Why waste money on an outboard? The Marinediesel VGT Series makes them obsolete!

Outboard engines are usually a complete waste of money for commercial and government vessels.


Quite simply, they are not designed for anything other than recreational, or occasionally, racing use.

Outboard engines are usually merely petrol car engines turned vertically and connected to a modified automobile transmission. They were designed to provide the torque that an automobile requires in order to operate efficiently. The problems start to arise with respect to torque: Boats are under far higher levels of resistance than automobiles, particular at lower rpm ranges. So, many companies have tried to use diesel engines in outboards, rather than gasoline engines. What was the problem? Simple. The gearbox and drive systems were not designed to withstand the amount of torque produced by a diesel.

There is an old saying in the marine industry: “Horsepower sells boats, but torque actually moves them”. A quick glance at the marketing materials produced by outboard manufacturers quickly confirms that fact: Virtually none of them mention torque or show torque curves.

Some advantages of a diesel inboard over gasoline outboards:

  1. Diesel fuel is normally cheaper than gasoline in most markets. For example, in Australia, August 2015, retail price of diesel is A$1.29 per liter average, petrol A$1.34. In some countries the differential exceeds 20%. For instance, during the same period in Thailand, diesel averaged US$0.64 per liter and petrol US$0.93.
  2. Diesel engines always consume less fuel than gasoline engines. For instance, a Mercury Verado 350 consumes a whopping 353 g/kWh at full throttle, compared to 221 g/kWh on the Marinediesel VGT300. This equates to over 1/3 higher consumption. At cruising speed of 5,000 rpm on the outboard, the fuel consumption drops to 190 g/kWh, but the power produced decreases by over 1/3, netting just over 220 hp. The VGT300’s cruising speed of 2,800 rpm yields a fuel consumption of 195 g/kWh, but still producing nearly 280 hp.
  3.  As far as torque is concerned, virtually no comparison is possible. The outboard produces barely 400 Nm of torque, under a drastic bell curve, whereas the VGT 300 produces nearly 600 Nm of torque along the majority of the rpm range. A huge difference in actual work performed by the engine.
  4. Gasoline outboards have a MTBO of under 400 hours, light recreational use (all of them). A diesel inboard will last up to 3,000 hours MTBO. You could overhaul an outboard five times before needing to overhaul the diesel.
  5. The outboards are lighter. 303 kg versus 515 kg. This is reflected in their lower torque produced.
  6. However, space is less of a constraint. The Mercury Verado 350 is less than 100 mm smaller on all dimensions. A quick look at the photo above shows a transom width of about 1 meter. The VGT engines will require approximately 300 mm more length, and no extra width. A mere 300 mm extra height is necessary. In other words, on the boat picture, two VGT engines WILL fit.
  7. Outboards cost less to purchase. MSRP on the Mercury Verado 350 was US$32,000 in August 2015. The VGT 350 MSRP was US$37,000. However, given the fuel cost and consumption differentials, this small difference in acquisition cost quickly disappears.
  8. Gasoline is far more explosive than diesel. The inboards are much safer.
  9. Diesel engines have higher maintenance costs, but require much less maintenance.

As is evident in the points above, a strong case can be made for equipping your vessel with the Marinediesel VGT Series instead of outboards. The cost differential at acquisition is minimal, and the diesels far outperform outboards in nearly every instance. In particular, on the higher horsepower outboards, the price differential starts becoming significantly less.

Finally, in the picture above, twin Yamaha 200 hp outboards are pictured. A single VGT 400 or VGT 450 will outperform those two engines, lessening the price differential, even after gearboxes and drives are taken into account on the VGT engines.



Mass Spectrometer Testing



On marine engines, the most critical systems are: Fuel, Lubrication, and Cooling. A failure in any one of these systems can mean very expensive or complicated repairs to your engines. In this blog, we often write about the impact of bad fuel, and so on, but how can you know if fuel is the problem?

The answer should be familiar to anyone who has ever watched CSI or any of the crime shows on television: Mass Spectrometry. A lab will use a mass spectrometer to determine the chemical composition of your fuel, lube oil, coolant, or any other fluid on your vessel. Though most companies do not own these devices themselves, there are many labs that offer inexpensive analysis of your fuel or oil. Essentially, a sample of the fluid is taken, and the mass spectrometer provides a detailed report about what is in that fluid, whether it is rust, different metals, or seawater.

So, this should be the ultimate arbiter in the decision making process, right?

Not really. The results of the test are dependent on where and when the sample is taken. For instance, if you suspect you have bad fuel, you should not take the sample with fresh fuel just put into the tank. Likewise, with lube oil, the sample should be taken before oil is changed.

