Engine horsepower ratings explained
by Dudley Dawson
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.
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.
Not only is it important to know which system has been used to specify HP, it’s also vital to know where and how it has been determined
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.