It’s Exhausting

Marine exhaust systems are actually fairly complex systems: much more complex than a layman would think. They are simply the “tailpipe” of the boat, right?

This misconception is often a problem. One critical part of the vessel exhaust system is the exhaust risers, and these can often fail. The function of the riser is two fold:

  1. They keep water from backing up into the engine.
  2. They channel cooling water to the engine.

The design of the exhaust riser is critical, and is a situation where it often is beneficial for the engine manufacturer and the shipyard or naval architect to work closely together. If the angle of the riser is incorrect, or it is modified, water can accumulate, leading to bad corrosion issues. Poor design, especially with aftermarket additions or modifications, can also create serious back pressure problems on the engine, greatly impacting engine performance.

Water and metal do not mix well. This is why the gaskets and the risers should be frequently inspected and replaced if necessary. Even in cases with no leakage, the risers should be removed and periodically cleared of any rust or scale that may have formed.

Some engine manufacturers provide risers with the engines, and other do not (they are optional with Marinediesel). In some cases, they must be custom designed in order to fit into an engine compartment. In any event, there is a golden rule with exhaust risers: NEVER use aluminium risers. Yes, they are cheap, and risers can be one of the more expensive components to buy, but they are short on life span and can cause serious problems over the life of the engine.

When does outboard power become ridiculous?

Outboard engines have their place and uses. However, a trend over the last few years has been to simply add more outboards to a hull in order to give it more power. No longer are there simply triple or quad outboard installations, but sometimes even five, six or more.

At what point does simply adding more outboards become pointless?

When the costs outweigh the benefits.

The vessel pictured is a pretty well-known photograph of a drug-runner caught in the English Channel (a “scandal” in the UK marine industry at the time). It essentially is engine and fuel tank. Now, most people are not buying boats for smuggling, but are intending to use the vessel over an extended period of time. What does adding extra engines actually do as far as performance?


  1. Added weight (not just the engine, but all equipment, such as mounts and the weight of extra fuel).
  2. Added drag.
  3. Reduced maneuverability.
  4. Reduced safety (often, the hull was not designed for such applications)
  5. Huge increase in fuel costs.
  6. Reduced running time and range (unless larger fuel tanks were designed or installed)
  7. Greatly increased maintenance expense
  8. Difficulty in ventilating the props.



  1. Greater power.
  2. Greater acceleration.


Note that the negatives are pretty big, compared to the positives and their associated costs. Of course, these negatives can be minimized by using diesel inboard engines.

Should a vessel be equipped with two 250 hp outboards, or a single VGT 500? What about instead of three 300 hp Mercury Verados, two VGT 450s?

Unless the vessel has absolutely no room for an inboard, the inboard option will win every time.

  1. Weight: Two Mercury Verados weigh 586 kg. A single VGT 500 weighs 515 kg. Even with gearbox and drive, the weight differential is only around 100 kg.
  2. Drag: Fewer engines mean less drag. All of the time.
  3. Maneuverability: Even numbers are more maneuverable, due to less torsional pull. However, the reduced drag and greater number of propulsion options mean that a single inboard will be as good as or more maneuverable than multiple outboards.
  4. Fuel: No contest. A single diesel VGT will save enough money to pay for the differential in price very, very quickly.
  5. Range: Likewise, the range can be longer, with smaller fuel tanks.
  6. Maintenance: Though an inboard diesel, depending on engine compartment and configuration, is less accessible, it will have a far higher life cycle and require less maintenance and access.
  7. Acceleration: Multiple outboards can accellerate very quickly. But guess what. A Marinediesel VGT engine, designed for high speed craft, will accelerate faster.
  8. Service life: A VGT engine will last as much as five times longer than even the highest quality outboards.

MarineDiesel Services Profile: Engineering



We at MarineDiesel often refer to ourself as an engineering company, rather than as a manufacturer. The reason we do so is that a substantial portion of our annual revenues come from engineering services, rather than engine sales.

