 | From Ocean Navigator #98 May/June 1999 |
The simple single-speed transmission, here in a Friendship sloop built in 1900, is typical of marine transmissions for displacement vessels.
The need for multi-speed transmissions in land vehicles is undisputed. Marine transmissions, however, offer only three choices: forward, neutral, or reverse. Boats rely upon the relatively inefficient coupling between the prop and the water to provide the equivalent of long-duration clutch slip, and this approach is, at best, inefficient. When starting from rest, many boats deliver initial acceleration that is similar to an automobile starting in too high a gear. If the engine is a diesel, clouds of black smoke testify to the overload being imposed.
During the past few years a number of racing boats have been equipped with two- or three-speed transmissions. Their performance advantage on the race course has resulted in their being banned from some competition classes.
An examination of the speed versus horsepower, torque, and propeller power absorption curves for a typical internal-combustion marine engine illustrate the technical considerations that make the use of multi-speed transmissions in boats desirable. For our representative engine, we have used the data for a nominal, 100-horsepower, turbocharged, aftercooled diesel engine, the Yanmar 4JH2?UT(B)E, in this analysis.
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The horsepower vs. speed curve (A, see page 88) illustrates the relationship between engine speed and developed horsepower, with less than 50% of maximum power available until the engine is turning over at 1,800 rpm. The torque curve (B) confirms the very limited ability of the engine to do much useful work at low speeds. The specific fuel consumption curve (SFC) (grams of fuel per horsepower-hour, curve not shown), shows that the engine is most efficient when operating at between 2,900 and 3,300 rpm. The propeller power curve (D) illustrates an important fact. A fixed-pitch prop can fully absorb the engine’s power at only one operating point, which is usually chosen to be at maximum allowable rpm. This is shown by the intersection point of curves D and A.
The variables in the prop selection process are diameter, number, and type of blades and blade pitch. For most installations, the goal is to select a prop that will allow the engine to at least reach, or slightly exceed, maximum rated speed when the hull is clean and the boat is loaded to maximum operating weight. At all lesser engine speeds the prop is progressively less efficient in absorbing the available power. This mismatch can be seen at point (1) on the graph (2,800 rpm) where the engine is capable of developing 88 horsepower, while the prop is only capable of converting 45 horsepower into thrust. These data curves make it clear that, at any engine speed less than maximum, the engine must turn more rapidly than is necessary to deliver the power required by the boat. This is the equivalent of driving a car in second gear when third or fourth would be correct for the load and speed conditions.
When the engine’s throttle in a boat with a single-speed transmission is rapidly advanced from idle, the engine is momentarily overloaded, an effect similar to that of starting a car in second or third gear. The initial load imposed by the prop, which was chosen for its performance at wide-open throttle, causes the engine to labor or lug, and, if it is a diesel, emit an impressive cloud of black smoke (which is largely composed of unburned fuel). Electronically controlled engines minimize the smoke by sensing the amount of air available and moderating the increase in fuel delivered to the cylinders. The penalty of using a single-speed gear box is still present, however. Once the engine accelerates to a speed at which it can deliver a reasonable amount of power the prop is too small to absorb all of the available energy.
The shape of the propeller power-absorption curve is of interest in understanding the performance limitations that use of a fixed-pitch prop and a single-speed gear box impose. The curve reflects one of the basic laws of physics: force = mass x acceleration; or force = mass x velocity x velocity, usually written as F=MV.
