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Marine Engine Horsepower Ratings

Counting Horses

What does your engine’s rating really mean?

A friend of mine, an astute observer of human nature, propounds the theory that when sailors get together they talk about weather and sea conditions, while when powerboaters assemble, the topic is boats—their boats. My own observations have borne this out over the years and I can add this: When powerboaters talk about their boats, they eventually turn to the topic of horsepower. This is hardly surprising, since engine output directly affects performance.  

It’s a pretty good bet that if you’re reading this column, you know a fair amount about the engine(s) in your boat, including the horsepower rating. But it’s also a safe wager that to you that rating is just a number that’s meaningless unless you can compare it to another engine’s rating. And indeed that’s true—up to a point. Because a quick look at the term and topic of horsepower can provide you with some interesting information—or at least a little trivia with which you can impress your boating pals.

First, the basics, and I promise to keep this part simple. For our purposes, the term horsepower emanates from James Watt, who is credited with the first truly successful steam engine. Being an entrepreneur, he of course wanted to promote his invention by comparing it to the most popular motive power of the day, the horse. After hours of observation and arcane calculations, Watt determined that a horse could turn a mill wheel 144 times in an hour, or 2.4 times a minute. Using the formula “power equals work divided by time,” where work equals force multiplied by distance, he came up with a rounded-up figure of one “horsepower” equaling 33,000 foot-pounds per minute.

Well, good for Watt, but what does that have to do with your Cats? Actually, quite a lot. Even today that figure stands when we’re talking about mechanical horsepower, which is what boat engines produce. (Other kinds of horsepower include electric, boiler, and hydraulic.) 

That would seem to end the subject right there except for one thing: There are lots of different ways of measuring horsepower—what engineers call engine power test codes. Codes address how an engine is rated and under what conditions—things like air temperature, humidity, and fuel temperature. Cool air is denser and allows more fuel to be added and thus more horsepower to be produced. Humid air contains more moisture, and that means more oxygen, a key component of combustion. And cool fuel is denser than hot fuel, so more of it can be added to the combustion chambers. 

Traditionally the way to establish horsepower was to measure it with a device that attached directly to the crankshaft’s output flange: an “engine brake,” which used water to provide resistance so that the engine would be fully loaded. Today most engineers rely on a more sophisticated device called a dynamometer, but regardless, the obvious drawback to mounting a device to the crankshaft is that it does not account for frictional losses that necessarily occur in a transmission, which can run as high as 15 percent. Even so, look through marine engine product literature and spec sheets, and you will still find engines rated in brake horsepower or bhp.

A better method to measure horsepower is by attaching the measuring device to the output flange of the transmission, which obviously accounts for frictional losses from the transmission and so paints a more accurate picture of what an engine is actually delivering in the real world. But for a boat, the most accurate horsepower measurement is the one taken at the propeller because it accounts for the frictional losses created by shaft logs, strut bearings, and such—accurate but impractical for anything but complete engine-drive packages like stern drives and outboards. And that’s why today, almost all inboard marine engines are rated in shaft horsepower. 

But back to the topic of rating regimens, a quick look at a few spec sheets will tell you that most marine engines today are also rated in metric horsepower, which is a term not of the Society of Automotive Engineers (SAE) but of the DIN, or Deutsches Institut für Normung, the German national organization for standardized measurement. Metric horsepower is also measured at the transmission output shaft but under slightly different conditions. Still, it actually ends up being pretty close to the equivalent SAE rating: One mechanical DIN horsepower is equivalent to 98.6 percent of a conventional shaft horsepower.

If you’ve learned anything from this little exegesis it should be that the conditions under which an engine operates have a significant effect on how much power it actually produces. So if your engine room is hot or poorly ventilated, or your fuel coolers are clogged or too small, your engine’s horsepower ratings may end up  being inaccurate. Or to put it in more real-world terms, if you run your boat in the summertime in Florida, it will not perform as well as it does in the fall up north. And that’s true no matter what the rating numbers say.The Mangusta 165 boasts triple 4,613-brake-horsepower MTUs.

The Mangusta 165 boasts triple 4,613-brake-horsepower MTUs.

This article originally appeared in the April 2013 issue of Power & Motoryacht magazine.