As a diesel engine maker, we are required to invest in a substantial level of infrastructure that is highly specialized. This infrastructure is used to customize engines and design engines for specific purposes, which makes MarineDiesel ideally suited to offer complete engineering services to our customers, whether they use our engines or not. Indeed, we consider several of our competitors also to be customers, since we have completed many such projects for them. This is a primary selling point for MarineDiesel: We can usually accomplish things both quicker and cheaper than our much larger competition cannot.

These services include, but are not limited to:

  • Torsional vibration analysis
  • Flow simulation
  • Engine Testing
  • Emissions Testing
  • Compliance Testing (eg. NATO or Intrinsically Safe)

As long as our facilities allow it, in terms of size (We do not have the facilities to test two stroke, 2 mW engines, for example), we can offer customers complete turnkey engineering solutions. Our facilities are state-of-the-art, designed by GM. We have the latest test cells, software, and equipment.

MD ENGINEERING hosts three engine dynos in its test facility, with each test cell having its own unique measurement range and options. This provides us a great span in terms of engine size and application to better meet our client’s requirements and specifications. All cells are equipped with high frequency, multi-channel data logging.

MD Engineering has been involved in complete engine control systems in a multitude of areas, some areas where we have deep knowledge and long experience:

• Control safety systems
• Torque management and control
• Air system diagnostics (MAF, EGR, Swirl)
• EOBD and OBDII controls and diagnostics
• Engine thermal management control and diagnostics
• After treatment systems control and diagnostics
• Stop/start control
• Boost system control for a wide variety of hardware setups:
• Single wastgate turbo and by-pass turbo
• Two-stage turbocharging
• Supercharger
• Twin turbo
• VNT turbo

Our engineers and product analysis experts can take a wide client brief for comparative benchmark and cross product comparison. This includes all aspects of the process from virtual analysis, desktop studies, static and dynamic repeatable test scenarios to full product teardown, measurement reporting and recommendations.

MarineDiesel, through the normal course of business, maintains partnerships with numerous naval architects, engineering firms, automobile manufacturers, other engine manufacturers, and designers. In fact, we are often the ones providing the project management services on complex projects.

To illustrate, we recently were involved in a project where we provided the engines for a new boat, along with the design, and handled the entire contracting process with the shipyard.

MD Engineering is a division of MD Group that comprises of three divisions with fields ranging from engineering to powertrain supply and marine propulsion engines.

For your next engineering project, please contact us today with the project details for a complete proposal.









Vessel Performance and Load



“Load” is one of those terms that gets thrown around in the marine industry, and the meaning is often blurred. What, exactly, is meant buy load, and how does it impact vessel performance.

Load can refer to either:

  • Engine load
  • Vessel load (tied to displacement)

These two things are also tied together.

Most high speed vessel are designed with a planning hull. In other words, the hull is designed to lift itself out of the water when power is introduced. It is a question of physics and opposing forces. In absolute simplest terms, the hull is experiencing:

A force downwards. This is displacement, or the weight of the boat. If more weight is added, the greater the force necessary to lift the boat.

A force backwards. This is resistance. As the propulsion system pushes forwards, the resistance increases. Additionally, anything sticking in the water, like the drive, rudder, propeller, or spray rails, produces drag, which increases resistance.

A force upwards. This is buoyancy.

A force forward. This is propulsion.

There are other forces and impacts on performance, such as trim of drives, wind, intakes, etc. The above are the most important.

So, as is evident, vessel performance is a question of balance. In order to make the boat move forward and up, enough power needs to be provided to overcome these forces. There is no magic or voodoo involved. It is simply a question of physics. Depending on the extent of the problem, a solution may or may not be able to be found.

So, on vessels with performance problems, the first diagnostic is usually weight. More weight means more displacement means the engine must work harder to make the boat plane. This is engine load. This is also why naval architects often pull their hair out at design changes. Any additional weight will impact performance. Put a marble floor in a recreational yacht? Performance will suffer. Extra guns and ammunition on a military boat? Performance will suffer. Add 10 additional passengers on a water taxi? Performance will suffer.

