Engine compartment ventilation

Ralph Johnstone · May 14, 2023

This was covered a few days ago :

Engine compartment blower re design what do you have in your boat

Completely refitting a new blower and aeroduct 3inch flexible ducting. Have a perkins 4.108 Does you boat have a blower escaping air out of the engine compartment,do you also have one the blows air into the compartment Does your blower come on as you turn the key and stay on. Do you have a...

forums.sailboatowners.com

Terry Cox · May 15, 2023

Our boat came equipped with an engine room fan.

Ralph Johnstone · May 15, 2023

BabuBoat said:
Figured the fan was there to remove fumes.

No need to remove diesel fuel fumes from an engine compartment other than they stink. An exhaust fan for a diesel engine is for supplying cooler air for the alternator,

Terry Cox · May 15, 2023

You're welcome. I'm assuming ours was placed to remove heated air when running both engines (main/generator) in warmer climates. Never had a reason to run ours.

Don S/V ILLusion · May 15, 2023

Seems like there is a misconception of what engine room ventilation accomplishes in small sailboat engine spaces.

when a diesel engine is running, it sucks in a huge volume of air into the compartment to allow proper diesel combustion. Inherently, this air flow serves to ventilate the compartment better than any artificial means produced by a fan. In some cases where a fan sucks air out of the compartment, combustion actually fights the air flow rather than serve any useful purpose.

Engine compartment fans serve a function but only when the engine is off when they can remove or displace hot air or any fumes trapped therein. Generally, this latter function is useful in gasoline field engines to ensure the ambient air in the compartment doesn’t exceed the lower explosive limit should gas fumes be present.

Calif. Ted · May 15, 2023

BabuBoat said:
Are the two scoops, one at each side of the cockpit, used to bring air into the engine compartment?

Common to Catalinas there is an exhaust fan on the transom in one hose while the other hose is passive intake, on ignition switch but a switch on bottom of engine instrument panel also. So when ignition switch ON fan runs if panel switch also on, does not run if panel switch off.
Jeanneau have the exhaust fan mounted above the engine attached to transom hose and is on ignition switch, also with a second passive intake on transom.

Don S/V ILLusion · May 15, 2023

BabuBoat said:
In my case it's just sheer ignorance.

So I should run the fan after shut down to cool off the compartment, correct? Otherwise, leave it off? There is a suggestion to wire the blower to the start switch so it will run when the engine runs, so there is some conflict of advice

Are the two scoops, one at each side of the cockpit, used to bring air into the engine compartment?

As to your first question regarding cooling the engine compartment, the answer lies in the purpose - if you want to cool the engine, there is no cooling water or oil flow once the engine shuts down. Just like your car. Trying to cool it with a fan is futile. Cooling a hot engine by ventilation is a function of temp difference and heat transfer. With a 500degree engine, it will cool at almost the same rate regardless of whether the ambient air in the compartment is 150degrees or 80degrees.

As to the function of scoops, they facilitate the engine air intake to fed air for combustion. Consequently, the compartment is inherently being ventilated while the engine is running.

Ralph Johnstone · May 15, 2023

Don S/V ILLusion said:
when a diesel engine is running, it sucks in a huge volume of air into the compartment to allow proper diesel combustion.

One would think so, but that's not quite the case.

Just how much combustion air does something like a Yanmar 2GM20F turning over at 2600 RPM need for combustion :

29.2 CFM is the theoretical air flow while the actual air flow will be somewhat less due to the inertia of the air entering the cylinders.

My Detmar blower fan for cooling the alternator is five times that volume :

This was the only reason I went ahead with the installation of the blower fan when I saw the engine compartment sitting around 160° F. After the fan was installed, the temp. went down to 120° F.

Also, no worries about starving the engine for air as the engine is wide open to the cold bilge area.

Ralph Johnstone · May 15, 2023

BabuBoat said:
There is a suggestion to wire the blower to the start switch so it will run when the engine runs, so there is some conflict of advice

If your concern is making life easier for your alternator, yes you want to run your fan while the engine is running. See post #11 for the reason. I do it because I have a heavy electrical load on the boat and the alt. is really humping when we start motoring after a few days at anchor.

BabuBoat said:
So I should run the fan after shut down to cool off the compartment, correct?

Yes, particularly if you have a bunch of nagging guests who are complaining about the hot summer weather in the first place. The engine will add heat to your cabin once the fan and engine are shut down. The skin temperature on your engine will be as high as about 180° F because the cylinders are surrounded by antifreeze at approximately 180° F. As long as you're pulling that heat out via the fan, it's not going into the cabin. I had considered adding a time delay to the fan circuitry but was too much of a chicken :poop:

worrying if the off-delay timer were to stay closed past its scheduled time (approx. 30 minutes) and the batteries were wasting their charge.

Engine compartment blower re design what do you have in your boat

Completely refitting a new blower and aeroduct 3inch flexible ducting. Have a perkins 4.108 Does you boat have a blower escaping air out of the engine compartment,do you also have one the blows air into the compartment Does your blower come on as you turn the key and stay on. Do you have a...

forums.sailboatowners.com

See post #2 for details.

