Horsepower vs Torque: What Is the Difference in Marine Engines?

September 10th, 2019 by

Discussions about horsepower and torque mislead, confound, and confuse boat owners, brokers, builders, and even readers. When it comes to these two terms, many people make the mistake of referring to horsepower when they are actually referring to the torque.

You don’t have to worry if you fall into this category. The engine horsepower and its torque are so related that it is very easy to get mixed up.

When you want to buy a marine engine, should you focus on its horsepower, its torque, or both? Do you know the difference between horsepower and torque in your marine engine? Learn more about the comparison of horsepower vs torque before you buy.

 

Horsepower

Horsepower is a unit of power, and it is stated in foot-pounds of force per second (ft-lbs/s). One horsepower equals 550 ft-lbs/s. Today, the term is usually used in connection with auto and boat engines, but originally it got used for measuring only the power of steam engines.

James Watt, a Scottish engineer, was the inventor of the steam engine and he’s the one who coined the term horsepower.

Watt’s steam engine was an improvement of another engine design by Thomas Newcomen. His engine used as little as a quarter of the fuel used by Newcomen’s engine to do the same amount of work.

Unfortunately, it wasn’t easy for Watt to compare his engine to that of Newcomen for marketing purposes. This is because many people were using horses for their mechanical works. So, Watt had to devise a way of comparing the work done by the steam engine to the work done by horses.

By performing several experiments, he was able to determine that a horse could take one minute to lift a weight of 33,000 pounds. This is how he ended up stating that one horsepower is equivalent to 33,000 foot-pounds of work done in one minute (or 550 foot-pounds of work per second).

Watt made some assumptions that made his figure to have some flaws. For example, he assumed that a horse could perform the work at a constant rate rather than get tired. However, such little details did not bother him, and neither were his customers bothered by those details.

The fact that Watt was able to compare the power of his steam engine to the power of a horse made him make good sales. This is because he was able to show his customers that his engine could perform the work of five horses at the same time duration. His invention played a significant role in the Industrial Revolution.

 

How to Calculate Horsepower

The measurement 550 ft-lbs/s didn’t stop with the Industrial Revolution. The unit has been converted into many other measurement units. For instance, in some experiments, one horsepower has been found to be equivalent to 746 watts.

This means, if you use a treadmill to generate electricity, then the one-horsepower horse will be able to produce 746 watts. Engineers have also done calculations to relate horsepower to torque. This is is especially important for marine engines and marine engineers.

 

Torque

Torque is also a measure of energy, but unlike horsepower, it has nothing to do with time. You can impart it on an object for as long as you wish.

Horsepower if force measured over time but torque is the process of converting that force into motion that’s useful – one that twists the propeller shaft or an axle, for example. Torque causes an object to rotate about an axis.

It is easier to explain torque by using an example. If you apply a 50 pound-force to a two-foot handle socket wrench, you’ll be applying a torque (turning force) of 100 pound-feet (lb-ft) to the bold.

 

Horsepower Vs Torque

Torque produces power in engines. So you should be able to relate torque to horsepower. To measure the torque of an engine, a device called a dynamometer is used. This device applies some load on the engine, measures the resultant power then uses the power to determine the torque.

Once the torque gets determined, it can be converted into horsepower. Horsepower and torque are connected by the formula:

Torque = Horsepower x 5252 / Rpm (where 5252 is a constant, and Rpm is rotations per minute)

The figure 5,252 is obtained from a series of calculations, and it is necessary to convert rotations per minute to radians per second.

By using a dynamometer, you will get the horsepower with the corresponding rpm values for a given engine. You can use these values to calculate the torque and peak horsepower.

The power of an engine reaches a maximum value at a certain point of rpm. This is referred to as the peak horsepower and is often written as …. HP at …rpm.

 

How Much Horsepower Do You Need?

Once you understand what horsepower vs torque is all about, you may be asking yourself how much horsepower your boat needs.

There are several factors you need to take into consideration before you settle on the value of horsepower needed by your boat – the number of people on the boat, the use of the boat, fuel efficiency, horsepower-to-weight ratio, and the manufacturer’s recommendations and limits, among others.

 

Application of Horsepower Vs Torque

The overall boat speed is a function of prop pitch and prop speed. Prop pitch gets measured in inches, and it refers to the total distance covered in one propeller revolution. 21 inches of horizontal travel in a single revolution is written as Ex 21P.

If you have a mid-sized boat, it is advisable to fit it with a higher-revving engine that’s able to generate high torque at low rpm.

You can see from the formula, torque = Horsepower x 5252 / Rpm, that torque and rpm are inversely proportional: the lower the rpm, the higher the torque, and vice versa.

If you want to get more power from your engine, you need to make it turn faster. So, when it comes to buying an engine, you should focus more on torque rather than horsepower. If you want to learn more horsepower vs torque issues or the technical aspects of your boat, be sure to contact us.

Posted in Boating, How-To