In a previous Power Voyaging column, we discussed the specific fuel consumption curve for a typical marine diesel. In this installment we look at more engine performance curves to see what they can tell us about the operating parameters of the marine diesel.
Once again, our example is a Yanmar 6LYA-STE, the engine used as the standard powerplant for the Nordhavn 35.
The curve in Figure 1 illustrates the engine’s torque capability at each operating speed. The three curves in Figure 2 show the total power available from the engine’s crankshaft without the marine gear, the amount available at the output of the transmission to turn the prop shaft, and the lowest curve shows the amount of energy a properly matched fixed-pitch propeller can absorb and convert into thrust at each engine operating speed.
The torque curve shows the engine’s ability to twist its crankshaft against a load, to produce torque – a force applied through a distance. The peak of the curve at about 2,500 rpm indicates the speed at which the engine’s mechanical efficiency is optimal. There is nothing wrong with running the engine at lower or higher speeds; the peak torque point simply shows where the total of the engine’s design decisions are most agreeable.
The relative lack of torque at low rpm limits the initial acceleration of the engine when it is under load. Although the curve does not extend below 1,600 rpm, the torque continues to decrease below this speed. The diminished torque results in the reluctance of the engine to accelerate from a stopped position. In an automobile we can select a high gear ratio, (low or first gear) to reduce the load on the engine, so it can accelerate to a speed where more torque is available. We continue to shift gears to match the load on the engine to the available torque. The prop slips in the water in much the same way we can slip the clutch in a manual transmission. The fixed-pitch prop equates, however, to a single-speed transmission and can match the engine’s power capability at only one speed.
Moving from the torque curve to the uppermost of the horsepower curves, the dotted line shows horsepower availability without the marine gear (a nice number to know if you are powering an electrical alternator). The curve immediately below shows the horsepower available to turn the prop shaft. The difference, usually on the order of 5 to 7 percent, is energy converted to heat by the friction between the gear teeth in the transmission. That’s why large marine gears have oil coolers.
The remaining curve shows the ability of the prop to convert the available power into thrust. This curve is obviously different from the horsepower curves in that it is convex. This shape is the result of the way in which a propeller converts the movement of its blades into thrust. The blades move through the water at an angle, creating a force vector that acts to push the blade forward along the axis of the prop shaft. This phenomenon can be seen and felt when you hold your hand in the slipstream out the car window. Your hand will be forced upward if held at the proper angle to the wind. The second effect results from Bernoulli’s principle: the movement of water across the convex surface of the blade induces an area of reduced pressure, creating additional forward-acting thrust. The fact that these phenomena vary roughly with the square of the velocity lends a parabolic shape to the thrust vs. rotational speed curve.
The most significant fact to be gleaned from the prop law curve is that a fixed-pitch prop, whether on a boat or an aircraft, can be made to absorb all the engine’s power at only one engine speed.
If the prop is too small (in diameter, total blade area or blade pitch), its curve will intersect the available power curve at an rpm greater than the engine’s maximum speed. The prop will not be able to convert all the available power into thrust, an unfortunate circumstance if you are trying to stem an outgoing current.
If the prop is too large, the engine will be overloaded by any attempt to operate above the match point. The result will be similar to trying to drive a car uphill in too high a gear. Marine diesels are speed-governed engines. Except for the latest electronically controlled engines, the fuel metering system will supply additional fuel in an effort to make the engine run at the commanded speed. If the load is excessive, the engine will be overfueled, resulting in quantities of black smoke, accumulation of carbon and eventually damage to the engine. The ideal condition exists when the prop curve intersects the output power curve at the engine’s maximum rated rpm.
Fortunately you don’t have to draw curves to find out if your prop is about the right size for your boat and engine. First, have the tachometer checked for accuracy, particularly at the maximum speed rating of the engine. If possible, do the check when the boat’s bottom and the running gear are clean. Do the test on a calm, preferably low-wind day. Thoroughly warm the engine and make a series of three- or four-minute runs each in various directions (to cancel out what wind may exist; you don’t care about currents). During each run, note the maximum rpm. The goal is to achieve the maximum rated rpm for the engine plus 50 to 100 rpm.
If the engine willingly exceeds more than max plus 100 rpm, the prop is likely too small. If the engine won’t reach max plus 50 rpm with a clean bottom, the prop is too large and is limiting the engine’s ability to deliver all the power it was designed to produce.
The margin above maximum rated rpm is intended to allow for the eventual (inevitable) accumulation of a modest amount of fouling on the hull surface. The effect of the fouling, an increase in the weight of the boat or the installation of a breeze-catching bimini top will be to move the intersection point between the prop law curve and the power curve to the left, to precisely where it is shown on the diagram.
The power and prop law curves in this example show that when the engine is running at 2,900 rpm, maximum rated rpm minus 400, the engine can deliver about 340 hp. However, at that speed the prop can absorb only 240 hp, about 70 percent of the engine’s capability. Of course, if your boat has a controllable-pitch propeller, you can do the prop-matching exercise continuously as it is done in aircraft equipped with controllable-pitch/constant-speed props.
Simply start out with the prop set to minimum pitch so it presents a minimal load to the engine. This will allow the engine to accelerate rapidly to a speed where it is capable of delivering a substantial amount of power. Then increase the prop pitch while monitoring the increasing reading on the exhaust gas temperature gauge. Stop increasing the pitch when the target temperature is reached. The prop is then properly matched and will absorb all the power the engine can deliver safely at that rpm.
Contributing Editor Chuck Husick is based in St. Pete, Fla. As a mariner and a pilot, he is quite familiar with all kinds of props.