Don S/V ILLusion · May 15, 2023

Ralph Johnstone said:
One would think so, but that's not quite the case.

Just how much combustion air does something like a Yanmar 2GM20F turning over at 2600 RPM need for combustion :

View attachment 215679

29.2 CFM is the theoretical air flow while the actual air flow will be somewhat less due to the inertia of the air entering the cylinders.

My Detmar blower fan for cooling the alternator is five times that volume :

View attachment 215681

This was the only reason I went ahead with the installation of the blower fan when I saw the engine compartment sitting around 160° F. After the fan was installed, the temp. went down to 120° F.

Also, no worries about starving the engine for air as the engine is wide open to the cold bilge area.

View attachment 215682

View attachment 215683

Curious about what size hole the air exhaust exits If it has any restrictions such as bends or rainwater hood. I can’t imagine having a 3 inch diameter hole anywhere near the waterline.

Ralph Johnstone · May 15, 2023

Don S/V ILLusion said:
I can’t imagine having a 3 inch diameter hole anywhere near the waterline.

If the water gets up to this level :

..................... I drop my hat on my head and wish anyone left behind a "fond farewell".

And because I'm so OCD, I've also included a :

....................... loop as a last line of defence.

Don S/V ILLusion · May 15, 2023

Ralph Johnstone said:
If the water gets up to this level :

View attachment 215690

..................... I drop my hat on my head and wish anyone left behind a "fond farewell".

And because I'm so OCD, I've also included a :

View attachment 215692

....................... loop as a last line of defence.

I’m assuming you had some form of cover over a 3inch hole which is why I asked as you may not realize that clam exhaust cover significantly restricts the claimed flow rate (As does the air resistance through that long hose).

If you have a flow meter and checked the rate, I’d be interested to know what the claimed/actual flow rate actually is out that exhaust cover.

I’m dubious this serves as significantly more than the engine air draw while running.

jon hansen · May 15, 2023

my $0.02, i would want the OP's air vents, one forward, one facing aft. ventilation. i run the blower during engine running to cool the compartment, not the engine. keeps the salon cooler not having the heat conducting into it i run the blower on hot days after shut down so the heat doesn't warm up the salon while the engine is cooling down. the vent for my blower is in the cockpit down low. i can then smell any bad fumes. there are no bad fumes during normal operation. bad fumes come from problems. therefore they can alert me to said problems.

after 49 seasons, i must also be getting plenty of air to the engine as it's never stalled do to air starvation.

Ralph, sweet looking engine compartment, nice job Sir

Hayden Watson · May 15, 2023

I have my blower ducting set up about the same as yours because with my fully sound insulated compartment and a high output alternator charging my 560Ah LFP bank, I needed a way to provide cooler air to the alternator. My wiring is set so that it is always on when every the key is in the on position. That way, I hear it spool up before I start and know that it is still working. I also have it connected to power with a switch that is parallel to the key. That way when I want to keep the cabin cool, I can manually turn on the blower to extract the leftover heat from the engine space. In cold weather, I leave it off to provide more cabin heat. I do not need to worry about forgetting it because my blower is a Seadog squirrel cage blower is loud enough to remind you that it is still running.

At 100-cfm, it does not move as much air at free flow as does the inline blowers but a squirrel cage blower has much higher static pressure than does any inline fan type blower. That is why you never see an inline blower on a ducted heating system. This type of blower is also rated for continuous duty cycle. Given that, it most likely moves more air through the ducts that a higher rated inline fan.

On my boat, with my tight and fully insulated compartment with a high output alternator feeding a large LFP bank, the blower running drops my alternator case temperature by 50ºF.

Ralph Johnstone · May 15, 2023

Don S/V ILLusion said:
I’m dubious this serves as significantly more than the engine air draw while running.

No doubt about it. The air flow is definitely less than the 158 CFM factory rated performance with atm. pressure at inlet and outlet. I'm still betting several times more than the 29 CFM of the engine.

Don't have a velometer at this time so not going to be able to prove this one way or the other. I do know I got a 40° F temp. drop with the fan so I'll have to hang my hat on that.

Csw1742 · Apr 4, 2025

BabuBoat said:
In my case it's just sheer ignorance.

So I should run the fan after shut down to cool off the compartment, correct? Otherwise, leave it off? There is a suggestion to wire the blower to the start switch so it will run when the engine runs, so there is some conflict of advice

"Are the two scoops, one at each side of the cockpit, used to bring air into the engine compartment?"

Csw1742 · Apr 4, 2025

On my 1990 P 42, the starboard "scoup" is for the galley ventilator, port is unknown. I dont see a way to duct the port " scoup" into the engine compartment without traveling through the overhead cabinet over the fridge/ freezer. Currently, nothing in mine but with an alternator upgrade, getting cooler air over the alternator is worth exploring options.