What is important to remember is that with the tools available to the marine industry today, operators can easily and inexpensively monitor their vessels for new or ongoing problems and prevent further damage.



Diesel Fuel Injectors: How They Work



In your engine, there is probably no more critical component than the fuel injection system. How critical? Without properly functioning injectors, the fuel combustion cannot be controlled and the engine will not function properly, not producing enough power, at best, and incurring severe damage, at worst.

But how do fuel injectors function?

MarineDiesel uses common rail fuel injection systems on our VGT Series, manufactured by Bosch. Common rail systems are now the standard on modern diesel engines, due to not only emissions regulations, but also the demand by consumers of more power for less fuel.

Common rail systems are under extremely high pressure. Fuel is forced to each injector (cylinder) under one single pipe (the “rail”). The injectors themselves look like big needles, with a nozzle in one end. The fuel enters the injector at pressure and is measured by a small needle valve, which is timed to atomize a set amount of fuel in tiny bursts. The reason that injectors are so susceptible to damage is that  the inlets and outlet, along with the valve, are easily blocked by dirty or poor quality fuel. Since injectors in common rail systems are under much greater pressure than injectors on older diesel engines, they tend to experience far greater wear and tear than injectors on older technology diesel engines.

This is also one reason that we always stress using OEM spares when replacing injectors. They are tested under the engine’s operating conditions and are manufactured under our specifications. Since replacing or repairing injectors is a highly technical, and usually expensive, procedure, we always recommend that this be done by our factory trained distributors.




Choosing the Right Engine



As an engine manufacturer, MarineDiesel would love to be able to answer the question, “Which engine should I choose for my project?” with a resounding “MarineDiesel, of course!” every single time. However, that is not the correct answer to give. Different engines have different strengths and weaknesses, and ours are no different. We often get requests to quote where our engines are simply not an appropriate match for a project. Sometimes, people are just price shopping, matching horsepower to horsepower, and sometimes a new project pops up where the shipyard does not have a lot of experience.

When determining the engine to choose, price should be among the last crieria that should be considered. There are far more important questions to ask:

  1. How will the vessel be used? Our engines tend to focus on fast boat applications. They are normally not a good match on tug boats, for instance (Though sometimes, occasionally, they are suitable).
  2. How much space is available for the engine? Smaller spaces require smaller engines.
  3. Is noise a problem? Engines are tested for noise when manufactured. Noise can be controlled through both silencing and insulation, in addition, yet those items also have costs in space and money associated with them.
  4. Is vibration a problem? Some engines, such as our VGT Series, produce much less vibration than inline models, due to their physical characteristics. Additionally, there are aftermarket ways to control vibration, like the use of different mounts or couplings.
  5. How capable is your maintenance team? Some engines are more complex than others. Some require a higher level of skill to maintain.
  6. Price. Price is important, and does play a role. However, consideration also needs to be given to the cost of spares, service, and training.
  7. Life Cycle / Rating. An engine used 2,000 hours per year needs a longer life cycle than an engine used for recreational purposes.
  8. Service network. Some engines may fit all criteria, but there is no service in your country available. Engine maintenance gets expensive very quickly when performed across continents.
  9. Warranty. How good is your engine’s warranty? Some manufacturers have better warranties than others.
  10. Fuel consumption. Fuel costs, on average, exceed 60% of any engine’s operating cost. Cost savings are significant over time.
  11. Performance expectations. You need to have proper calculations made with bona fide data: Not just guesses. Horsepower and torque requirements can vary drastically with small differences in hull design.
  12. Emissions requirements. This can be important. Laws and regulations vary widely between regions / nations.

We realize that choosing an engine is complicated. Contact your local MarineDiesel dealer for personalized assistance on your project.





Bad practice: Engine idling




A common misconception in the marine industry is that you must allow diesel engines to idle before putting them under load. With modern diesel engines, this belief is 100%, completely incorrect.

In fact, allowing an engine to sit at idle for long periods of time has the opposite effect: It increases wear on the engine, increases emissions, and wastes fuel.

How did this belief get started? Quite simply, people observed truckers at truck stops allowing their engines to idle and thought it was “Best Practice.” On older diesel engines, this may have been the case with fuel efficiency, and it is certainly the case with gasoline engines. Yet, a glance at a fuel consumption curve will show that a diesel consumes more fuel at idle, over a greater time period, than at startup. Additionally, in colder climates, it is believed that idling maintains the temperature. However, all modern diesel engines that are used in cold climates have sufficient heating to allow an easy start.

