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.


Small boat? Electricity from your alternator.




Designing small boats is often very challenging. The advantage of small vessels is usually the high speeds necessary for completing different missions. What small vessels gain in terms of speed, they also give up in terms of size.

Operators of small vessels sometimes have problems getting enough electricity to power external devices onboard. Large boats usually have engine rooms and are equipped with gensets, negating the problem of finding current. Small vessels, such a RHIBs, have tiny engine compartments and are highly weight restrictive, making genset installation impractical.

So, what do you do in order to get power, particularly on vessels, such as rescue boats, that often need ancillary equipment and current to power such devices?

MarineDiesel has solutions for this problem.

We can optionally provide larger alternators, or additional alternators, to provide a constant power source. This solution is relatively inexpensive, but typically requires additional pulleys and belts, or modified patterns of placement (depending on engine compartment size).

MarineDiesel commonly equips new engines in this manner, especially on government vessels or commercial vessels.

If your vessel has a requirement for additional power, simply contact your local MarineDiesel distributor and give them the specifications of your needs.



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]






Engine Physics 101: Compression Ratio



Continuing our series about the physics of engines, today we focus on compression ratio.

Compression ratio is directly related to the amount of power an engine produces. In general, the higher the compression ration, the more powerful the engine. This ratio is measured from the top of the stroke to the bottom of the stroke. So, what is compression ratio?

Compression ratio is the ratio of volume within the engine’s combustion chamber from greatest to smallest capacity. For instance, the compression ratio of our VGT 500 is 18:1. That means that the volume of the bottom of the stroke is 18 times larger than the volume at the top of the stroke. It is how much air is compressed.

What does this have to do with power?

Since diesel engines use heat and pressure to combust fuel, the more pressure applied, the greater the amount of energy produced. So what is the limit? The amount of pressure that the cylinder head and piston are designed to withstand. On diesel engines from most manufacturers, the compression ratio ranges from 14:1 to as high as 22:1. On most petrol cars, by way of comparison, compression ratios are nearly always under 14:1, and usually no higher than 10:1, since they use a spark for combustion.

It is for this reason that diesel engines of a similar size to their petrol counterparts normally produce much greater levels of power and torque.

The formula for determining compression ratio is here:

mbox{CR} = frac { tfrac{pi}{4} b^2 s + V_c } {V_c}

whereb; = cylinder bore (diameter)

s; = piston stroke length

V_c; = clearance volume. It is the volume of the combustion chamber including head gasket.






515 Kilograms


515 Kilograms. That weight is the equivalent of:

  • An average adult polar bear
  • 1.1 times the weight of a grand piano
  • 30% of the weight of an average sized sedan
  • About 10% of the weight of an adult Tyrannosaurus Rex
  • 12 times the weight of a toilet
  • The weight of MarineDiesel’s VGT Series of marine engines

MarineDiesel manufactures the lightest, most compact, most powerful marine engines in their class. No competitors even come close. Though the comparison above is somewhat lighthearted, vessel displacement is a highly critical factor in vessel performance, and MarineDiesel leads the way in high power at low weight.







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.





Eat my wake!



When you first board a vessel equipped with MarineDiesel’s VGT Series of engines, you may be thinking, “OK, this is just another sea trial or boat ride.”

Five minutes later, and this has actually happened, you might be surprised to hear yourself exclaiming, “Eat my wake!!!!

Don’t be embarrassed.

We understand.

You simply cannot appreciate the compact raw power of a MarineDiesel VGT engine without experiencing it first, especially after years of operating boats equipped with the wimpy, inline six cylinder engines made by our competition. The difference is astounding.

The raw exhilaration that comes from extreme acceleration.

The noticeable increase in performance.

Feeling secure in the knowledge that other boats, however they are equipped, cannot match your power.

Give it a try. See for yourself.

At MarineDiesel’s headquarters in Ängelholm, we have several company boats that we use to demonstrate our engines. We have an “open door policy” for customers and potential customers who wish to try our engines out. Simply contact us with the details of when you will be in Sweden, and we will be happy to schedule an appointment.







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.



Genset Engines



Here at MarineDiesel, we often receive inquiries as to whether or not we manufacture generators. The answer is “no”, though we do manufacture engines that can be used to power gensets. Why would someone who needs a generator just purchase an engine?

Quite simply: We can customize the engine for the application.

Most (though not all) generator manufacturers do not manufacture the engines used in the generator. The reason is because providing electrical power is a different specialization than providing mechanical power. There are dozens of different variables in power generation, and the production of standalone models requires many different options and a different manufacturing process.

So, when asked to provide an engine for a generator, we nearly always receive this type of inquiry from either a generator manufacturer or from someone who needs a very specialized generator with needs that “off the shelf” models cannot meet. For instance, we can manufacture engines with customized levels of continuous power or multiple engine maps for different uses of the same genset.

MarineDiesel has, over the years, provided both marine and industrial engines for gensets and applications as widely varied as for electricity on large mining equipment to small fishing boats. We also can often refer people to our customers who are generator manufacturers and specialize in electrical power.