How big of a difference in speed does 50 horsepower make?

Power, power, power.

A glance at the different marine engine manufacturers’ website shows a constant focus on horsepower. However, how do you separate the marketing spin from reality?

What difference does horsepower make?

Shouldn’t torque be more of a concern?

Yes it should. As should an examination of both the engine’s power curve and torque curve. What matters is how much torque, its’ distribution along the operating rpm range of the engine, and how much of that torque is transfered to the shaft.

Let’s take a look at an example. We will show an 11m boat (RHIB), equipped with twin VGT 450 and twin VGT 500, showing you the difference 50 horsepower makes in vessel performance.

Vessel LOA: 11m

Beam: 3m

Displacement (full): 6.5t

2 engines

surface drives

Equipped with 2 X VGT 450, top speed 50 knots.

Equipped with 2 X VGT 500, top speed 52 knots.

This is a differential of only 2 knots in speed. Actually, since there are two engines, the differential on the vessel is 100 hp total.

Is the engine price differential worth the extra increase in speed? Only the customer can answer that question. Sometimes yes, and sometimes no. It really depends on the vessel’s mission and how it will be used.

Now, compare the same vessel, equipped with a Cummins QSB 6.7 engine, rated at 450 hp (The one we replaced). That vessel achieved a top speed of only 49 knots. Why the difference?


The torque curves are different. Though the Cummins engine produces a slightly higher maximum torque than the VGT, that extra torque is concentrated in the middle of the rpm range. So, as the vessel continues to move faster after planning, the acceleration is lower, since the torque produced rapidly drops. This is fine when equipping a truck. However, boats encounter far more resistance than trucks, even after planning.

The VGT 450, on the other hand, produces a flat torque curve, much more evenly distributed along the operating rpm range of the engine. This allows a greater increase in speed that other engines.

VGT450 curves 100714

So, when powering a vessel, it is not simply a matter of horsepower. Torque and its’ distribution is far more important.

All of our curves

We recently had a request from a customer to show all of the power and torque curves of our VGT Series of engines on a single graph, for “at a glance” comparison purposes.



Take a look at the graph. Note how very flat power and torque curves are on our engines, and compare them to the curves of our competitors. The one below is for a Cummins QSB 6.7, medium intermittent duty (Comparable to VGT 450):

cummins torque 6.7


Note the spike in torque on the Cummins. Why does this matter? Well, the Cummins may have a higher maximum torque level, but it occurs at a spike, at a single RPM level. The VGT provides more torque, over a wider RPM range. This translates into lower cruising RPM (thus less fuel consumption) and more power at those speeds.


Engine Physics 101: Power Curves



This article is the final posting in our short series about the physics of diesel engines. Today, we discuss power output and power curves. Whenever you purchase an engine, you are given a data sheet that shows a curve with the power output of the engine, the torque produced, and, normally, the fuel consumption of the engine at specific speeds. These curves are not derived out of thin air, there are formulas used to determine the shape of the curve and the power produced by the engine at different speeds. All engine manufacturers adhere to strict ISO standards when testing the engines and producing these graphic depictions of power.

So, how is the power output of an engine determined? Here’s the science:


power output


P = engine power [W]

ρa = air density [kg/m3 ]

Vs = engine swept volume [m3 ]

S = engine speed [revs/sec]

formula1= fuel:air ratio [no units]

Qlhv = lower heating value of fuel [J/kg]

η = efficiency [expressed as a decimal]

Thus, this formula is repeated along the entire power curve at each speed and the results plotted along the curve. But what about turbochargers and their impact on air density? Simple. The change in air pressure is adjusted according to the amount of pressure produced by the turbocharger.

For torque, the formula is also relatively simple:

torque for


Ti = engine indicated torque [Nm]

imep = indicated mean effective pressure [N/m2]

Ac = cylinder area [m2]

                L = stroke length [m]

z = 1 (for 2 stroke engines), 2 (for 4 stroke engines)

           n = number of cylinders

           θ = crank shaft angle [1/s]






Maintenance Tip of the Week: Mounts 05/11/2015



Maintenance Tip of the Week – Engine Mounts

Engines vibrate considerably when operating, and bolts on the mounts can sometimes work themselves loose. Periodically re-torque the mount bolts to ensure that they are properly tightened.




