Measuring and Managing the Power that Helps You Enjoy Boating

By Mike Milne

Most power boaters simply see horsepower as the factor that makes a boat go faster. For some, that’s all they want to know. But for others, knowing how horsepower is generated, calculated and applied will help them get the most out of their boat and engine.

If you are in the don’t-want-to-know group, relax. Designers, boatbuilders, engine manufacturers and dealers – plus industry organizations and government regulators – are all in your corner. Designers try to specify powerplants that will deliver the kind of performance needed for the way a boat will be used, whether it’s a lightweight fishing boat, a day-cruising pontoon, or a heavy cruiser.

There are proven formulas – most based on the Savitsky planing algorithm for planing boats – that help designers choose likely power solutions. Boatbuilders, working with engine manufacturers, then do extensive water-testing. By the time a production boat, equipped with recommended engine or engines, reaches the boat-buying public, it usually works well. A test drive will give a buyer the assurance that a boat meets his or her needs.

Regardless of its engine’s horsepower rating, a boat has to meet its performance goals. For planing boats, that means accelerating to plane quickly and smoothly, running efficiently at speeds required for intended purposes such as fishing, tow sports or cruising, and operating at top speeds within the engine’s wide-open-throttle range.

Another key part of the performance package is the propeller or water-jet impeller that pushes the boat on a plane easily and allows it to reach top speed within the engine manufacturer’s specified full throttle range. Planing usually occurs comfortably between 2,000 and 3,000 rpm on most sterndrives or higher on higher-revving outboards or water-jet drives. Choosing the right propeller is not simple, but it is significant.

Two manufacturers of water-jet-powered craft, BRP and Yamaha, don’t include horsepower in technical specifications. Reasons for that are blurry, but may include marketing approaches and liability issues. Power losses in water-jet propulsion drives can range up to about 20 percent, make horsepower ratings problematic and may also be a factor.

Regardless, Yamaha’s WaveRunners and sports boats and BRP’s Sea-Doo PWC perform very well. But horsepower is not specifically used to quantify that.

When it comes to horsepower ratings that match boats and motors, regulations, standards and certification all help.

“From the consumers’ point of view, if they are following the capacity plate or the manufacturer’s recommendation . . . they are going to have a safe product, ” says Michael Vollmer. A Burlington, Ont., based professional engineer and yacht designer, Vollmer does vessel inspections for NMMA (National Marine Manufacturers Association), IMCI (International Marine Certification Institute) and Canadian Coast Guard certification.

Some boaters, though, want to know more.

That alphabet soup of voluntary certification and regulation helps to ensure that safe boats are delivered to the public. Some boaters, though, want to know more.

To learn more about horsepower, start with a definition.

First used to compare steam power to the work of horses, mechanical horsepower includes the elements of time and torque (measured in pound-feet). In the 1800s, steam engine developer James Watt figured out that a horse could do 33,000 lb-ft of work in one minute.

One pound-foot (lb-ft) is the amount of work done by one pound of turning (or twisting) force applied to move or displace a distance of a foot. The same force can also be expressed as 746 watts.

Engine specs often show the metric equivalent to horsepower in watts. Metric horsepower, sometimes listed as PS, is about 98.6 percent of mechanical horsepower.

To get horsepower, multiply the torque (lb-ft) being produced times the rpm (revolutions per minute) and divide by 5252. (That’s a calculated constant number.) The amount of horsepower being produced varies depending on the rpm at the time of measurement.

But torque is also an important element in quick planing and fast, mid-range acceleration.

Right off the bat, it’s clear that torque is a key ingredient of horsepower. Cruisers who operate large diesel-powered boats already understand that. But torque is also an important element in quick planing and fast, mid-range acceleration.

A graph showing a torque curve (illustrating torque over engine rpm) illustrates where an engine develops horsepower. What is usually desirable for a boat is a flatter or “fat” curve, with plenty of torque over a wide rpm range.

Generally speaking, diesels do this best and two-stroke outboard engines also have desirable torque curves. Four-stroke outboard manufacturers these days are building engines that develop torque in similar patterns.

“Torque is what’s going to get you up and moving.”

