Engine compartment ventilation question

[Panoguy]

Hi:

I'm designing a 3-compartment engine hatch/storage areas, the lid will act as a sundeck/fishing platform.

I'm wondering if the ventilation scheme that I've come-up with will be sufficient? In particular, I'm wondering if my air-in should be beefed-up... perhaps a 4" or 5" diameter vinyl hose instead of the 3" that I spec-ed, or Is this overkill??

Any thought/suggestions?
As always, thanks in-advance!

 

[infrared]

Fuel vapors are heavier than air. The hose that is "sucking" should be low as possible

 

[Panoguy]

Fuel vapors are heavier than air. The hose that is "sucking" should be low as possible

Thanks for the quick reply, infrared.

Yep - the hose with the fan on it (sucking air out when the fan is on) would be placed down low, into the bilge... but I'm worried about engine heat when under-way (and with the blower turned off) - that's why I also placed a passive "up-high" exhaust hose.

Perhaps air being drawn-into the carb makes this heat buildup worry a non-issue?

Buuuut, with the 2 hose exhaust scheme, whilst I feel that I'm addressing heat build-up in the compartment, I'm afraid that I'm introducing the possibility that all air may travel directly from the intake (which is up high), to the (passive) "up high" exhaust, thereby allowing the possibility of fume-pooling in the bilge.

So, is it a bad idea to have that upper exhaust hose because of fumes & If I don't have it, will the engine compartment get too hot??

 

[GA_Boater]

It won't get too hot unless the motor is overheating. Blower and vents are to extract fuel vapor as stated earlier, not for cooling. The thermostats used in boats have much lower opening temps than a car - Typically 140-160 degrees, give or take so it isn't as hot as under a car hood.

The "high" hose could prevent proper fuel vapor venting. I would duplicate the original venting intake and output venting.

Are the engine bay sides of the new storage areas closed and sealed (vapor-proof) from the engine bay or open? If you aren't careful the storage compartments could become an unvented vapor collector.

 

[infrared]

Are the engine bay sides of the new storage areas closed and sealed (vapor-proof) from the engine bay or open? If you aren't careful the storage compartments could become an unvented vapor collector.

This is important. If there are batteries in there the hydrogen gas needs to be vented out

 

[Panoguy]

Thanks for the info, GA_Boater, so... I'll have both intake & exhaust hoses down low, drawing from the bilge only.

My side compartments are fairly well sealed from the engine hatch, but are far from airtight, some air is able to get in there from the sides (forward of the compartment) - still, battery gasses are something to think-about, thanks for that, infrared - I'll be absolutely sure that the side where the battery is located definitely has ventilation.

 

[infrared]

... I'll have both intake & exhaust hoses down low, drawing from the bilge only.

You should have:

• Ventilation at the top. For intake and exhaust of any gasses that rise.
• Blower motor with a duct pulling gasses from the bilge
• Ventilation for any battery compartments

I just looked at your diagram again. I think it's OK. However, the blue arrow on the left is indicating exhaust so I was confused and I immediately made my first post about how it has to be in the bilge. It sounds like you've always known about this requirement so I hope I wasn't insulting your intelligence by stating the obvious. If the diagram had arrows showing flow in both directions I think that would have eliminated my confusion.

If this were my boat... besides the obvious blower duct in the bilge, I would have both the remaining vents up high. And if there's a spot on the port vent to attach another duct, I would utilize it. These vents are for gas exchange, not for cooling (as GA_Boater stated).

 

[Panoguy]

Hey infrared - no worries about insults, and I appreciate any thoughts & advice!