Ski Boat and Parasail Boat Engines


The market for commercial marine engines is quite varied. There are dozens of different types of commercial vessels and uses, each with their own specific performance requirements.

One market niche where MarineDiesel engines are extremely competitive is the ski boat and parasailing boat market. These boats have very specific engine power requirements. Commercial, yet recreational also, these vessels need to be operational on a seasonal basis (usually), and every minute they are not operating can cost the owners thousands upon thousands of dollars in opportunity costs that are lost forever. They often use gasoline or petrol engines, and the Marinediesel engine’s compact size gives the operators a high torque, lower operating cost, diesel alternative.

Additionally, ski boats and parasail boats typically have highly compact engine compartments and need to minimize weight. The VGT Series is the most compact engine in its’ class on the market. In most cases, it is the ONLY marine diesel engine that can even fit into these types of vessels.

Ski boats and parasail boats require a torque curve that gives the optimal power at the lower RPM range for rapid takeoff, decreasing in need after the vessel is planning (This is why they often use gasoline engines, designed originally for automobiles). The NIRA ECU used by MarineDiesel allows us to optimize the performance specifically for these types of operations, giving the operator the torque that is needed, when it is needed. Add in the lower cost of diesel fuel (usually) and the much longer life cycle of a diesel engine versus a gasoline engine, and the best decision becomes immediately clear: Marinediesel’s VGT Series.



Variable Geometry Turbocharger Facts


MarineDiesel’s VGT Series of engines was named as such for a very good reason: We use Variable Geometry Turbochargers (V.G.T.) in their design.

What is a VGT?

A VGT is a type of turbocharger that has internal vanes that open and close, changing the amount of air and exhaust that enter the engine. Thus, the geometry of the turbocharger changes, based on RPM.

Old, non-turbocharged diesel engines that are naturally aspirated were inefficient. Adding a turbocharger increased the amount of power and torque produced by the engine. By changing the geometry of the turbocharger, the compression ratio can be changed, thus making the engine more efficient, and substantially increasing the amount of power and torque produced, particularly at the low RPM range. Additionally, standard turbochargers tend to produce a lag at low RPM. The VGT solves this problem by further compressing the air into the engine.

The use of the VGT produces higher, sustainable torque across the engine speed range. In part, this is why our VGT Series of engines has a much higher level of overall torque, rather than the spikes, peaks, and valleys common in the toque curves of engines made by our competitors.

Since the power of the engine is increased by the VGT, it means that we produce, at only 515 kilos, the lightest, most powerful, 500 hp marine engine on the market.

Honeywell (the manufacturer of the Garrett VGT we use), has an interesting video that has an explanatory animation. We hope you find it interesting.





Keep Calm and Get Torqued


For the last year or so, a popular meme has been making the rounds through Facebook, and so on.


This was originally based on a poster from the UK that was made during WWII, but never actually used. A few years ago, someone found it, and the image spread all over the Internet.


So in the spirit of perpetuating a modern legend (or irritation, depending on your viewpoint), here at MarineDiesel we invite everyone to Keep Calm and Get Torqued.


Because the engines you have been using are weak. They don’t have the power.

Because the MarineDiesel VGT Series gives your vessel the highest level of torque, at the lowest weight, in the market today.

Because you demand high performance, at a reasonable price.

Because you should never settle for second best.



What is a Dynamometer? Why do we use it?



When purchasing a diesel engine, one may often hear the term “dyno test”. A dyno test is, quite simply, a test that measures the torque and power output of an engine. In order to accurately measure output, a device known as a dynamometer is required.

So, what exactly is a dynamometer?

In purest terms, a dynamometer is an instrument that measures mechanical force. Wikipedia has a fairly decent article describing dynamometers HERE.

Why is a dynamometer necessary?

When MarineDiesel manufactures an engine, we need to have the ability to test whether that engine is producing the power and torque we advertise. Engines are complex pieces of equipment, with hundreds of different parts. A dynamometer allows us to adjust an engine, or verify any design changes that have been made. A dyno test also allows us to catch mistakes made during the assembly or manufacturing process, in addition to seeing any faults in components or parts that we have used.