Load on the engine. Load on the hull. In both cases, speed, maneuverability, and overall performance take a hit.






Vessel Performance Problems – Who’s to Blame?



Designing high performance vessels is both an art and a science. Performance is one of those terms that can be defined in many different ways. However, when dealing with vessel design, performance is measured in terms of a customer’s expectations when a boat is designed. On recreational boats, built by large yards as a production run or as a specific model, performance means a standard that can be advertised to potential buyers. On military, commercial, or government service boats, performance is strictly defined in terms of expected speed, vessel load, and vessel use.

So, what happens when a boat is built and it does not perform up to expectations?

Enter the blame game.

  • The yard will blame the engine manufacturer.
  • The engine manufacturer will blame the propeller manufacturer.
  • The propeller manufacturer will blame the yard.
  • The yard will blame the naval architect.
  • The naval architect will then say that the customer’s demands were not realistic.

A cycle that is common, and usually is completely unproductive when solving the problem: WHY is the boat not performing?

Vessel design is a question of balance and the laws of physics. Numbers do not lie, and the laws of physics apply to everyone. What happened?

In our experience, the vast majority of performance issues are related to lack of communication between all of the different manufacturers of the vessel systems. In general, the following items are performance critical:

Vessel Weight / Displacement. This is the most common problem. It is either calculated incorrectly, or different materials were used than specified in the design (usually for cost reasons). Sometimes, it is not the yard’s fault. The vessel owner will often make changes after construction was started, sometimes ignoring the advice of the yard. This will put the builder in a delicate position of keeping the customer happy or achieving performance. It is not just with recreational vessels, either. MarineDiesel has seen many projects fail when a military commander decides to change the weaponry or vessel mission without any understanding of the physics involved, sometimes to comical results.

Bad Propeller / Propulsion Design. This is also not always the fault of the manufacturer. Propeller makers (or drive makers, or jet makers) can only make calculations based on the information that they are given. If they are given incorrect information, then the vessel performance will not be correct.

Bad Engine Selection. Engines cost money. Power costs money. As an engine manufacturer, the guarantees we make are based on the power of the engine we provide. So, if you buy 500 horsepower, we guarantee that the engine you buy will produce 500 horsepower. We do not make guarantees based on a project’s performance. Why not? How can we guarantee a performance level when we did not design or manufacture the hull, propeller, or drive? In some cases, a customer will try and save money by buying an engine that is under-powered for an application in order to save money. This returns us back to the laws of physics. There is no such thing as a free lunch.

Bad Hull Design. This is also quite common, especially on new, or prototype, hulls. On high speed or high performance vessels, design mistakes are often magnified, due to the increased forces on the hull at high speeds. We have seen such mistakes in water intakes, spray rails, steps (especially), foils, and simply bad designs. This is also why many yards insist on having designs that are proven, with other customers using the vessel.

It has been our experience that when vessels do not reach their required performance levels, it is usually a case of “all of the above” to varying degrees. It is important for vessel buyers to understand what they are buying and for everyone to be upfront and honest with all information. Unfortunately, in the competitive marketplace, costs are often the driving criterion on projects, and the ever present desire to reduce costs oftem is the real culprit, leading to bad decisions and poor engineering.


Best of 2014 – Engine Room Ventilation



We wish all MarineDiesel customers a happy holiday season. Our factory will close from December 22 through January 5. For the balance of the year, we will be re-running our most popular articles from 2014, based on the number of visitors. We will start new daily articles in the New Year. We hope that you continue to find them interesting.

MarineDiesel designs its’engines with reliability and service life being key concerns. Using a Duramax block as a foundation for our VGT Series of engines, the product is reliable and trouble-free as long as regular maintenance is performed when due.

There are, however, two situations that can greatly reduce engine life. The first is the use of dirty fuel. The second is inadequate ventilation of the engine compartment.

This situation is most prevalent in tropical, or hot, climates.

All engines produce a tremendous amount of heat. That is how they operate and why they produce power. In order to operate continuously, they must be adequately cooled, with ample ventilation provided for continued operation.

