Effciency with low drag

Sailors can be a little unreasonable regarding propellers: they want them to be ideally efficient when under power and gone, missing, or absent when under sail. Today’s sailors are little advanced over their predecessors on the very first propeller-equipped ships of the early 1800s. Some ships of that era were equipped with quick-release couplings between the prop and the drive shaft. Lifting tackle then allowed the prop to be raised vertically until completely out of the waterthe ultimate low-drag position, unequaled by any of today’s folding or feathering props.

The complexity of this approach, along with the problem of finding volunteers willing to undertake the coupling and uncoupling of the prop from the shaft, led to the early development of feathering props. It turns out that adjustable-pitch, variable-pitch, and controllable-pitch props were also tried, with varying success, before the middle of the 19th century.

When under power, a propeller moves our boat. When sailing, however, it is nothing but a drag on our progress. Other than the impractical lifting propeller mentioned above, there are two practical ways to eliminate a large part of the drag of an unpowered prop: fold the blades in a streamline position or turn them about their axes so that they present only a narrow edge to the water flow.

There are a number folding props available on today’s market. In addition to being reasonably efficient in converting engine torque into forward thrust, they must meet a couple of additional critical performance parameters. In order to prevent possibly damaging out-of-balance moments on the prop system, their blades must fold and open simultaneously. They must open promptly and provide effective thrust in reverse. The Martec folding prop uses individually hinged blades, relying upon centripetal acceleration to move each blade separately from the folded to the full open position. This prop is relatively inexpensive; however, there is no positive assurance that the blades will either fold or open in synch. Props from Austral, Flex-O-Fold, and Gori force their blades to move together by interposing gears between the blades. Blade configuration varies among manufacturers, and some, like Gori, offer both cruising designs, in two- and three-blade versions, as well as a racing prop optimized for absolute minimum folded drag. When motoring with a light power load or motor-sailing, the three-blade Gori prop can be manipulated, with movement of the shift and throttle, into either a normal- orhigh-pitch bladesetting. Although control is a bit cumbersome and indirect, this capability does provide a bit of the advantage of a controllable-pitch prop. The engine can operate at speeds lower than normal when modest amounts of power are required. Few, if any, props provide symmetrical thrustthe same in forward and in reverse. In general, folding props suffer even more thrust deficit in reverse than non-folding propellers. In addition to the complexity of the folding prop, there is usually a weight penalty and often a significant price difference when compared with a conventional propdrag reduction is always costly.

Feathering the blades

Prop drag when sailing can also be reduced by using a feathering propeller, in which the blades turn in the hub so as to present an edge-on aspect to the water flow when not in use. In practice, a typical feathering prop may offer a bit more drag than a folding prop; however, its greater efficiency when powering offsets this disadvantage. In addition, the reverse thrust of a feathering prop may be significantly better than that provided by a folding prop. Some of the firms offering feathering props include Mar-Tec, Max-Prop, Cruising Designs, and Autoprop.

The Max-Prop, available in both two- and three-bladed versions, relies on water pressure acting on the opposing sides of its pivoted blades to force the blades into a streamline configuration when the boat is moving forward with the engine off and the shaft locked. Another benefit to to the Max-Prop is the virtually instantaneous shift from the blades’ forward position to reverse (and vice versa). This shift, which is driven by torque from the shaft, happens in only three-quarters of a turn of the shaft.

The internal blade angle stops of the Max-Prop are adjustable, permitting the pitch of the propeller to be finely adjusted to best match the hull/engine characteristics of the boat. The blade pitch adjustment requires partial disassembly of the propeller, a task best done with the boat out of the water. The V.P. version of the three-bladed Max-Prop incorporates an external blade pitch adjustment. Setting the final pitch angle when the boat is in the water and after test runs have been made greatly improves the fine-tuning of the prop to the boat. When rotated in reverse, the pivoting blade design allows the blade geometry to match that used for forward propulsion. Reverse thrust should more closely match forward thrust than with either fixed blade props or folding props.

The pitch of at least two other feathering props, by Mar-Tec and Cruising Designs, can be adjusted in a few minutes with the boat still in the water by a swimmer using a few simple hand tools.