Dynamometers are expensive pieces of equipment, often costing in the hundreds of thousands of Euros. At MarineDiesel, we have three dynamometers in our factory, each in their own test cell (One pictured above), and each configured to measure power at different ranges. These test cells not only include dynamometers, but other pieces of test equipment that allow us to perform diagnostics on thousands of different data points, measuring everything from power and torque with the dynamometer to electrical signals from sensors, fuel consumption, and engine emissions. Indeed, these test labs are highly proprietary. It is the one place in our factory where we do not allow outside photographs.

MarineDiesel batch tests production engines that use common parts. So, for instance, engines made from common lot numbers of parts are tested on average every ten engines or so. For customized engines, the testing can occur with every engine. Every engine manufacturer dyno tests at different levels. To use an example from the automotive industry, BMW runs a dyno test every 10,000 engines or so in the production process. Ferrari, with hand-built engines, may test every single engine made.

When looking at test data, the curves that we provide in our brochures were made by a dynamometer. When engines require classification, these dyno tests are usually supervised. Occasionally, customers may want dyno test data. We never provide batch test data to customers, for a very simple reason: Batch test data is generated for a batch of engines, and never a single engine. When this type of data is requested, we open our labs to the independent agency of the customer’s choice. This allows the customer to receive data from his specific engine. It is bona fide proof of engine performance.




Comparing Torque Curves – VGT 400 vs Volvo Penta D6



MarineDiesel’s VGT Series of high speed marine engines are the lightest and most powerful engines in the marine market today. Don’t just take our word for it, though. Compare for yourself! In this article, we compare the torque curves of the VGT 400 with the Volvo Penta D6:

Volvo Penta D6 Torque Curve

Volvo Penta D6 Torque Curve

VGT400 torque

VGT400 torque

Note the “flatness of the VGT’s torque, as compared to the “spike” on the Volvo Penta.




The Displacement Game. Fact or Marketing?


Engine displacement is, perhaps, one of the most misunderstood terms when it comes to an engine. Especially when comparing one engine’s power to another engine’s power.

On this web site, we often mention displacement, or make comparisons between our engines and a competing manufacturer’s product. Displacement is important, and can give you a general idea about an engine’s overall power, but it is not the only factor that determines power output.

First off, what is displacement anyway? Quite simply, displacement is the volume of air displaced by the pistons in the cylinder when the engine is operating. The more air displaced, in theory, the more powerful the engine. For those with an interest, the formula for determining displacement is here:

 mbox{displacement} = {piover 4} times mbox{bore}^2 times mbox{stroke} times mbox{number of cylinders}

So why is an engine with a larger displacement automatically more powerful?

In the old days, before the advent of turbochargers, superchargers, and whatnot, displacement was a pretty reliable determinant of power. However, engine power is a product of combustion, and there are many factors that determine combustion.

For instance, our 6.6 litre VGT series is much, much lighter than other engines made by our competitors. However, we produce far more torque and are at a much lighter weight than any other competitor on the market. Why? Others have the same displacement, but do not produce the same power or torque. The answer is that displacement is only a single component of the compression ratio. Our VGT turbocharger can be altered in order to change the amount of air let into the combustion chamber, and at what force.

Likewise, there are many 500 horsepower engines sold on the market for workboats, for instance the Scania DI13-80M. It produces the same 350 mhp as our VGT 350, but at a much lower RPM (1,800 vs 3,500), and produces far more torque. It also weighs over twice the weight of our VGT 350. The Scania engine has a displacement of 12.7 litres, versus our VGT 350’s 6.6 litres displacement. For small tugs requiring lots of bollard pull, the Scania engine is a far better choice than our VGT. Likewise, on a fast RHIB, the VGT 350 is a much better choice.

Over the years, marketing departments have used the quick to define displacement as a way to show high power. Think TV commercials that say something like, “With a whopping 5 litre displacement power!” and so on. It is best to base engine choices on the manufacturer’s rating, rather than simple rules of thumb. As in our example above, if you own a small tug, you want to look for engines with lower RPM and higher displacement. The CAT C7 or C9, with higher displacement, or the Hyundai Seasall offerings.