Engine Ventilation

I have looked for archive material on this subject and found none sufficient.
This is not for venting gas bilge fumes.

Having just read 'High Speed Motor Boats' (John Teale), I am informed my twin 229 cu.in V6's require 398 cu. ft. per minute intake air at only 1500 rpm!
Well, my inlet vent is a plastic gunwale manifold with just two 3" diameter inlet holes!
According to Mr Teales calculation - assuming 100 lineal feet per minute through natural ventilation - my boat will need 398/100 = 3.98 sq.ft. cross section ventilator !!!!!!!!

I'm just near the rebuild of this 26' planing boat. I haven't known its past performance. Does this mean it won't work unless the engine covers are off?

 

One can always remember 50 sq inches per 100 HP - It will meet every engine manufacturer's specs.
As to CFM, engines consume about 500 CFM per 100HP produced.
Dave Gerr, BTW, suggests minimum vent area (sq inches) = hp divided by 3.3, and minimum air flow (cu ft/min) = (2.75Xhp) - 90. He's a pretty smart dude.
It doesn't make as much sence to calculate by engine displacement, IMO. By no standard, however, is your mouse-lung of air enough and you will greatly shorten the life of your engines. Lift a hatch-cover while running and you will feel that your engines are trying to eat it - another easy test. Get out the SawZall!

 

Excellent Mark.
Your figure calculates to a 13" x 13" vent and Dave Gerr's to 10" x 10".

The boat is a 29 year old Fletcher Zingaro with twin OMC 3.8 170hp V6's.
As I mentioned, I have nearly finished a complete rebuild after 10 years on dry land, so I assume the manufacturers original vent to engine capacity was a bit inadequate.

What concerns me now is the length of louvred vent to be cut into the hull under the gunwale. The existing vents, one to each side, are about one foot long by 4" deep with two 3" diameter inside pipes.
Would the correct sizes weaken the structure, or would you suggest 4" blowers to the carbs or how would you suggest the air comes in?

BTW, being an illiterate Limey - what's a SawZall?

 

You can always use a lot of small vents instead of one big vent, if you're afraid of weakening something. If the total vent area adds up to something around 40-50 sq.in per 100 horsepower, it'll likely be more than adequate.

I don't like any systems that direct outside air directly on the engine's intake. Invariably, the incoming air will bring salt, water and crud with it, and that air is also needed to keep things cool in the engine room. Air intakes should be designed so that spray and crud, if ingested, will end up in the bilge and not on the motor.

A Sawzall is a reciprocating saw- think giant handheld sabre saw with a six-inch blade that cuts anything.

 

Are you saying pipes connected to louvered hull vents back boxes (manifolds) configured as P traps and ending near the hieght of the engine, against the hull wall, would do the job?

 

if that suits you, yes it will, as long as the area is sufficient.

Many engine air inlets are too small, but making then route too far is not good either as there is resistance in small tubes that will affect the outcome.

Certainly though, do make use of confined areas as you see fit.

 

Got the message - thank you, Andy

 

Mark,
On the rule of thumb that it takes 500 cfm per 100 hp produced.

Does this imply that a carb needs to flow 500 cfm for each 100 hp produced by the motor?

That intutively doesn't seem right but that's a feeling not a fact. If so, does it have to do with something like the fuel air mixture @ 500 cfm only contains enough energy to produce 100 hp in an internal combustion motor with the conventional levels of effiency?

I've been puzzling over this for a couple of days, enquiring minds want to know.

 

No, some of that is for ventilation and these are numbers from I don't know where (Cummins intallation manual, I think, and I have never owned a Cummins but their book is great). Eric (Ski) on Boatdiesel is the expert on this stuff. I talk with him if you want to know theory - I am certainly not versed in this. What I know is that engines are happy with a lot of air and I use these numbers as a guide.

 

Thanks

 

Air weighs about 0.074 lb per cuft
Then take the weight of fuel per gallon
use your fuel to air burn ratio of about 13:1 by weight and if you know how much fuel you use you can calculate how much air you use, give or take....

If you can get the BTU's of the engine external heat you can then work out with a few HVAC formulas how much air to move to keep the engine room at a certain temp over the ambient.

or
in theory at 100% VE
your 229ci engines pump half that per revolution being a 4 stroke
114.6 cuin/rev
*60 = 6870 /minute
* 5000rpm = 412200 cu in/minute
ci-cft = 238cf/m
and if you could get 100%VE from a 229 at 5000 you would have one serious race motor (unlikley to get 100% ve at that high a rev)

hows my maths?


OEM Installation manuals for Mercruisers have always come from with the data to do the sums including the minimum USCG approved engine box volume for non pressure fed engine boxes to maintain an acceptable temp.
I suggest you ask a Merc dealer if thay have an OEM manual and just use the Chev v6 data.

 

For survey standards of a motor boat you are required to have the minimum of 1 sq /inch per 1 hp !
100 hp you need unrestricted air flow through a 100 sq inch hole !! Easy to remember ?
Note its better not to have a blower fan to assist as it needs to run real hard when the motor if under full load at max rpms .

Best way to check if you think the motor is not getting enough air is to run full out as fast as it will go and watch the tacho !! then lift the engine cover !! If the motor picks up a few rpms you have a breathing problem . The cover could also be hard to lift to begin with and thats another sign !!
Theres lots a formulars but this is easy to remember 1 hp = 1sq/in of unrestricted airflow


The motor will love you for getting more COOL air

Boat builders are not Boat designers ,who are not Engineers who are not builders ,who are not designers.....and they should all know better !!!

 

"Note its better not to have a blower fan to assist as it needs to run real hard when the motor if under full load at max rpms ."


IT is NEVER a good practice to pressurize an engine space , as the smells or exhaust can be blown into the rest of the boat.

FF

 

You would be amazed at the number of boats that do not have enough engine room ventilation or it has never been planed out to work properly !
What do i mean bu work properly ?? anyone have a guess ?
Hint !!:?:
Its meant to work even when the hot motor has been turned off after being used !!

 

FAO states in their very informative Fisheries Technical Paper number 383, page 34 that 8 cm2/HP is required for water cooled engines to provide sufficient combustion air.

This is for guys like me who are unable to do inches and feet.....and ounces and stones and miles ....I give up.

(BTW, the document is a goldmine of thumb-rule information for commercial/fishing boats)