BUYING BOAT POWER

Comparison shopping for boat engines requires a thoughtful approach.

“Apint’s a pound the world around,” goes the old rule of thumb. That works in many instances. A US pint equals 16 ounces, and an imperial pint weighs a close-enough-for-some-folks 1.25 pounds. But the last time I was in London, a pint cost about £5.50 at the Lamb and Flag in Covent Garden. This goes to show that context is all with regard to rules of thumb, and especially when making comparisons.

It’s natural and normal to want to look for a simple, easy-to-remember basis of comparison. Unfortunately, universal rules or standards for comparing one thing against other similar things, as we boaters do when shopping for marine power, prove rarer to find than blenders and umbrella drinks at a UK pub.

Horsepower seems a natural refutation of my assertion. But there’s a rub: Horsepower can be measured in different ways. There is brake or crankshaft horsepower measured at—you guessed it—the crankshaft. Both are essentially the same thing, except that brake horsepower, rendered as bhp, is a more traditional term (how we boaters love tradition) still used by diesel engine-makers for referring to the force required to brake an engine when water or other weight is applied as drag on the crankshaft. Nowadays, a dynamometer is used instead of a brake. There’s also prop-shaft horsepower measured after the transmission. And there are other derivations. Each will render a different number when applied to the same engine.

Generally, diesel-inboard ratings are in brake horsepower, which amounts to crankshaft power. Gas inboards, sterndrives and jets are typically rated at the crankshaft too. It makes no sense to rate inboards or sterndrives at the shaft because any engine can power a variety of shafts, gear ratios, transmissions and drives. Outboard horsepower output is rated at the prop shaft because these marine engines are all-in-one. Knowing this, you might think, “Hey, great! I now have a yardstick with which to compare outboard to outboard.”

Allow me to digress.

Outboard-makers can utilize an industry-standard 10 percent swing in power ratings. An engine rated at 250 hp might actually make somewhere between 225 and 275 hp, for example. So, dockside tales of such-and-such making a “strong” 300 or a “weak” 115 compared to other engines might owe their origins to this 10 percent rating swing. Reasons for the 10 percent rule include test method and installation variables, and the improbability of manufacturing an engine that neatly delivers power in the increments to which boat buyers have become accustomed. Think about it: The number 400 looks cooler on an engine cowl than 392.61, right?

One of the best ways to compare engines (and boats) is to review our database of boat tests at boatingmag.com. You can use those tests as a basis of comparison that shows boats of similar size and weight powered by similar-rated engines and draw your comparative conclusions from actual recorded-on-the-water data.

Enjoy the issue.

Horsepower seems a natural refutation of my assertion. But there’s a rub: Horsepower can be measured in different ways.

Kevin Falvey, Editor-in-Chief editor@boatingmag.com