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Old 31-01-2021, 18:56   #46
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Re: How much ventilation for diesel?

Odd how it took so long for this thread to get past just the need to feed the engine intake. Diesels require cooling air as well. I have attached Volvo's requirements from the installation manual. output.pdf

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Old 31-01-2021, 19:16   #47
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Re: How much ventilation for diesel?

Years ago I attended a Mack Boring Yanmar boat owner's diesel engine class. The instructor - who had been with Mack Boring since a young man told a story of when he was in his 20's starting out in the 1970's

One day his boss told him that a celebrity Yanmar owner - my memory is hazy but it may have been Art Carney (my apologies if I am wrong) was having trouble with his Yanmar engine. He was dispatched to the marina to find Mr. Carney onboard with three beautiful Hollywood starlets - who soon took off all their clothes.

Trying not to be distracted by the young ladies, he opened the engine hatches and had Mr. Carney start the engine. Ran perfectly. So they motored out of the harbor with the hatches still open - engines still running perfectly. After half an hour he was stumped and told Mr. Carney to head back to the dock - closing the engine hatches. Within a few minutes the engines died. He rushed to open the hatches and there was so much suction he couldn't open them.

He then went below and found that all of the engine vents had been blocked. He said "Mr. Carney - why are these vents covered" The answer "The girls didn't like the noise".

So one practical solution for the OP would be to cover the vents and see if the engine gains power as he remove the covers. I believe the starlets are optional.
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Old 01-02-2021, 19:02   #48
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Re: How much ventilation for diesel?

Funny thing is if I run my engine with the engine room door open it runs hotter than when closed I can only presume it sucks in all the hot air out of the engine room then starts sucking it out of the bilge ! I have no engine room vents only gaps from the bilge !
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Old 01-02-2021, 20:08   #49
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Re: How much ventilation for diesel?

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Originally Posted by Emmalina View Post
Funny thing is if I run my engine with the engine room door open it runs hotter than when closed I can only presume it sucks in all the hot air out of the engine room then starts sucking it out of the bilge ! I have no engine room vents only gaps from the bilge !

Have you got 3 naked starlets tho?
Sorry I couldnt resist
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Old 05-02-2021, 10:39   #50
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Re: How much ventilation for diesel?

I think the bigger issue is keeping a good airflow through the engine compartment for cooling and removal of fumes. A car or truck engine is virtually in the open air and we know how hot and smelly they get. Imagine that sealed up in a box! My 140A alternator quickly overheats when working hard unless it is in a good airflow. So I have outside air blowing into the back of the alternator and an extractor taking the hot air out of the top of the engine compartment. I probably have about 3000L/min blown into the engine compartment, 2000L/min sucked out by the engine and the other 1000L/min sucked out by the extractor. It keeps everything cool and sweet.
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Old 05-02-2021, 11:12   #51
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Re: How much ventilation for diesel?

For what it is worth, I have had blowers with vents on 40hp diesel engine compartments. I believe it was due to regulations for gas engines that flowed over to diesel. I did not ever use them. The engines have run fine with only the air flow from the floor boards for over 2000 hours.
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Old 05-02-2021, 11:24   #52
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Re: How much ventilation for diesel?

Indeed, I guess ventilation must be much more important for gasoline engines.
Whether you need blowers presumably depends to some extent to how much air 'leaks' into your engine compartment. My diesel certainly needs a good draught through, specially in hotter climates.
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Old 05-02-2021, 12:25   #53
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Re: How much ventilation for diesel?

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The original Herreshoff Meadow Larks had gas engines. The Vaitses Meadow Larks all seem to have been one-offs, with very little standardization. Curlew may well have had gas engines to start.

What I'm thinking I might do, one I have her in the water, is to seal the vents, run the diesel with a cockpit lockers open, then close the locker and see what happens.

It may be that there is no real need for vents.

My prior Vaites '87 Meadowlark 37 had interesting vents on the the cockpit comings in the cast bronze winch base, which had screw-down caps over the ~4" vents in the base. The rounded, smooth outside of the bases made a good turning point for hauling up the leeboards, with a jam cleat termination, and freeing the jib-sheet winches/cleats. Westerbeake 25 4-cyl engine.
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Old 05-02-2021, 16:06   #54
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Re: How much ventilation for diesel?