Two cooperating effects are at work in converting the rotational energy delivered to the prop into a force that propels the boatthrust. The rotation of the blades, which move at an angle to the water stream, creates a thrust vector in the same way your hand is forced upward or downward if held at an angle into the wind passing a moving car. At the same time, the airfoil section of the rotating blades creates a pressure differential from the front to the back of the blade. This pressure differential is accounted for by Bernoulli’s principle, the same effect that allows a sail to draw a boat forward into the relative wind and to lift an aircraft off the ground. The amount of thrust developed by these two effects is largely determined by the number of water molecules that flow over the prop blade surfaces and the speed of those molecules. We are back to F=MV. The velocity squared term in the equation largely accounts for the curved shape of the propeller curve (D). It is clear that a fixed-pitch prop can match an engine’s power output at only one specific speed. It is also clear that the same prop, rotating at a lower speed relative to the driving engine (a higher gear ratio in the gearbox, usually termed, from automotive practice, as a “lower” gear) will match the engine’s power capability at another point. As in a vehicle starting out in low gear, the engine is better able to accelerate to a speed at which it can develop significant power. In addition, when faced with the increased load of a steep grade, it can operate at a speed that delivers a high percentage of its available power, without being overloaded. For a boat, a typical two-speed gear might offer a low ratio of 2:1 and a high ratio of 1.5:1. The prop would be sized based on having the gear in high and the throttle wide open, just as with a single-speed transmission. When the transmission is in low gear, the engine will be able to accelerate quickly and when powering into head seas will have sufficient reserve power to maintain boat speed against the varying resisting force of the waves. Using these ratios, in low gear, the prop would match the engine’s power curve at about 2,700 rpm. Use of a two-speed gear will also eliminate some low-speed operating problems. Many sport-fishing boats have difficulty moving slowly enough when their engines are in gear and the throttles are at idle. In compensation, these boats are often fitted with so-called trolling valves, which in reality are nothing more than intentionally slipping clutch modes. Excessive use of the trolling valve can cause clutch overheating. Many large boats have minimum speeds, with the engines in gear and throttles closed, which are excessive for maneuvering in congested areas. A two-speed gear, in low range, reduces the boat’s minimum speed while simultaneously enhancing initial acceleration capability.
A controllable-pitch propeller is a viable alternative to the multi-speed gear box. It can yield the same advantages and correct the same shortcomings. With the controllable-pitch prop blades initially set to minimum pitch, the engine can accelerate rapidly to a speed at which it is delivering significant power. As the boat’s speed increases and the engine speed reaches maximum, the blade pitch can be increased, absorbing the excess available energy and ultimately converting all available engine power into thrust. The overall effect is to match the prop at all points on the engine’s power curve. Using a controllable-pitch prop also allows the operator to set the engine speed to the minimum point on the SFC curve and then, by adjusting the prop pitch, absorb all the power available at that speed. The result is maximum fuel efficiency. Controllable-pitch props, some with full-blade pitch-reversal capability, are common on large yachts, many cargo ships, and most new ocean liners. They are standard equipment on all turbo-prop aircraft and multi-engine piston aircraft and most high-performance single-engine piston aircraft. Controllable-pitch props have been made for use on small sailboats. Thousands of 27-foot, Albin Vega, Swedish-built sailboats were equipped with a direct-drive, 12-horsepower engine and a controllable-pitch, fully feathering prop. When the combined throttle “gear” lever was in neutral, the engine was at idle speed and the blades were set at zero pitch, producing no thrust. When the throttle was moved to the ahead direction, the engine speed and the blade pitch were increased simultaneously. When reverse was required, movement of the throttle to the astern position advanced the throttle; however, the prop blades were repitched to provide reverse thrust.
Perhaps the interest shown by racing enthusiasts in the new approaches to conveying engine power to props will encourage others to apply their skills to the improvement of boat propulsion.
The horsepower vs. speed curve (A, see page 88) illustrates the relationship between engine speed and developed horsepower, with less than 50% of maximum power available until the engine is turning over at 1,800 rpm. The torque curve (B) confirms the very limited ability of the engine to do much useful work at low speeds. The specific fuel consumption curve (SFC) (grams of fuel per horsepower-hour, curve not shown), shows that the engine is most efficient when operating at between 2,900 and 3,300 rpm. The propeller power curve (D) illustrates an important fact. A fixed-pitch prop can fully absorb the engine’s power at only one operating point, which is usually chosen to be at maximum allowable rpm. This is shown by the intersection point of curves D and A.
The variables in the prop selection process are diameter, number, and type of blades and blade pitch. For most installations, the goal is to select a prop that will allow the engine to at least reach, or slightly exceed, maximum rated speed when the hull is clean and the boat is loaded to maximum operating weight. At all lesser engine speeds the prop is progressively less efficient in absorbing the available power. This mismatch can be seen at point (1) on the graph (2,800 rpm) where the engine is capable of developing 88 horsepower, while the prop is only capable of converting 45 horsepower into thrust. These data curves make it clear that, at any engine speed less than maximum, the engine must turn more rapidly than is necessary to deliver the power required by the boat. This is the equivalent of driving a car in second gear when third or fourth would be correct for the load and speed conditions.