Unlike gasoline engines, modern diesel engines are designed to heat up under load. The fuel is under compression by heat, combusting more efficiently, and idling does not generate that necessary heat. Sitting at idle does not provide this load, and merely increases the friction in the cylinders. The load is generated on a boat under throttle. Idling for a period of time any more than an initial 30 seconds or so after the engine is started does nothing. No extra lubrication. No benefits. All marine engines manufactured by MarineDiesel are sufficiently lubed in this time period.

Finally, add in the fact that many jurisdictions in the world restrict unnecessary idling, due to emissions, this practice is one that should be eliminated. It just creates waste.



Marinediesel Dealer Profile: Finland


Marinediesel have had a long business association, since 2007, with our distributor in Finland, Tekno Marine.

Tekno Marine are a fully qualified and factory trained MarineDiesel distributor, and we have completed many projects over the years in the country. In addition to representing MarineDiesel, Tekno Marine are boat builders, building many unique craft under their own brands, including a line of airboats.

Additionally, they represent Bukh, Hyundai Seasall, and Sole engines in Finland, and offer complete naval architecture and marine engineering services.

Their MarineDiesel web page, in Finnish, is TM, and we encourage our customers in Finland to contact Tekno Marine for a quote on your next project.

Teknomarine AB

PL29, Marinsatamantie 3
FI Espoo
Phone: +358 9 819 07 70
Fax: +358 9 819 07 750







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: Thermal Efficiency


Today’s article continues our short series about the physics behind diesel engines. The word “efficiency” is often mentioned when looking at diesel engines. All internal combustion engines are heat engines; they convert heat energy into mechanical energy. Indeed, a thorough understanding of thermodynamics is critical when it comes to engine design. This topic is far more involved than can be covered in a simple, short blog article. In general terms, the engine’s efficiency is simply the ratio of how much of the heat produced is converted into usable mechanical energy. The formula used to determine the thermal efficiency of an engine is here:



hth = thermal efficiency

Pb = brake power [kW]

FC = fuel consumption [kg/h = (fuel consumption in L/h) x (ρ in kg/L)]

CV = calorific value of kilogram fuel [kJ/kg]

ρ = relative density of fuel [kg/L]

This formula works well, in an ideal world, where there are no limits on efficiency, and no losses. However, forces, such as friction, are always present and there will be some energy loss in any heat engine. This limitation is known as Carnot’s Theorem, named after the physicist who figured it out. It describes these limits with the following formula:

eta_{text{max}} = eta_{text{Carnot}} = 1 - frac{T_C}{T_H}

Where TC is the absolute temperature of the cold reservoir (the engine when it is cool) and TH is the temperature of the heat reservoir (the maximum engine operating temperature).

In diesel engines, the Diesel Cycle is what defines the amount of energy received from combustion. Since diesel fuel ignites after introduction into the combustion chamber as it is needed, the compression ratio of the engine further determines that engine’s efficiency.

eta_{th} = 1-frac{r^{1-gamma}(r_c^gamma - 1)}{gamma(r_c - 1)} ,

Therefore, whenever you see material or data sheets produced by engine manufacturers describing the efficiency of their engines, these terms are not merely marketing spin, but determined by the physics of the engine itself.            






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.






Engine Physics 101: Engine Displacement



This week’s articles will describe some of the basic physics as related to diesel engines. Everyone has heard the marketing terminology and spin produced by manufacturers, but do you really understand what that terminology really means? Many people don’t, so this short series will explain some of the basics.

On this blog, our website, and marketing materials, MarineDiesel often refers to the engine as a 6.5 liter, 6.6 liter, or other displacement engine.

What is engine displacement, though? Many people believe that it is related to fuel consumption or power. In marketing terms, the term “displacement” is usually used to convey power, or how large an engine is.

Engine displacement simply refers to the amount of air, by volume, displaced (moved) by all of the pistons in one complete cycle. The amount of air displaced depends on the bore (diameter of the cylinders) and stroke (distance the piston travels) of the engine block. In general (not always), the greater the volume displaced, the more powerful the engine.  As a rough guide, the greater the displacement, the larger and heavier the engine.

Here is the formula:

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

When looking at an engine purchase, it is important to look at the displacement to determine which engine to use. This is why we often ask how an engine will be used. You can have two engines sitting side by side, each producing 500 horsepower. Yet, one could be our VGT 500 engine, at 6.6 liters displacement, and the other could be a 12 liter engine. If you own a fast boat, the VGT is preferable, by far. If you own a tugboat, requiring slower speed, a different torque curve, and higher bollard pull, the 12 liter engine is preferable.