through the water. While prompt blade erection is desirable when the prop shaft begins to rotate for forward propulsion, it is also important that the blades remain open and not move into the folded position during the usually brief time it takes to shift from ahead to reverse, for example, when docking. Any malfunction at such a time will be notable.
The need to minimize the drag of the folded blades can limit the propulsive efficiency of a folding prop when compared with a fixed blade prop of equal dimensions. Fluid dynamics research, however, has created some very efficient designs (confirmed by some of the test data commented on below). Some folding props, including the two-, three- and four-bladed models offered by Volvo Penta claim efficiencies in both forward and reverse very close to or equal to fixed props.
Interesting test data comparing the forward and reverse thrust developed by folding and fixed props is available at a number of websites. The Flex-O-Fold Web site includes a set of comparison data under the heading “Test Results” that presents data from German laboratory tests from 1997 (http://www.flexofold.com/Test_Results.php
). The German tests at the Technical University in Berlin were conducted in a test tank and compared a three-bladed self-pitching, Flex-O-Fold; another unidentified three-bladed, geared folding prop; a three-bladed, no-twist feathering prop; a large blade (size unspecified) three-bladed fixed-pitch prop and a third-geared three-bladed folding prop. The German tests, conducted with a zero-degree propeller shaft angle (required by the test condition in the tank) showed the Flex-O-Fold prop to be 64 percent efficient for forward propulsion, 2 percent better than the fixed-pitch prop and 4 percent better than the self-pitching prop.
The data showed the Flex-O-Fold prop to create a drag of 2.7 newtons at 8 knots, equal to another of the folding props and very close to the drag of the no-twist feathering prop. The drag of the self-pitching feathering prop was 16.9 newtons; that of the fixed-pitch prop 94.5 newtons.
Based on this limited set of test data it’s possible to conclude that the self-pitching feathering props did best in providing balanced forward and reverse thrust. The closely controlled test conditions in the German laboratory test show that the efficiency differences between the best of the various types of propellers are not overwhelming, averaging about 2 to 4 percent. This test measured the ability of the props to convert horsepower into thrust and therefore did not include any effect from the driving engine’s power curve. The fact that the German test was conducted with a zero-angle prop shaft, typical of a sail drive (a very popular propulsion system in Europe where few boats remain in the water year round, less so in the U.S. where the drives are continuously immersed) may skew the data somewhat. The props intended for use with sail drives include an electrically insulating rubber bushing to prevent a galvanic reaction with the metal used for the drive housing and to reduce shock loading on the drive.
The process of choosing between a folding prop and a feathering prop may very likely wear out a significant number of your little grey cells (or conversely provide them with much needed exercise). The problem is multi-dimensional. How willing are you to rely upon a moving mechanical device that will be out of reach if it needs service while you are using your boat? To what degree are you willing to rely upon the manufacturer’s claims that their props are ultimately reliable? How important are the performance differences in forward and reverse propulsion and drag coefficient between feathering and folding props? Is the ability to precisely adjust the pitch of a feathering prop to match the boat and engine of value to you? Voyagers need to decide how many dollars they’re willing to invest in a device that will likely add from 10 percent to 20 percent to their speed under sail.
Contributing editor Chuck Husick is a sailor, pilot, writer and photographer.