You almost certainly know the horsepower rating or displacement of your boat’s engine. But some marine-engine manufacturers have begun marketing torque over horsepower. In a break from standard practice, Indmar Marine Engines is even naming its Wake Series engine models based on torque rather than horsepower or displacement. We think that’s smart—though, it might make comparison shopping cumbersome if not universally adopted—because for most boat owners, engine torque is more important than horsepower. To understand why, you need to understand the difference between these two important engine characteristics.
Torque is a measure of force, usually expressed in weight times distance. Hang a 10-pound weight from the end of a 5-foot-long horizontal lever and you have created 50 pound-feet (10 x 5) of force. In engine spec charts, torque is stated in pound-feet (lb.-ft.) or newton meters (Nm).
Horsepower (aka kilowatts) is power, a measure of work accomplished in a specific period of time. If you lift five 10-pound weights off the floor and set each on a shelf 4 feet high, you’ve moved 50 pounds of weight 4 feet, which is 200 foot-pounds (4 feet x 50 pounds) of work. If you finish this task in two minutes, you have produced 100 foot-pounds of work per minute. One horsepower is defined as 33,000 foot-pounds of work per minute. The formula used to calculate the power of a rotating engine is: horsepower equals (torque x rpm)/5,252. For example, an engine producing 300 lb.-ft. of torque at 5,000 rpm is making 285.6 hp at 5,000 rpm (300 x 5,000)/5,252 equals 285.6.
That number on the cowl of your outboard represents its rated peak horsepower. That peak will occur at or near the top of its rpm range. Most boat owners do not spend a lot of time running wide open. But we do want that boat to get smartly on plane every time we advance the throttle from idle, and this is where torque becomes important. The more torque at the prop shaft, the larger the propeller the engine can turn, and the more thrust will be generated. (Larger can mean any combination of increased pitch, blade area or diameter.) Thrust is what moves the boat forward and lifts it on plane. Once planing, that bigger propeller moves the boat farther with each revolution, with more efficiency than a smaller propeller. Because horsepower is a function of engine speed, an engine that makes more peak horsepower does not necessarily produce more torque where we need it most. The ideal boat engine would make as much prop-shaft torque as possible at 2,000 rpm, and then maintain that level of torque—a so-called “flat torque curve”—for much of the rpm range.
Study the power curve of a 510-peak-horsepower Roushcharged Raptor 575 by Indmar engine. This engine makes 520 lb.-ft. of torque but only 198 hp at 2,000 rpm. Torque climbs steadily to its rated peak of 580 lb.-ft. at 3,000 rpm and remains above 570 lb.-ft. until it starts to decline at about 4,500 rpm—this is a pretty flat torque curve. The horsepower curve makes a steady climb diagonally up the chart as rpm increases to its 5,200 peak.
Tow-sports enthusiasts are a group of boaters who don’t give a whit about top speed. But it takes a lot of thrust to push a 6,000-pound boat with 5,000 pounds of ballast—plus the lead ballast that many carry and some newer boats include, plus 2,000 pounds of crew and fuel—at 11 mph against a wall of water. Torque at 3,500 rpm, not the engine’s peak horsepower, is what matters, and this happens to be right in the meat of the Raptor 575’s torque curve. This is why builders of tow-sports engines, including Indmar, Pleasurecraft, Malibu and Ilmor, brag on torque. Similarly, diesel inboard engines meant to propel large cruising and sport-fishing boats also promote torque.
Courtesy Invincible Boats
When Mercury Marine introduced its V-12 Verado 600 outboard, we asked how much torque the motor makes, and the company refused to divulge. To be fair, its two-speed transmission makes that a more complicated question because any gear reduction between the crankshaft and the prop shaft will amplify prop-shaft torque. But we don’t think any outboard manufacturer has ever published a torque rating. Mercury says one reason is that there is no industry standard for measuring engine torque. One engineer cited the example of an Evinrude advertisement that bragged on prop-shaft torque—but at engine rpm rather than prop-shaft rpm, thus taking the gear ratio out of the equation for an advantageous comparison. There may be more to it than a testing standard. For example, does a 4.2-liter 250 outboard make more midrange torque than a 4.2-liter 225? And if not, maybe it would be smarter to buy the less-expensive 225. That would be valuable information for boat buyers.