This is where problems can arise. The VGT Series, in particular, being so compact, is often used in very small craft, such as RHIBs, that have very small engine compartments as part of their design. Small, tight, engine compartments tend to lack much ventilation, and therefore ventilation must be provided in order to ensure trouble free operation.

From MDS, our service team:

Engine power is affected by a number of different external factors. Among the most important are air pressure and volume, air temperature and exhaust backpressure. Deviations from the normal values affect engine performance, function and reliability.
Diesel engines require a large amount of air compared to petrol engines. Reductions from the required values show up first of all as an increase in exhaust black smoke. This can be particularly noticeable at the planing threshold when the engine torque demands are high. If the deviations from the required values are great, the engine will lose power. This power loss can
be so great that a planing boat cannot pass through the planing threshold. For the engine to function properly and give full power, it is absolutely essential that both the inlet and outlet air ducts are sufficiently dimensioned and installed correctly.

Two main conditions must be fulfilled.

1. The engine must get enough air (oxygen) to allow efficient combustion.
2. The engine room must be ventilated so that the temperature can be kept down to an acceptable level.

Ventilation is also important to keep the engine’s electrical equipment and fuel system temperature at an acceptable level and for general cooling of engine components.

Basic design.

Engine space ventilation should be considered at an early stage and well before the engine is installed as it is often has to be integrated into the boat structure. Guidelines for air intake area are provided in the installation data and we have provided basic formulae in this section if you wish to calculate your own. Air intake area should never be underspecified, it is always better to have too much than too little. Intake air should always be directed to the bottom of the space and exhausted at the highest part preferably on the opposite diagonal to promote good circulation and natural convection.

There are two schools of thought concerning engine space ventilation, that of the engine manufacturer and that of the boat builder. Most engine manufacturers recommend forcing air into the engine space to provide positive pressure to ensure adequate air supply and ventilation for the engine. Boat builders on the other hand tend to favour extracting air from the engine space to provide a small negative pressure, this can prevent engine odours and fumes entering the passenger compartment through cable and hose ducting, etc.
Either system can be used for MarineDiesel engines but we prefer forcing air into the engine space and having properly sealed engine rooms to prevent odours and fumes. If air is to be drawn out using a fan then we recommend adding the CFM of the fan to that of the engine when working out your air intake area.

Engine room depression.

The maximum engine room depression is 0.5 kPa at full speed, this should be checked in every circumstance irrespective of the type ventilation system used.

Dimension of air intakes and ducts.

The engine itself sucks in air very effectively and naturally will take in air from any direction. Should the inlet or outlet air ducts be under dimensioned, the engine will consequently suck air from both ducts and no ventilation air will go out through the outlet air ducts. This causes dangerously high engine room temperatures and potential engine damage. Most of the radiant heat from the engine must be transported out of the engine room. This is an absolute requirement to keep the engine room temperature below the permitted maximum limit.

Engine room temperature.

Remembering that the engine’s performance figures apply at a test temperature of +25°C, it is important that the inlet air temperature is kept as low as possible. The temperature of the inlet air at the air filters should not be higher than +25 °C for full power output.

There is always a loss of power with increased temperatures and if the engine’s inlet air is constantly above +45°C the engine ECM will de-rate the engine as a safety measure. During sea trials the air temperature in the air filter should not exceed 20 °C above ambient temperature or 45°C maximum.

Location of air ducts.

Air intakes should be located where there is a clean flow of air and away from low pressure zones of the boat structure. They should be designed in such a way as not to allow water ingress into the engine space and provide a dry air supply for the engine(s). Care should be exercised with multiple engine installations to ensure air is delivered effectively to all the engines. If louvers are used, the air inlets should be louvered forward and the air outlets louvered towards the stern, this will encourage ventilation on naturally vented systems. Blowers and/or extractors can also be incorporated if deemed necessary. The channels or ducts for the engine air supply should be routed up as close as possible to the air filters but with a minimum distance of 20–30 cm (8–12″) as a precaution should water enter them.

