Few discussions confuse, confound, and mislead boat owners, builders, brokers, and readers as much as that of horsepower and torque.
When distilled down to its most basic form, horsepower is a measurement of work over a period of time, while torque is simply a measurement of force irrespective of the time over which its applied. Torque is an element of horsepower; however, its distinctly different.
Well over a dozen different types of horsepower measurement have been used since the term was first coined by James Watt, the developer of the improved steam engine, in 1782. He determined that ponies used to carry coal (or water) out of vertical mines could lift, via a rope and pulley, 22,000 foot-pounds of force per minute (more on what this means in a moment). The pony turned a mill wheel 144 times in one hour (2.4 times per minute). Watt also estimated that a pony could pull with a force of roughly 180 pounds. The wheel had a 12-foot radius, which meant the horse traveled 2.4 x 2? x 12 feet in one minute, which, using Watts formula of force multiplied by distance divided by time equaled 32,572 foot-pounds of force/minute, which was rounded to 33,000 ft lbf (pounds of force)/minute or 1 horsepower. Some say Watt carried out these calculations for ponies then increased the figures by 50 percent, assuming horses were that much stronger. There are competing stories involving brewery horses and other historically obscure individuals, but the unit of measure for horsepower, which involves a calculation of force over time, remains.
The reason Watt went through the trouble of devising the horsepower calculation was profit-driven. He needed a way to calculate for his clients the amount of coal they would save by using his improved steam engine (improved over Newcomens steam power plant), his fee being based on a percentage of the coal not used by his customers after making the switch. However, the commission program wouldnt work for mines that used horses rather than Newcomens steam engine and thus, the term horsepower was born. The inference being, for instance, a 20-horsepower steam engine would replace 20 horses, although its not likely the comparison was that empirical.
Other measures of horsepower have been proffered by other nations since Watts, including those abbreviated as PS (German), CV (Italian, Spanish, and Portuguese), pk (Dutch), ch (French), and others, all of which translate to horsepower in English, with one significant twist: they represent metric horsepower (MHP). The good news is, 1 PS (which stands for Pferdestärke, or horse strength, in German), CV, pk, etc., as well as all of the other metric horsepower abbreviations is equivalent to .99 non-metric or imperial horsepower, sometimes abbreviated as hp(I). Thus, theres not much of a difference unless you are talking about really large ships or locomotive engines, for instance.
Occasionally, these suffixes may still be used on European and other foreign engines, however, with the adoption of European Union standards for member countries, all horsepower must be rendered in watts (hp may be provided as well) for engines produced or sold in those countries, coming full circle to watts or kilowatts (kW), one of which equals 1,000 watts. One horsepower is equivalent to 746 watts, or 0.746kW. Therefore, a 100hp(I) engine produces 74.6kW.
That, however, is not the end of the horsepower story. In addition to defining horsepower, its form of measurement must also be defined. There are several definitions, including drawbar horsepower (dbhp), used for measuring locomotive power plants; indicated horsepower (ihp), a theoretical measurement of a perfectly efficient engine; brake horsepower (bhp), used to measure an engines power without any accessories such as transmission, belts, water pumps, hydraulic PTO pumps, etc.; and shaft horsepower (shp), which is a measurement of the power available at the transmission output coupling. For the most part, the latter two are most commonly used for measurement of marine engine power, although its important to understand that in both cases, accessories such as the aforementioned alternators and pumps, are not included in most engines hp ratings and none of them take into account drag induced by shafts, stuffing boxes, or cutless bearings. Although it varies, the friction losses imparted by the transmission are typically between 3 percent and 10 percent, with reduction gears and V-drives leaning closer to the higher end of that range. Thus, the difference between bhp and shp is typically small, although losses imparted by add-on equipment and running gear can be significant.
When belt and friction losses are taken into account, its not unusual for a high output alternator to absorb, at full output, as much as 10hp. If its doing so while the engine is idling, then its worth considering that you may not have enough power for maneuvering or hydraulic thruster operation.
In short, the unit or method of measurement of horsepower for a marine engine is less significant than the importance of comparing like measurements and units. If you are comparing engines or completed vessels engine, make certain you are also comparing bhp to bhp or kW to kW, etc., and take into account add-on accessories such as alternators and hydraulic systems.
