Engine horsepower ratings explained

When comparing diesel engines, too often the rating is simply given as horsepower, or hp, but more is needed to fully evaluate one engine against another to determine what we’re getting for our money. There’s also shaft horsepower and brake horsepower, in both Imperial and metric units, their equivalents in kilowatts rather than horsepower to consider.

Horsepower’s development

We owe the concept of horsepower to a Scotsman, James Watt, who sought to market his steam engine to Cornish miners in the 18th Century.

The miners used horses to power the pumps that drained mines, so Watt faced the challenge of determining how many horses could be replaced by one of his engines. A unit of measure was needed, and Watt coined ‘horsepower’.

Watt carried out experiments to determine how much work a Cornish horse could do and found that, on a continual basis, they could lift 330 pounds of water an average of 100 feet each minute. Multiplying 330 times 100, Watt set his ‘horsepower’ at 33,000 foot-pounds/minute.

This number still stands today, officially sanctioned by the Society of Automotive Engineers as the standard value of one SAE (or English) horsepower.

Thus it was until someone saw fit to immortalise James Watt by naming a unit of energy in his honour. The ‘watt’ was born, and quickly became the standard for rating the energy consumption of everything from light bulbs to microwave ovens. Along the way, the watt was also adopted for rating diesel engines, but as a single watt is so small, the term kilowatt was adopted (one kW is equivalent to 1,000 watts). SAE established an equivalence, setting one horsepower as equal to 745.7 watts, or 0.7457 kilowatts. As an example, a 1,000hp (SAE) engine has a power output of 745.7kW.

This was all done prior to the formalisation of the metric system and its adoption as the official system of measurement by many of the world’s countries. When that occurred, feet were converted to metres, and pounds to grams. The conversion of horsepower didn’t result in a nice round number, so unlike SAE and the conversion to watts, the metric officials established a new value that was close, but not truly equivalent.

They decided that one metric horsepower would be defined as 4,500 kilogram-metres/minute. That converts to 32,549 foot-pounds/minute, meaning an SAE horsepower is slightly more powerful, by 1.4%, than a metric horsepower. A 1,000hp (SAE) rating would thus be equivalent to a rating of 1,014 metric horsepower (mhp) for the same engine.

You might also see ‘PS or ‘CV’ in place of mhp on some engine specifications. They all represent the same value, simply by a different name. ‘PS’ is often found on engines from Germany, standing for pferdestarke, or ‘horse strength’. Similarly, CV is often found on engines from Italy, standing for cavallo, or horse.

Diesel engines are the most common power source in marine usage, but all of the definitions and values given are equally applicable to other types of engines, including outboard motors, gasoline inboards, sterndrives, gas turbines and others.

There is no truth to the factoid that petrol horsepower is somehow inferior to diesel horsepower. What is true is that the continuous duty rating of a diesel engine is often at a higher percentage of its maximum rotational speed (revolutions per minute, or RPM), so a 1,000hp diesel may be capable of delivering more power day in and day out than a 1,000hp petrol engine.

There’s also the matter of torque, which is inversely proportional to the RPM at a given horsepower. As a result, diesels, which generally turn at a lower RPM than gasoline engines, deliver more torque for a given horsepower.

Judging horsepower

Not only is it important to know which system has been used to specify the horsepower – metric or SAE – it’s also vital to know where and how the horsepower has been determined, and under what ambient and loading conditions.

Most engines are certification-tested at the factory on a water-brake dynamometer, often simply called a ‘brake’. The results of that testing are reported as ‘brake horsepower’, or bhp. It’s important to read the fine print on the specification sheet, as it should tell you whether the engine was tested with or without its accessories, such as water pumps and alternators, and at what air and cooling water temperature.

If tested with air and water at a cool temperature, and no accessories, you’ll never see that power when cruising the Caribbean or Mediterranean. The best manufacturers can supply a curve of power loss vs ambient temperature, but you’ll probably have to ask for it.

The most useful rating, reported by some suppliers and not by others, is the shaft horsepower (shp). This is the power measured at the propeller shaft coupling flange, aft of the reduction gear, and usually includes the ‘parasitic losses’ of the engine accessories as well as the reduction gear. These losses are usually in the order of three per cent, meaning an engine with a brake horsepower of 1,000bhp will likely have a shaft horsepower of about 970shp.

These various terms are equally applicable to the SAE alternatives, so we can also speak of metric bhp or metric shp, as well as brake kilowatts or shaft kilowatts.

What’s important is that in evaluating engines and choosing a model for your yacht, is that you don’t compare apples to oranges. Don’t look at metric shaft horsepower for one engine and SAE brake horsepower for another. You can make the conversions yourself, with the information given here, or you can ask your dealer to do it for you.

In either case, just make sure all the numbers are in the same units, whatever they are, and are corrected to reflect the same rating conditions, before deciding which engine is the right one for you.