Understanding your engine's performance curve can help you improve performance without wasting precious fuel. Every engine has a unique set of performance curves, so it's important to make sure you have the correct curves, including the proper injector designation and rating classification.

The set of curves shown here is for a Detroit Diesel model 12V-92TA, operating at its maximum power rating. The curves are specific to this engine, popular in many larger sportfishermen of recent vintage, but the secrets they hold are illustrative of most diesel engines.

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To understand the curves, we must first decipher the terminology used on the graph. To the upper left is a scale of horsepower, and it refers to the top three curves, one dotted and two solid. Along the bottom is a scale of engine speed in rpm, so we can determine the horsepower at any given rpm.

The uppermost curve, (shown as number 1), is rated BHP, or brake horsepower. This is the horsepower measured at the flywheel by the engine manufacturer on a test brake, or dynamometer. For this engine, the maximum rated SHP, or shaft horsepower, at the engine's top speed of 2300 rpm is 1,080 horsepower. From this power, we must deduct a small percentage for friction losses in the reduction gear to get the SHP, shown by the second curve from the top (2). This is the power available at the output flange of the transmission where it connects to the propeller shaft, and in this case is 1,040 horsepower after accounting for gear losses of 3.7 percent.

Ideally, this engine would be operating at its full 2300 rpm with the throttles at wide open and the boat fully loaded for sea. If it is turning less than 2300 rpm, it is a sign that the propeller has too much pitch, the boat is overloaded, or both. If it is turning more than 2300 rpm, the engine is underloaded, and a bit more propeller pitch or cup might yield a bit more speed.

Because this is an ideal situation with the propeller properly sized, the third curve from the top, the propeller load curve (3), intersects the SHP exactly at the top engine speed of 2300 rpm. This curve shows how much horsepower is required to turn the installed propeller at various engine speeds. You'll note that as the rpm is reduced, a gap opens between the power available-SHP, curve 2-and the power absorbed by the propeller, curve 3. Therefore, the propeller is not using the full power available from the engine at lower speeds.

This margin between the horsepower that is theoretically available and that which is actually used is intentional to ensure the engine has a reasonable life. This is the reason full-rpm operation of high-performance engines, such as this one, is time-limited by the manufacturers under their warranties. The peak power rating for this engine for use in long-range cruising yachts is only 790 horsepower, and for commercial workboats, which might operate 24 hours per day, it's a mere 510 horsepower. You can get only so much out of a ton of iron, and you can have it as horsepower or as hours, but you can't have both.

The scale on the lower right refers to the lower set of curves numbered 4 and 5, labeled "Fuel Consumption" and "FuelPropeller Load," and measures how much fuel the engine burns in gallons per hour. The upper curve (4) is how much the engine would burn if it were pulling full power throughout its rpm range, but it's not. It's burning only the amount of fuel required to turn the propeller, indicated by the lower curve (5).

Using the curves, let's have a look at what happens when you pull back on the throttles. Normally, from the top speed of 2300 RPM, you might slow to a cruise speed of 2000 RPM. As mentioned last month, the power used and the fuel burned drop in proportion to nearly the cube of the rpm, so as you can see from curve 3, the power drops from 1,040 SHP to 684 SHP-(2,000/2,300) x (2,000/2,300) x (2,000/2,300) x 1,040 = 684), a reduction of 34 percent. Using curve 5, we see the fuel burned drops a similar proportion, from 60 gph to just under 40 gph, per engine. If you drop back to 1800 instead of 2000 rpm, the savings is even more, with power going to 499 SHP, a reduction of 52 percent, and the fuel rate dropping to 29 gph (visit www.yachtingmagazine.com for more fuel-saving tips).

Power varies roughly as the square of the speed, or inversely, speed varies as the square root of the power. This means that if our theoretical sportfisherman is making 35 knots at full power, it will cruise at about 28.4 knots at 2000 RPM or 24.2 knots at 1800 RPM.

What does this mean in real terms? For a trip of 250 nautical miles, our twin-engine sportfisherman will take 7 hours, 9 minutes at 35 knots (2300 RPM) and burn 857 gallons of fuel ($4,714 at $5.50 per gallon). The trip will be rough on the engines and boat, and maybe the crew, too. At a more reasonable 2000 RPM (28.4 knots), it will take 8 hours, 48 minutes and burn 700 gallons of fuel ($3,850). At a fuel- saving 1800 RPM (24.2 knots), it will take 10 hours, 20 minutes and burn 600 gallons of fuel ($3,300) at 24.2 knots. In this example, arriving three hours earlier costs an additional $1,414 in fuel. Only you can decide the true value of your time, but now you have a way to evaluate the cost in dollars as you nudge those throttles forward.