All channels and ducts must be routed so that the least possible flow resistance is obtained. The bends must not be sharp but softly rounded. The smallest radius should be equal to the internal area. Restrictions must always be avoided.
The ducts should be cut obliquely at the ends to assist flow.


Air intakes or outlet holes must never be installed in the transom. The air in this area is turbulent and usually a mix of water and exhaust fumes and must therefore never be allowed to enter the engine or boat.

Function of air intakes.

Air intakes and outlets must function well even in bad weather and must therefore have efficient water traps. Soundproofing must usually be built in. The air intake and outlet should be placed as far away from each other as possible so that a good
through-flow is obtained. If the intake and outlet are too close, the air can re circulate resulting in poor ventilation.

Engine’s air consumption.

The engine consumes a certain amount of air in the combustion process. This requires a minimum internal area of air supply ducting, the minimum area can be calculated by using this formula.

A = 1.9 × engine power output in Kw
A = Area in cm²

The area of the outlet ventilation ducting can be calculated to be a minimum of a third of the air intake ducting area. The value applies for non-restricted intake and up to 1m (3.3 ft) duct length with only one 90 degree bend. The bending radius should be at least twice the internal area. If longer ducts or more bends are used, the area is corrected by multiplying a coefficient from Table
1 below.

eng vent1

Ambient temperature.

The ambient air temperature, (outdoor air temperature) is assumed to be +30°C (86°F). Correction factors as per Table 2 below should be applied as required by multiplying the calculated area by the correction factor.

eng vent2

A - Air should exit the engine bay and the upper section B – Air should enter the engine bay at the lower section

A – Air should exit the engine bay and the upper section
B – Air should enter the engine bay at the lower section




5 vessel power mistakes that kill performance



When designing a vessel, the choice of the proper engine and propulsion system is the most critical aspect of vessel performance. Choose right, and you have a finished vessel that can complete its’ intended mission. Choose wrong, and you have a whole slew of performance problems and failed missions. The following list is comprised of situations MarineDiesel has encountered, normally after a problem already exists and we have been contacted to solve the issue:

1. WEIGHT: This is, perhaps, the most common, and critical, issue that we encounter. Naval architects and shipyards often have the habit of under-estimating the finished vessel weight when the vessel is being designed (VERY seldom is the actual vessel weight lighter than predicted). Predicting weight is not an exact science. There are many variables and situations that occur. Often, it is the vessel owner who demands changes to a design after it has been designed, and the yard builds what the customer wants. Note that on a 10m boat, a difference of 1,000 kg can have an impact of 3 or 4 knots (or more) of speed, so the question is not merely academic. As a manufacturer, we can sometimes adjust the engine ECU to compensate, but we are limited by the Laws of Physics by what we can do, in many cases.


2. Inadequate Ventilation: Proper engine ventilation is critical to heat dispersal and engine life. Due to space constraints, this aspect of vessel design is often overlooked. The design of louvers and vents also falls under this category.


3. Looking at Horsepower, rather than Torque: Sometimes, it is a simple mismatch. Most engine dealers are forthright, but sometimes, the pressure to make a sale leads to poor engine matches.


4. LCG: The LCG of the vessel is impacted by engine weight, but also hull design. On every vessel, there is a “sweet spot”: The location that is a precision location for LCG. Poor planning can result, with an impact on speed. This is also related to vessel trim and propulsion choice.


Under powering due to price: Performance costs money. We usually see this more in the recreational market. Vessel owners try to get performance from smaller, cheaper engines: This almost never delivers the desired performance. Additionally, expectations may not be a good match with reality.



Bespoke Engine Services



Bespoke engine services are certainly a specialty of MarineDiesel. We can design, build, and customize virtually any engine in virtually any application, marine or industrial. Of course, through our partnership with GM, we have a wide range of blocks and engine types to provide us a basis from which to start. Additionally, we have connections in the engine market to purchase a wide variety of engine blocks (sometimes even from competitors) to modify for specific customer applications.

