Yanmar Pleasure Boat Marine Engine Help

Ventilation and Space

Once upon a time, a long, long time ago, I was asked to check on a twin engine installation that wouldn't give full power. As I got on the boat I saw black lines along the white, shag pile ( I said it was long ago didnt I? ) carpet. We cast off and took the vessel for a run so the owner could show me how his twin 3208 Cat's weren't performing. At full throttle I went and heaved open the engine room door. The owner was tossed back onto the steering station seat as the vessel suddenly accelerated and the fact I was hanging on to the door handle kept me from being flung into the engine room among the rotating machinery. There was a burning smell, which on closer inspection turned out to be the air cooled turbochargers setting fire to the wooden beams running over the top of them. They were so close that the installer had cut away the beams to get the engines to fit. The black lines on the carpet were dust, caused by the two 265hp 'vacuum cleaners' sucking air through the gaps in the removable hatches that made up the saloon floor!
Air is needed for the engine to work ( by weight, approximately 15kg of air to 1 kg of fuel ) and for cooling, if you dont want to swim home one horrible day, make sure there is enough space between the engine and the nearest parts of the boat, especially if they're hot!
Another reason for having adequate space is that the owner never expects the engine to be removed to do something simple, like change the water pump impeller or set the tappets. I have seen this problem far too many times and it is always the owner, not the designer, who pays!
The maximum temperature of the engine room should never exceed 60 deg C within 20mm of any electrical equipment and 45degC at the air intake.
With high performance engines, while doing a seatrial, we could measure the engine room temperature. If the temperature dropped considerably we would know the air supply was too small. The great increase in air velocity through the vents would lower the temperature in the engine room, too great a drop and we would re-measure the air intake duct area as it was usually smaller than the minimum allowed.

Ventilation and Space

Vents: two vents are needed, one in and one out. When the intakes are on the side, a 'dorade' box needs to be built so that any water goes into the scuppers or overboard, not the engine room. There is no excuse for water in the engine room except via a stern gland that is supposed to drip while running. (OK, I accept something broke...)
Any scoop on the exterior should face forward and the exit scoop should face aft. If possible install any exterior engine room vents on the cabin topsides. If you look at TeWhara III you can see the port engine room vent on the topsides, just aft of the saloon window. The duct goes down to the bilge so that if you do ship a 'green' one the water will go into the bilge, not down the engine air intake. Dont connect the duct to the engine air intake filter!
On smaller vessels the engine room vents can be put into the side lockers or in the companion way ladder, soundproofing the dorade box keeps the noise at bay. Make sure the locker, etc. is also adequately vented to the outside.
For those of you who kept telling me the limber holes ( drain holes through the ribs ) were all that were needed for an air supply, the team are ready to overhaul your engine anytime, just front with the cash!

What size duct do I need? ( x 2 )

The table below is for vessels without forward facing intake vents. For vessels with forward facing air intakes that exceed 19 knots, the duct area can be reduced, depending on the boat speed. Tip: multiply cm² by 0.155 to get inches²

Note: TWO ducts the same size are required, one inlet and one outlet.

100mm inside diameter tube duct area is 78.54 cm² ~ 50mm inside diameter tube duct area is 19.7 cm²
4 inch inside diameter tube duct area is 12.57 inches² ~ 2 inch inside diameter tube duct area is 3.15 inches²

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