Engine Room Ventilation



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