When the engine’s throttle in a boat with a single-speed transmission is rapidly advanced from idle, the engine is momentarily overloaded, an effect similar to that of starting a car in second or third gear. The initial load imposed by the prop, which was chosen for its performance at wide-open throttle, causes the engine to labor or lug, and, if it is a diesel, emit an impressive cloud of black smoke (which is largely composed of unburned fuel). Electronically controlled engines minimize the smoke by sensing the amount of air available and moderating the increase in fuel delivered to the cylinders. The penalty of using a single-speed gear box is still present, however. Once the engine accelerates to a speed at which it can deliver a reasonable amount of power the prop is too small to absorb all of the available energy.
The shape of the propeller power-absorption curve is of interest in understanding the performance limitations that use of a fixed-pitch prop and a single-speed gear box impose. The curve reflects one of the basic laws of physics: force = mass x acceleration; or force = mass x velocity x velocity, usually written as F=MV.
Two cooperating effects are at work in converting the rotational energy delivered to the prop into a force that propels the boatthrust. The rotation of the blades, which move at an angle to the water stream, creates a thrust vector in the same way your hand is forced upward or downward if held at an angle into the wind passing a moving car. At the same time, the airfoil section of the rotating blades creates a pressure differential from the front to the back of the blade. This pressure differential is accounted for by Bernoulli’s principle, the same effect that allows a sail to draw a boat forward into the relative wind and to lift an aircraft off the ground. The amount of thrust developed by these two effects is largely determined by the number of water molecules that flow over the prop blade surfaces and the speed of those molecules. We are back to F=MV. The velocity squared term in the equation largely accounts for the curved shape of the propeller curve (D). It is clear that a fixed-pitch prop can match an engine’s power output at only one specific speed. It is also clear that the same prop, rotating at a lower speed relative to the driving engine (a higher gear ratio in the gearbox, usually termed, from automotive practice, as a “lower” gear) will match the engine’s power capability at another point. As in a vehicle starting out in low gear, the engine is better able to accelerate to a speed at which it can develop significant power. In addition, when faced with the increased load of a steep grade, it can operate at a speed that delivers a high percentage of its available power, without being overloaded. For a boat, a typical two-speed gear might offer a low ratio of 2:1 and a high ratio of 1.5:1. The prop would be sized based on having the gear in high and the throttle wide open, just as with a single-speed transmission. When the transmission is in low gear, the engine will be able to accelerate quickly and when powering into head seas will have sufficient reserve power to maintain boat speed against the varying resisting force of the waves. Using these ratios, in low gear, the prop would match the engine’s power curve at about 2,700 rpm. Use of a two-speed gear will also eliminate some low-speed operating problems. Many sport-fishing boats have difficulty moving slowly enough when their engines are in gear and the throttles are at idle. In compensation, these boats are often fitted with so-called trolling valves, which in reality are nothing more than intentionally slipping clutch modes. Excessive use of the trolling valve can cause clutch overheating. Many large boats have minimum speeds, with the engines in gear and throttles closed, which are excessive for maneuvering in congested areas. A two-speed gear, in low range, reduces the boat’s minimum speed while simultaneously enhancing initial acceleration capability.
A controllable-pitch propeller is a viable alternative to the multi-speed gear box. It can yield the same advantages and correct the same shortcomings. With the controllable-pitch prop blades initially set to minimum pitch, the engine can accelerate rapidly to a speed at which it is delivering significant power. As the boat’s speed increases and the engine speed reaches maximum, the blade pitch can be increased, absorbing the excess available energy and ultimately converting all available engine power into thrust. The overall effect is to match the prop at all points on the engine’s power curve. Using a controllable-pitch prop also allows the operator to set the engine speed to the minimum point on the SFC curve and then, by adjusting the prop pitch, absorb all the power available at that speed. The result is maximum fuel efficiency. Controllable-pitch props, some with full-blade pitch-reversal capability, are common on large yachts, many cargo ships, and most new ocean liners. They are standard equipment on all turbo-prop aircraft and multi-engine piston aircraft and most high-performance single-engine piston aircraft. Controllable-pitch props have been made for use on small sailboats. Thousands of 27-foot, Albin Vega, Swedish-built sailboats were equipped with a direct-drive, 12-horsepower engine and a controllable-pitch, fully feathering prop. When the combined throttle “gear” lever was in neutral, the engine was at idle speed and the blades were set at zero pitch, producing no thrust. When the throttle was moved to the ahead direction, the engine speed and the blade pitch were increased simultaneously. When reverse was required, movement of the throttle to the astern position advanced the throttle; however, the prop blades were repitched to provide reverse thrust.
Perhaps the interest shown by racing enthusiasts in the new approaches to conveying engine power to props will encourage others to apply their skills to the improvement of boat propulsion.