“Torque is what’s going to get you up and moving,” says Adrian Rushforth of Mercury Marine Canada. “It’s just not talked about a lot. With horsepower, you’ve got the big number on the back and people can look at that. But you have to dig deeper to find the torque. And torque ratings are going to be different if you put a different gearcase on it or a different gear ratio, or enhance different parts of the engine; it’s going to deliver more torque to the propeller.”

If your head is spinning already, let a reputable marine dealer lead you through the forest of engine choices. But a little knowledge about horsepower is not a bad thing.

Setting torque aside for the moment, let’s take a look at horsepower ratings – the various ways engine manufacturers quantify horsepower. Torque can be measured on a dynamometer that puts a load on an engine. Horsepower is then calculated and expressed at its peak output point. Diesel engine specs often state the rpm level at which the rated horsepower has been measured, and the rpm level where the maximum torque is developed.

Internationally accepted standards are used to measure and express horsepower. But with a marine engine, the first variable is where on the engine (or engine and drive) that measurement takes place.

That allows any power losses through the gearing and lower unit to be taken into account.

With inboard engines, measurement takes place at the crankshaft and an ISO (International Standards Organization) standard is often used. But since the mid-1980s, outboard engine manufacturers have been measuring horsepower at the propshaft. Sterndrive horsepower is also measured at the propshaft. That allows any power losses through the gearing and lower unit to be taken into account. In most cases, manufacturers use the ICOMIA (International Council of Marine Industry Associations) 28/83 standard, in accordance with NMMA practice.

The standard allows an engine’s rated horsepower to be up to 10 percent more or less the rated amount. That means an engine rated at 150 hp could be actually putting out as little as 135 hp or as much as 165 hp.

“I’m hoping that all of our motors, and the engines of all the manufacturers, are at the horsepower rating or 10 percent above, not 10 percent below,” says Rob Bradley, marine special services representative with Yamaha Motor Canada.

So what about the higher-output outboards that are part of several manufacturer’s lineups? Are those just engines with electronic control units (ECU) designed to make more horsepower? Not necessarily.

In Yamaha’s lineup, for example, the performance-oriented VMAX SHO VF150 is built on a 2,785-cc block while the F150 four-stroke is built on a 2,650-cc powerhead.

“They are two completely different engines,” says Bradley, adding that “they are within that 10 percent, they are both 150 horsepower, but their power curves are different.” Yamaha’s two 115-hp engines are built on the same block, but the SHO version has a different cylinder head and develops more of its horsepower in the upper end of the engine speed range.

Mercury Marine also offers outboards with identical horsepower ratings, but with one model promising added performance. The Mercury OptiMax 250 and OptiMax Pro XS 250 are two examples. Both are 3 L V-6 direct-injection two-strokes.

“Mercury has always prided itself on over-delivering on the horsepower.”

Adrian Rushforth says, “Mercury has always prided itself on over-delivering on the horsepower, so if it’s a 250 it’s putting out 258 or 260, to err on the higher side.

“But comparing the Pro XS to the regular OptiMax I don’t think there’s so much a difference in horsepower as a difference in other components.” For example, the engine has a Torque Master gearcase and carbon fibre reeds.

So, instead of focusing on horsepower, pleasure boaters should consider an engine’s torque and ensure the propeller or propellers deliver it effectively. Torque is what makes diesels the top practical choice for large yachts. But torque also contributes to the fun factor in high-speed fishing boats, sport boats and performance boats, powered by gasoline engines.

High horsepower can offer “top speed potential for bragging rights,” says Mark Weigl, former offshore racing champion and owner of performance boat builder Tuff Marine, based in Cambridge, Ont.

“However, torque is the most fun. If you have something with more torque and it accelerates aggressively because of the torque you will have felt like that was much faster.”

According to Weigl, some four-strokes are better than others at building torque. Bigger displacement adds torque, he says, but superchargers and turbochargers generate it without adding much weight.

Weigl is currently building a Tuff 28 that will be equipped with twin Mercury Racing Verado 400 outboards. The engines are supercharged, with plenty of torque.

The horsepower should mean a top speed over 100 mph, says Weigl, but the torque fun factor will mean the owner won’t always have to opt for top speed.

Most recreational boaters won’t need that kind of speed, horsepower or torque, but it’s nice to know how it is created, if you want it.

This article is featured in the Summer 2015 issue of Boats&Places.