Engine room Air Intake/ Ventilation

Hi
I have a boat that has sealed decks with engine is half under the deck and rest in engine box.
the boat is 31ft and has a 130HP engine.
I havent got any vents in my engine box because i was trying to keep down the noise.
So my theory was to run 3" tubing from the air intake under the deck then up through the floor into a cabinet at the side of the boat, then vent it from there with a big clam shell vent and a dorade box to stop any water. This would also be cooller air being supplied to engine
I would also vent the other the side of the boat the same to to allow air to under the deck / engine compartment.
To creat a air flow I would also need a extraction fan set up to produce a air flow is that correct?
The other worry is that by making the air intake the much longer with tubing will it restict the amount of air that can be pulled in by the engine ??

 

Gas or Diesel?

Gas requires (USCG) an exhaust fan for explosive fumes, not required for diesel.

Forget the hose, the engine will draw the air through whatever air path you provide as long as it's big enough.

Are you talking about drawing air from the hull sides? If so I would forget the scoops and go with something louvered and flush, otherwise a piling is probably going to just scrape it off.

Gasket your engine box, important for noise.

Steve

 

If it is a diesel it will require 65 square inches for the intake. Cooling may require extra if the engineroom is getting too hot.

 

REMEMBER 1 square inch of unsestricted air inlet for every 1 hp of the motor !! This is rule of thumb!!! no blowers !,no suckers !,
Air coming into a engine room goes to the bilge !!!!!! air going out leaves from above the engine height . 130 hp 130 square inchs UNRESTRICTED air inlet !!! easy to remember and dont forget !!

 

There's something wrong with these rules of thumb.
First of all Gonzo's air intake is only half that of tunnels. I converted it to metric to visualize the sizes: these are escape hatches!

My 140 hp Kia diesel has to breath through a 3 inch plastic duct with 3 short 90 degree bends and an air filter. The average cross section is approximately 7 sq.in., yet the engine has no shortage of breath. The 120 hp Fiat engine in my RV has a similar intake setup, 3 inch seems to be the proper size for a 2.5 ltr diesel engine.

In closed confinement I would choose the air inlet opening somewhat larger, or even better to make two openings to keep the engine room temperature down. Ideal would be a separate 3 inch intake hose all the way to the transom so the engine can breath the cooler, denser outside air. Two louvered ventilation openings for the engine bay, perhaps even a small fan with a Klixon thermostat for the really hot days.

 

My farther was a cat man for many years doing new installs and repowers and repairs and thats what they worked on . easy to remember
Never starve a engine of breathing air!!! Never !! petrol or diesel . turbo or non turbo . injected or non injected !!!
130 hp =130 INCHS !! ONE SQUARE FOOT =144 SQUARE INCHS AND THATS NOT VERY BIG YOU SHOULD HAVE TWICE THAT FOR VENTILATION AND COOLING ANY WAY . COOL AIR MOTORS RUN BETTER AND HAVE MORE POWER !!!! The hotter the air the more slugish they are .

 

Often it's best to negatively pressurize the engine compartment. The easy way to do this is with a blower that sucks air out of the compartment. A 3" or 4" in line blower drawing compartment air, typically aft and out the transom or aft deck through clam shells or louvers.

What this does is suck air through the bilge, down behind ceilings, etc. so that air is always entering the compartment, but most importantly not escaping into living spaces, such as the cabin. If your boat is a planning craft, you can arrange venting to do this naturally, without blowers, but you have to be at speed for it to work, meaning that in low speed or idling situations, compartment smells and fumes can leak into living spaces.

The rule I use for minimum vent area is the HP divided by 3, which in your case is 44 sq. inches. If you want to be overly cautious then add 10%, making the vent area 48 sq. inches. I think 130 sq. inches is way over the top for your engine, no disrespect Tunnels, though you can't go wrong with this much venting either.

The rule I use for minimum air flow (blowers) is HP times 3, then subtract 100. This would be 290 CFM (cubic feet per minute). These formulas I've used for many years and are in line with Dave Geer's recommendations too. His formulas are just like mine, though his figures will be slightly lower. Of course these are the minimums. You can't have too much compartment ventilation.

If you brought your boat to me for this job, I'd install two 3" Rule 135 CFM in line blowers and send it through a 5"x10" louver or two 4"x6" vents. They'd be wired on a ON-OFF-ON switch, with one side slaved to the oil pressure sender. The blowers would come on automatically within a second or two of engine start, assuming you didn't already have the switch turned on and would shut down at engine off or by the switch. If desired, they could be wired to a delay, so they'd continue to run for a few minutes after engine off to vent fume build up.

Tunnels is correct in that compartment cooling is usually woefully lacking. Engines last longer, run better and are more efficient with lots of air. Again you can't go wrong going big. My simple formulas provide more then enough air and in many cases a lot more then the manufacture's provide.

 

That's why I'm in favor of a separate intake hose or duct.

 

On most of my personal installations I have a "cold air pack" installed as well as several other "hot rod" tricks, such as a fuel cloud tower above the carburetor throat, port matched intakes and exhaust ports and runners, much better cooling, etc.