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Sorry old mate, I have to disagree with you on this one, The air capacity of any engine is dependant on the bore x stroke x rpm. Any two engines of a comparable size will draw similar volumes when run at similar revs, sure a diesel has approx double the compression rate, this is a function of piston crown height to cylinder head. Running higher compression ratios possibly increases internal losses down the bore than a lower compression engine, and coupled with the fact that diesels run slower than petrol engines, it is probable that the diesel in fact uses less air than the petrol equivalent.
The exception would be if the diesel was a turbo model.
Absolutely correct. Intake is derived form piston vacuum (unless forced with turbo/blowers). Air requirement = Engine Cubic capacity x max RPM. Exhaust flow is the intake plus thermal volume expansion so the intake requirement is less than the exhaust. I’d work on a minimal sectional area half the exhaust elbow.
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Old 05-02-2021, 21:29   #55
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Re: How much ventilation for diesel?

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Originally Posted by PineyWoodsPete View Post
My prior Vaites '87 Meadowlark 37 had interesting vents on the the cockpit comings in the cast bronze winch base, which had screw-down caps over the ~4" vents in the base.
Mine was built in '78, and the cockpit coaming is different. Molded fiberglass about 2-1/2 inches across, and the vents are on the outside.

Still too narrow to mount a winch. The boat only came with one, on the coach roof.

I've been musing about how to install winches for the sheets, despite the prior owners saying he'd never missed them.

And I've considered building some kind of platform, over where the vents are, that would combine acting as a base for a winch and a baffled vent.

What you have looks accomplishes the same thing.
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Old 05-02-2021, 22:11   #56
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Re: How much ventilation for diesel?

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Originally Posted by Uncle Bob View Post
Sorry old mate, I have to disagree with you on this one, The air capacity of any engine is dependant on the bore x stroke x rpm. Any two engines of a comparable size will draw similar volumes when run at similar revs, sure a diesel has approx double the compression rate, this is a function of piston crown height to cylinder head. Running higher compression ratios possibly increases internal losses down the bore than a lower compression engine, and coupled with the fact that diesels run slower than petrol engines, it is probable that the diesel in fact uses less air than the petrol equivalent.
The exception would be if the diesel was a turbo model.
I 2nd this ... with the caveat that intake valve duration has an impact. Diesel camshaft lift and duration tends to be less than gasoline engines (esp performance engines) which constricts air intake on a diesel during the intake stroke more than a gasoline engine. Excluding the fact that diesel has 15% more power per unit volume than gas, HP of the engine is not material as the ideal stoich ratio of diesel (14.5:1) is similar to gasoline (14.7:1) so for a given amount of fuel squirted into the cylinder, both engines require roughly the same amount of air for efficient combustion.
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Old 06-02-2021, 00:35   #57
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Re: How much ventilation for diesel?

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Originally Posted by Compass790 View Post
Depends if his car is petrol or diesel.
Diesels require roughly twice as much air as the same cc petrol engine, the clue is in the compression ratio.
Not true. A petrol and a duesel of the same displacement will use the same amount of air per cycle. The diesel will just compress it much more.
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Old 06-02-2021, 09:35   #58
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Re: How much ventilation for diesel?

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I guess that technically, yes, however the simple reality is that an engine running at x rpm will consume y cfm of free air.
Okay ... just what does RPM have to do with volume?
So it moves more cfm in a given time frame, NOT more cfm per stroke.
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Old 06-02-2021, 12:38   #59
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Re: How much ventilation for diesel?

I have that engine in my Colvic 27, no external vents - the engine space is open to the boat space. Via the bilges and through space on one side of the cabin aft across the battery space.
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Old 06-02-2021, 13:58   #60
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Re: How much ventilation for diesel?

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Originally Posted by Jdege View Post
Mine was built in '78, and the cockpit coaming is different. Molded fiberglass about 2-1/2 inches across, and the vents are on the outside.

Still too narrow to mount a winch. The boat only came with one, on the coach roof. I've been musing about how to install winches for the sheets, despite the prior owners saying he'd never missed them.

And I've considered building some kind of platform, over where the vents are, that would combine acting as a base for a winch and a baffled vent.

What you have looks accomplishes the same thing.

That sounds like a solution.

We found the small non-overlapping jib w/boom and traveler too small and clobbered up the foredeck - replaced with a stock full-hoist Catalina 25 150% genny on a CDI Flexible furler - great improvement in performance, with about a 125% overlap, but a bit more crew work with the leeboards, and the winches def. needed for the headsail.
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