Torque, as mentioned previously, is also a measure of energy, however, it has nothing to do with time; it could be imparted over one minute or one year. If horsepower is energy measured over time, torque could be thought of as the process for transforming or converting that energy into a useful motion; one that twists, like an axle or propeller shaft.
The definition and understanding of torque can be a bit tricky. For illustrative purposes, lets say its simply a force in pounds multiplied by distance. Youve almost certainly demonstrated this yourself by using a longer wrench or an extension pipe slipped over a socket wrench when removing a stubborn nut or bolt (the extension is used in this case on the handle, it should not be confused with extensions used on the driven or socket end of the wrench). The extension multiplies the torque applied to the fastener by virtue of the length or distance that the force is applied by your hand from the point where its applying twisting motion. Ten pounds of force applied to a 1-foot-long wrench imparts 10 pound-feet (its pound-feet in this case by the way, not foot-pounds; the latter represents work or an expenditure of energy) of torque, or 13.5 Newton meters (Nm) in the metric system, while 10 pounds of force applied to a 2-foot-long extension enables you to subject the fastener to 20 pound-feet of torque or 27 Newton meters. Its one of those rare cases where Mother Nature seems to be offering up a free lunch.
Engine torque is measured using the following formula: (5,252 x hp) ÷ rpm. In order to get more power from an engine, and because horsepower is a measurement of power over time, it would seem then that one way to squeeze more of that power from an engine would be to make it turn faster. In fact, this approach works well and its why the small, light, yet powerful engine on my Italian motorcycle spins up to 8000 rpm, to develop more power from a smaller power plant package. Math-types will have noticed in the above formula, however, that as rpm increases torque decreases, and there goes the free lunch. This is why traditional, large, slow-turning diesels deliver mountains of torque. Its why my Ford F-250 Powerstroke diesel can haul a 4,000-pound boat up a ramp at idle speed (with a little help from its reduction gears, Ill get to that in moment) and why Westwind Tugboat Adventures vessel, Union Jack, is powered by a six-cylinder, 400hp engine that turns at 340 rpm and produces a whopping 6,180 pound-feet of torque. Its a heavy slow turner, weighing in at 38,000 pounds. (For more on this engine and the Union Jack, see Steves article, A Perfect Union, PMM Nov/Dec 04.) By way of comparison, a modern, high-speed 400hp diesel may produce around 600 pound-feet of torque (measured at the engines crankshaft output) and weigh somewhere around 1,000 pounds.
Does this mean that if you want torque, which is what really turns the prop, then you must have a large, heavy diesel? The short answer is no, because while Mother Nature cant be fooled, she can be cheated. What if we could keep the engine rpm high, to maintain horsepower, while slowing down the shaft/prop rpm in order to coax more torque out of the equation? In fact, this is done regularly by using a component known as a reduction gear, which is bolted to, and often appears to be, part of the transmission. Reduction gears do just that, reduce rpm at the shaft using gears, somewhat like the transmission in the F-250 mentioned above, while allowing the engine to continue to turn at higher rpm. For example, a 150hp engine that turns at 4000 rpm produces 197 pound-feet of torque. Not bad for a small, light, high-speed diesel engine. However, with a 2:1 reduction gear installed, the shaft rpm will be reduced to 2000 rpm, making the torque available to the propeller 394 pound-feet, a significant increase. There are trade-offs for the reduction gear and rules that must be followed, however, Ill save discussion of those for another column. In general, the price paid for this gear magic is twofold. First, theres the inefficiency and friction induced by the reduction gear, which may account for a 3 to 5 percent loss. Second, the reduction gear adds weight, complexity, and expense to the installation. In the end, its a net gain and one that many engine manufacturers and boatbuilders embrace. While heavy, slow-turning diesel engines are desirable in many ways, among other things they often last a very long time, they also have drawbacks and as such the higher-speed engine and reduction gear combination simply makes good sense in many applications.
When comparing engines and power output, ensure like units and measurement methods are being used, and remember, its not about horsepower alone.