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Old 09-06-2017, 03:14 AM   #21
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Originally Posted by markpierce View Post
Phew on turbocharged engines. I trawler needs no more power than to reach hull speed. Eighty horsepower is sufficient for 40-foot plus. If engine-compartment inlets are insufficient to supply air to the engine, that boat is wrong.

Forced air intake could blow hot engine-room air into living quarters.
As above my last post it's not about what the engine requires it's about all the other ancillary requirements (did you read the post?)as listed in 1,2,3

Again the correct system uses intake and EXHAUST fans so no air is ever forced into living spaces (quite the opposite actually)

Newer manufacturers recognize this importance where older vessels and there builders did not place enough emphasis on these issues, Engine rooms with out a correctly installed ventilation systems tend to have more issues with ancillary equipment as a part of being to hot (common sense)

Cheers Steve
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Old 09-06-2017, 07:01 AM   #22
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Engine room blowers for these types of boats are simply not needed. I have designed propulsion installs from around 27hp up to around 2800hp and also troubleshot boats where it was claimed the ventilation system was inadequate. In all cases it was a non-issue regarding air inlet temperature to turbo compressor suction, which is where it matters.

The amount of air getting drawn in on the larger engines is tremendous, and actually suction temperature drops quickly to near ambient once up to power. You would need like 5hp fans to move enough air to make any difference, and it is simply not needed.

At low power engine room temps do climb, but at low power settings the engine is not affected by the higher temps. Ancillary equipment (pumps, alternators, generators) generally are designed for these temps and when measured in worst case temps, they were below mfr limits.

The one situation where engine room temps often become an issue is running the generator after a main engine hot shutdown. The gen end is air cooled and the gen engine simply does not use enough air to draw in much cool air. In those cases we did see some excessive temps and then forced ventilation did matter. But it could be a relatively small fan as the added heat rate was low.

On small non turbo engines running trawler service, those are simply not sensitive to elevated intake temps. In that duty cycle, there is so much excess air in the process that the density change from going from say 80F to 120F does not matter much. Note that the density change must be considered in absolute terms (deg Rankine) so the density difference is 80+460/120+460 or 540/580=0.93 or a loss of 7% of available oxygen. If no soot out the tailpipe, not a problem.
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Old 09-06-2017, 07:38 AM   #23
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My final word (I Promise) you would be surprised at how may insurance claims i attend for engine room equipment failures and how many are thrown out on THE GROUNDS of not meeting the manufacturers ambient operating temperatures,

I stress again it's not ONLY about the Turbo or not requirement, Engine-room ventilation falls into two categories, those required for cooling the space, and those needed for combustion. The air used to cool the compartment improves not only the combustion efficiency, but also the longevity and efficiency of gear in this space, including engines, generators, compressors, and electrical and electronic components (and also makes it a more comfortable place to work). Where generators are concerned, while the engine portion is typically liquid cooled, the electrical generation end relies on air from the engine-room for its cooling (a small number of generators rely on liquid cooled stators). When engine-room temperature is excessive, generator manufacturers will often lower the unit’s rated output.

Additionally, heat takes a toll on “soft goods” such as generator couplings, belts, motor mounts,torsional vibration dampers, transmission damper plates, hoses, insulation, voltage regulators, and other electrical and electronic components, as well as batteries.

As a general rule of thumb, (these apply to wet exhaust engines) to determine the necessary air flow rate, in cubic feet per minute, use this formula (2.75 x hp) – 90. Having said that, it’s wise to defer to requirements established by engine manufacturers, or whichever is greater.

Caterpillar’s standard guidelines for forced ventilation call for 2.5 ft3 (.07m3), per horsepower, while one generic rule of thumb for passive combustion air inlet surface area specifies 0.55 in3 (9cm3) per horsepower (including generators). The Cummins guideline for passive vent surface area size uses the following formula, rated engine air intake, specific to each model, in cubic feet per minute, multiplied by 0.272, equals required area in square inches. Failure to follow these formulas is a common violation for new builders, particularly where designs evolve to accommodate larger engines with no changes made to intake size. It’s important to note that the cross section must be measured at its narrowest restriction (and louver area must be subtracted); it’s not unusual to find large louvers that supply air via significantly constricted duct work.


Due to the nature of my position as a (MIIMS) Lloyd's Maritime surveyor -(NZ Chief End 1)specializing in engineering installations/refits, I get to see a host of so called "Professional Installations" that dont meet manufacturers specifications and have spent CONSIDERABLE TIME in courts as an "Expert Witness" facing well known international boat builders out lining there failures and the belief in there systems.

"You can lead a horse to water but you cant make it drink"

Cheers Steve
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Old 09-06-2017, 10:07 AM   #24
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Well explained and in total agreement,. When I did my e.r. my 2 x120hp lehmans needed some 240cfm each, so I installed 2x400cfm blowing the outside ambient air at 85-90f at the intakes. (low down in bilges) Performance improved , more power at lower rpm, saving fuel etc. I also put 2x650 cfm extraction fans to achieve neg e.r. pressure. Actually impossible the E.R. main deck door is always open to allow lots of airflow. I got my E.R. temp from the 140f to operating around 110F. Heres a interesting website for engine CFMs maybe this will peak some of the other members interests about improving there E.R. efficiency.
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