Understand the importance of engine torque
You almost certainly know the horsepower rating or displacement of your boat’s engine. But some marine engine manufacturers have started marketing torque rather than horsepower. Contrary to common practice, Indmar Marine Engines even names its Wake Series engine models by torque rather than horsepower or displacement. We think it’s smart, although it could make comparison shopping tedious 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 features.
Torque is a measure of force, usually expressed as weight multiplied by distance. Hook a 10-pound weight on the end of a 5-foot-long horizontal lever and you’ve created a force of 50 foot-pounds (10 x 5). In engine specification tables, torque is listed in pound-feet (lb-ft) or newton meters (Nm).
Horsepower (aka kilowatts) is power, a measure of work done in a specific period of time. If you lift five 10-pound weights off the ground and place them each on a 4-foot-high shelf, you have moved 50 pounds of weights 4 feet, or 200 foot-pounds (4 feet x 50 pounds) of work . If you complete 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 horsepower of a rotating motor is: horsepower equals (torque x rpm)/5252. For example, a motor producing 300 lb-ft. of torque @ 5,000 rpm produces 285.6 hp @ 5,000 rpm (300 x 5,000) / 5,252 equals 285.6.
Courtesy of Indmar
This number on the cowling of your outboard represents its maximum horsepower rating. This peak will occur at or near the top of its RPM range. Most boat owners don’t spend a lot of time racing wide open. But we want this boat to plan smart every time we move the throttle forward from idle, and that’s where torque becomes important. The higher the torque at the propeller shaft, the larger the propeller the engine can turn and the more thrust will be generated. (Bigger can mean any combination of increased pitch, blade area, or diameter.) Thrust is what drives the boat forward and lifts it in plane. Once planed, this larger propeller moves the boat farther with each revolution, more efficiently than a smaller propeller. Since horsepower is a function of engine speed, an engine that produces more peak horsepower doesn’t necessarily produce more torque where we need it most. The ideal boat motor would produce as much driveshaft torque as possible at 2,000 rpm, then maintain that level of torque – a so-called “flat torque curve” – for much of the rpm range. .
Courtesy of Indmar
Study the power curve of a Roushcharged Raptor 575 with 510 horsepower per Indmar engine. This engine produces 520 lb-ft. of torque but only 198 bhp at 2,000 rpm. Torque climbs steadily to its rated peak of 580 lb-ft. at 3,000 rpm and stays above 570 lb-ft. until it starts to decline at about 4500 rpm – that’s a pretty flat torque curve. The power curve rises steadily diagonally up the chart as the rpm increases to its peak of 5,200.
Towing sports enthusiasts are a group of boaters who don’t care 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 peak engine power, is what matters, and that happens to be at the heart of the Raptor 575’s torque curve. That’s why tow engine builders, including Indmar, Pleasurecraft, Malibu and Ilmor, boast of the couple. Likewise, inboard diesel engines intended to power 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 engine produced, and the company refused to divulge it. To be fair, its two-speed transmission makes the matter more complicated because any reduction in speed between the crankshaft and driveshaft will amplify driveshaft torque. But we believe that no 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 ad that boasted of driveshaft torque, but at engine rpm rather than driveshaft rpm, thus removing the gear ratio from the equation for an advantageous comparison. There may be more than one test standard. For example, does a 4.2 liter 250 outboard produce more mid-range torque than a 4.2 liter 225? And if not, it might make more sense to buy the cheaper 225. This would be valuable information for boat buyers.