The Autoprop takes a different approach to both the thrust-producing and minimum drag aspects of propeller operation. It is a feathering, variable, automatic pitch setting propeller. The blades of the three-bladed Autoprop are pivoted around the spanwise axis of each blade. Low pivot friction is assured by the use of a double-row ball bearing assembly. The hub diameter is quite small, keeping form drag to a minimum. With the boat moving ahead and the prop shaft stopped, differential water pressure on the two sides of each blade moves the blade into the lowest possible drag position. Any deviation from this position will unbalance the forces on the face and back of the blade, creating a force which will restore the minimum drag configuration. In this position, the prop creates little more drag than a fully folded propeller.

To appreciate the operation of the Autoprop, or any other manually or automatically adjusted, controllable-pitch prop, it is desirable to understand the interaction between the engine and the propeller in the transfer of energy to the water. Propellers must deal with the reality that an internal combustion engine develops full rated power only when rotating at or near maximum design speed.

A typical speed/engine horsepower curve illustrates the point. In this case, the engine develops its maximum, 48 prop shaft horsepower at 3,600 rpm. The power-versus-speed relationship is reasonably linear; 2,400 rpm, 75% of maximum rpm, produces 35 hp, 73% of maximum power. However, a fixed-pitch propeller, matched to the maximum power output/speed of the engine, can transfer only about 15 horsepower to the water at 2,400 rpm. A fixed-pitch prop transfers total engine power to the water only at one point, usually chosen to be the engine’s maximum speed. At any lower engine speed, the energy transfer is inefficient.

One way to satisfy a desire for improved energy transfer efficiency at varying speeds is to choose a more highly pitched prop, whose performance is optimum at a rather low speed, and then add a multispeed or continuously variable ratio gear box, which will allow the engine to match the prop’s speed requirement.

Controllable pitch

Few marine power installations use this alternative. Just as in aircraft practice, controllable-pitch propellers are used in place of the variable ratio gear box. When applying power, the prop is initially set to a low or fine pitch. This allows the engine to accelerate to a speed at which it is capable of producing substantial powermore, in fact, than the prop can transfer to the water. When the boat achieves some forward speed, the prop pitch can be increased, improving energy transfer efficiency and absorbing more of the available energy potential of the engine. When full cruise speed is achieved, engine rpm is set to the desired value, usually with reference to the specific fuel consumption curve (pounds of fuel per horsepower-hour or grams per kilowatt-hour) and perhaps to the vibration characteristics of the engine/hull combination. At this point, the prop’s blade pitch angle is increased until, often by reference to an exhaust gas temperature gage, the engine is optimally power loaded.

Controllable-pitch props are common on large ships, where they simultaneously provide superior maneuverability in port and great efficiency at sea. Although propellers of such flexibility would be very useful even on sailboats, they are produced in very limited numbers and are, therefore, very expensive.

The Autoprop more or less automates the prop pitch versus engine speed and load balance problem. When power is applied to the Autoprop, rotation creates a changing balance of forces, with centripetal force working against hydrodynamic force to position the blades in an optimum “lift-over-drag” position. At low speeds, the prop blades are in a low or “fine” pitch position. As rotation speed increases, the pitch angle becomes more aggressive (a “coarser” pitch angle). As noted above, this action has two benefits: the prop’s “bite” will reasonably match the momentary loading condition, tending to present an optimum load to the engine; and low initial pitch allows rapid engine acceleration. Once the vessel is moving well, changing loads imposed by wave or wind action will constantly cause small changes in prop loading. The Autoprop responds with small pitch changes. The prop accomplishes all of this without external controls, couplings or power. It is not clear if the overall result is as efficient as what can be obtained with active external control of blade pitch. However, according to numerous published reports from apparently reputable testing laboratories, it works quite well.

As with any automated device, there are costs to be paid. The Autoprop and all of the other feathering and folding props mentioned are not cheap. They can cost four to six times as much as a conventional fixed-pitch prop. In addition, they are heavier and impose additional loads on the prop drive and shaft bearing system. Props with gears between the blades can suffer from fouling of the gear teeth.

Proper rotation of the blades of the Autoprop requires that the bearings remain in a clean, free condition, which might be difficult in some waters for boats that are not used frequently. Any of these expensive props will deserve and demand first-rate galvanic corrosion protection. Reasonably frequent inspection of the zincs is mandatory.

Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.

By Ocean Navigator