Engineering is really our specialty. We have designed power solutions for many different applications, often where an “off the shelf” engine does not exist. Some examples of previous applications we have designed:

  1. UAVs
  2. USVs
  3. Tanks
  4. Submarines
  5. Alternative, or Dual Fuel
  6. Cranes
  7. Prototype Boats
  8. Prototype Automobiles
  9. Customized Gensets
  10. Trains
  11. Off Road Vehicles

Our dedicated engineering team in Sweden can customize just about any project.

Contact us today for more information.





There are always trade-offs



The economist Milton Friedman was fond of using the cliche, “There’s no such thing as a free lunch” when discussing economics. When deciding on powering a vessel or industrial application, there are always trade-offs involved. There really is no such thing as a free lunch.

When choosing how to power a vessel, pump, vehicle, or generator, the first step in providing the appropriate power source is listing what the important functions should be:

  • Is weight critical?
  • Is reliability most important?
  • Is fuel economy most important?
  • Is reliability most important?
  • Is the availability of parts and service most important?
  • Is torque most important?

The questions go on and on.


Just like Friedman’s free lunch is bound by economic law, power applications are bound by the laws of physics. They are unalterable.

If you are designing a boat that will be used as a tug boat, MarineDiesel’s VGT Series is not a good match, even if compared to a similar, say 500 hp engine. Why? The VGT Series is designed for high speed operation, and Bollard Pull is not one of the VGT’s benefits. The engine was not designed to operate under that load. Were you to use it in that application, your service life would decrease sharply due to overload. Likewise, a 12 litre engine would power a small tugboat adequately, but would be far too large and heavy to operate on a light, high speed craft. The trade-off? Pulling force is sacrificed for weight on the VGT.

To further illustrate, suppose the engine is being used to power a RHIB with a tiny engine compartment. What happens if you install an engine and all of the major components are inaccessible? Your service costs will certainly go up the first time a water pump needs servicing. The VGT was designed with ease of service in mind in small engine compartments. A few thousand EURO savings on purchase may not be such a good deal when you need to pay many thousands of Euro in cutting an engine out of a hull in order to replace that pump.

Performance is one of those terms that is thrown around rather carelessly in the engine business. Performance really is a single term that asks, “Does this engine do what I need it to do?” There is no right or wrong answer to this question. MarineDiesel designed our VGT engines to be very flexible in performance. That is why we use the programmable NIRA ECUs, for instance.

However, though we can program the engine electronics, there is always a trade-off between benefits. You can never have it all. Want more high-end torque? OK. We can do that. But your service schedule and life cycle will change, and your fuel consumption will increase. Want longer service life? We can do that, too. But your top speed will suffer.

It is all a question of trade-offs, rather than simply price.





With Boat Design, You Cannot Ignore Physics



The graphic above lists the forces experienced by a planning hull. When we are asked to provide engines or a complete propulsion system, these are the forces that we need to take into account. As engine manufacturers, we are obligated to recommend a specific engine for a project. Price or the amount of a sale never enters into these discussion. Our focus is on providing ample power to deliver a required performance level on a specific vessel. If one of our products is a good solution, then we recommend it. If not, we would rather pass on a project than deliver a customer something that will not deliver the proper performance level.

What the picture above illustrates is basic physics. For each force shown, there must be a minimum of force pushing against it in order to make the boat move through the water. We cannot provide something that violates these physical laws: That is magic and not engineering.

Vessel performance is a question of balance. For instance, if displacement is increased, we must either increase buoyancy, provide more power either through engine or propulsion, or shift the LCG of the vessel. Sheer horsepower is a variable of the equation, but not the only variable. Though the equations can get complex, here is the “quick and dirty” version:

Speed =( Horsepower per engine*number of engine)^.551/(Displacement )^.476 *2.74


By using what is known as the speed length ratio, or Froude Number:

text{Speed Length Ratio} =frac{v}{sqrt {text{LWL}} }


v = speed in knots
LWL = length of waterline in feet