Engine bay blower question

[poconojoe]

Mercruiser I/O.
I have a bow rider with the large sunpad covering the engine bay.

I start the blower and run it for the suggested 4 minutes before starting the engine or whenever idling around the lake.

On my initial startup of the day, I open the engine hatch to check on everything and flip on the battery switch.

Something I've always wondered... is it ok to keep the hatch open while the venting blower is on or is it somehow better for air flow to close the hatch? I'm no air flow expert or HVAC guy. I was just wondering if the air intake has a better, maybe stronger flow if the hatch was closed.

Maybe I'm just being too fanatical?

 

[matt167]

It should not matter because the blower draws off the lowest point of the bilge. But I would still leave it closed

 

[Scott Danforth]

Open or closed doesn't matter

Has fumes are heavier than air, so they settle to the bottom

That is why most bilge blowers pull from the bottom

 

[garbageguy]

I don't think that's fanatical.
My regular procedure: Open, look, run blower, start, close and run blower more til out and running.

I agree with others, the hydraulic design parameters (gases are fluids too) are likely for closed-hatch conditions, but since the gases you want to evacuate are likely heavier than 'air', blower-hose inlet at bottom of bilge is what you want

 

[poconojoe]

Yeah, the blower definitely pulls from the lowest point.

Maybe I'm just overthinking it.
Thinking maybe with the hatch closed there would be a stronger air flow where it's needed, considering maybe the engineers design might have located the inlets for maximum flow to the lower bilge and having the hatch open might inhibit that designed air flow.

 

[airshot]

On my Islander, my fuel tank was well forward so I added a squeeze ball in the fuel line to prime the line and not have to crank the engine for long periods of time. Before pulling out of my barn, I always squeezed the ball and checked for fuel vapors before heading to the ramp. When backing the boat in, I flip on the blower so by the time I parked my trailer and got back to the boat it was ready to start. When on the water, I would flip on the blower and smell the exhaust vent for fuel smells. If there were any vapors, an inspection was in order before starting. Cant be to carefull with gasoline in a boat...no place to run to !!

 

[H20Rat]

On my Islander, my fuel tank was well forward so I added a squeeze ball in the fuel line to prime the line and not have to crank the engine for long periods of time. Before pulling out of my barn, I always squeezed the ball and checked for fuel vapors before heading to the ramp. When backing the boat in, I flip on the blower so by the time I parked my trailer and got back to the boat it was ready to start. When on the water, I would flip on the blower and smell the exhaust vent for fuel smells. If there were any vapors, an inspection was in order before starting. Cant be to carefull with gasoline in a boat...no place to run to !!

You just turned your boat into a potential bomb... (and violated coast guard regulations) There is a reason primer bulbs aren't allowed on inboards. Way too high of a potential for the rubber to dry out, crack, and leak.

You shouldn't need to crank very long, there is an anti-drainback valve on the fuel tank, yours might not be working.

 

[vroom ZOOM]

Time for a little rant of mine here...
First of all, no engineer will ever bother doing aerodynamics calculations for a bilge blower. Maximum is they will quickly check if they need a 4 inch or 3 inch blower - so basically going with "good enough". And still, I was working on a 26ft boat with a huge bilge and a BBC and it came from the factory with a 3 inch blower... probably a 4 inch would have been better on that boat. Boat manufacturers just buy a regular run-of-the-mill inline bilge blower with the cheapest possible hose they can find, stick one end in the bilge, and call it a day. So hatch open or closed - the guys that built the boat themselves probably don't know...
Heck, some manufacturers do really stupid stuff... like positioning the bilge blower vents under the camper top canvas so all the fumes go in and suffocate you... I don't think a single engineer ever designed that... so if they do stuff like that, you can't really be too sure that the blower will remove all the fumes within 4 minutes or not. IMHO there is no design or calculations at all involved in the bilge blower system. This is why I strongly recommend buying J1171 compliant electrical parts, and not using generic automotive ones. And yes, still run the blower, its better than nothing.

UGH... turned into another really long post

 

[Lou C]

Add a gas vapor detector to any gas inboard