N40475 Chapter 5 Engine room ventilation The engine room must be ventilated for two reasons: 1. To supply the engine with air for combustion. 2. To provide a flow of air through the engine room to prevent an excessive temperature build up, which may cause components such as the alternator to overheat. Note: The air temperature entering the engine should not exceed 52 O C (126 O F). The air temperature entering the engine room should not exceed 60 O C (140 O F). In most applications in temperate climates, the engine will draw air from the engine room. If this is the case then, as a rough guide, it can be taken that every horsepower produced by the engines requires, as a minimum, 161 sq.
mm, (6.34 sq.ins.) of vent area. If the boat is likely to be used in hot climates, and if engine room ventilation fans are fitted, then a vent area of 322.58 sq.mm (12.7 sq.ins.) per horsepower should be provided. Wherever possible a flow of air through the engine room should be encouraged by using forward facing intake vents to take advantage of ram airflow, together with other vents to allow hot air to escape. Figure 1 Noise absorbing chambers (1) with deflector baffles and noise absorbent material (2) are positioned to direct ventilation air flow over a large surface area of absorbent material. Cross sectional area of air flow path must not be too small.
With an effective ventilation system the engine air intake temperature will be no more than 10 O C higher than the outside air temperature. Note: For the minimum cross section of air duct per engine see ‘Reference data’ at the back of this manual. The air entry vents should be situated where spray is not likely to enter them and some form of water trap is desirable (see figure 1). Preferably the air ducts should reach the engine compartment at the sides of the hull so that water will fall into the bilge. When the engines are shut down after a run at high output in high ambient temperature conditions, it will be found that very high air temperatures will build up in the engine compartment.
In boats with open cockpits this is usually of no real consequence but if the engines are mounted below a wheel house, then unpleasantly warm conditions may result. In these circumstances engine room ventilation fans are beneficial, preferably arranged to exhaust air from over the engine. Page 15