The cold air pack is similar to what's now seen on cars. It's a duct that brings cool outside the engine compartment air, directly to the filter or arrestor. The passageways are intentionally clean, as straight as practical with large radiuses, etc. The air filter is usually a high volume unit, generally reusable, oil bath types. I even have this arrangement on my garden tractor which has more power and has lived twice the life span of a typical engine in it's configuration.

Raising the arrestor or air filter well above the carb throat is an easy way to improve efficiency and throttle response on any carbureted engine. The tube that conects the arrestor or filter should be smooth and if possible radiused at the top as it takes delivery of the air charge. On muscle cars this would be called a viscosity stack. It has no moving parts and is a cheap, easy way to get better flow.

Ducting cool outside air to the arrestor or filter is a natural improvement too.

What is often over looked is the exhaust. You can get huge gains with simple preformance enhancements. Reversion cones are something all drag racers know about. It's an exhaust port shaped cone that fits inside the exhaust manifold. It does two things; creates a venturi effect, which keeps the hot gasses centered in the port and it also prevents exhaust gas contamination in the combustion chamber. You see most modern engines have a substantial amount of valve "overlap", which simply means the intake valve is opening as the exhaust is closing. Without the reversion cone, exhaust gases can sneak back into the chamber, as the piston goes back down and the intake valve is opening. This can change the air/fuel ratio substantially, but a reversion cone will eliminate the issue. Any hot rodder that's looked at a set of headers for their V8, has seen the reversion cone option on the higher end products and of course the race versions.

On a marine manifold, you'll need to weld on some cones, but it's not hard if you can handle a welder. Also reducing restrictions in the exhaust path is another way to improve engine efficiency.

Port matching is a time honored method tweaking a few more ponies out of an engine, as are radiusing corners, removing casting "flash" and the usual host of go fast stuff.

A full blown improved flow in and out on an Atomic 4 as an example will get you 10% to 15% more power, mostly for free.

 

whats wrong with
get btu rating of the engine/gearbox ( that why they give you this data) plus air consumption at full power, ambient air temp and then your max temp rise ( for engine rating)you are shooting for and then get the right size blowers, then air speed for db which will give you required vent cross section
Any AC guy could do that for you on the back of a napkin

 

Gee, its just a small boat. Generally ventilation in, ventilation out, size is determined by the available space you have to work with.

With engine room ventilation...bigger is better.

Choose the diameter that matches the common marine blowers....3 inch sounds good. 4 inch even better

General thoughts are....... cool air enters the machine room down low and receives its cool air supply from a completely weatherproof , stormproof intake vent.


Since heat rises, exhaust this hot air from the top of the machine space.

Remember in heavy weather you will storm shutter the exhaust vent, but the intake vent must be free. A spray proof, self draining dorade box type is common for the intake vent.

Heat prematurely ages all equipment . Your machine room temp will spike up as soon as you shut down the engine. You must evacuate this heat. It may take hours to cool the machine space.


Since you are a small craft electric power is limited. Three hours of powerful blower action will kill your batteries. A workable compromise to conserve electricity is TWO intake vents and one exhaust vent. Intake ONE, you primary air feed, is powered by the appropriate blower. Intake TWO is powered by a low volume Computer style cooling fan. The system is set up so that once the engine is stopped the main air feed blower runs perhaps one half hour or one hour or whatever via a timer switch then shuts down to conserve energy and the low volume intake TWO takes over.

Up north your climate is cool so dissipating machine space heat is easier. Down south its very difficult to dissipate heat.

Pay particular attention to exhaust pipe insulation. If in doubtre insulate. Exhaust heat is the number one heat source.

A simple home heating style air filter on the primary intake side of your vent system , incorporated in your dorade box style intake, will keep your machine space clean and salt particle free.

 

Simply put give the motor plenty of air !!! its working really hard for you so why are you trying to strangle and choke it to death .
Blowers should only be used to blow air past and round the starter just before you start the motor . You should never have to rely on a blower to feed your motor air while it is running that is poor design !

 

Wrong!

You never push air into the engine room, you pull it out. Under-pressure prevents fumes going where you do not want them to go.

A powerful marine blower takes about 2.5-3 amps to do its job, a battery can easily deliver that for many hours. No need for a timer, just a Klixon thermostat, 120 F. or 50 C. is a suitable value. The fan will not work below that temperature and will keep running until the temperature is 10% below the threshold value to remove residual heat from the engine block. What more can you want?

 

Again as I (and others) previously mentioned, a negative pressure compartment is what's desired.

Powerabout, every single engine manufacture lists minimum venting requirements, but they also don't know several very important usually mitigating factors, such as other equipment, exhaust routing, the size and type of compartment, etc. which all affect the venting.

It would be nice if all boats could afford to have naturally vented compartments, but this is the rare exception, particularly with noise, smell and equipment requirements aboard today. Most folks don't want to know the beast is anywhere on the boat.

The best systems provide more then enough air, they do so with little fuss, automatically, with limited noise and other concerns.

 

General machine room vent system considerations.


http://www.gregorypoole.com/product...ments/LEBW4971-02 ENGINE ROOM VENTILATION.pdf


positive or negative pressure is your choice. Negative pressure is energy intensive for a small craft because it need two blowers..in and out. Positive only one blower.

Also note that you never direct cool air at a hot component or you will raise the temp of the machine room.