Vent placement on Gentry

[Trackhappy]

My Gentry was hurriedly "completed" for IAGG, and has had a break while I am working on other projects. She is away at present getting veneer laminated onto the dash and getting a name on the transom. She will be back next Tuesday (can't wait) and we will be doing the rest of the finishing to hopefully have her ready for a local wooden boat show at Bribie Island in May.
One of the finishing items is the ventilation. I want to put vents so that fresh air is forced into the internal space as she travels along. I can't decide whether they would be ok on the engine hatches, which in my opinion would look better, or should they be further forward to get the air into more of the boat. Just in front of the horns maybe. Note that the asymmetry of a single horn is doing my head in, so I am going to get a matching one for he other side shortly.
Also, given that there is open space in the cockpit area, I am assuming that I won't need exit vents other than the electric extractor fan.
Glen-L has two sizes on their site but I also can't decide which size is better on an un-stretched Gentry, anyone got any comments???

Gentry Forward deck

[Rich Coey]

I think you can put the vents on the forward part of the hatch, thats where they are on my boat and it has worked fine for seven years. I agree with the open space in the cockpit area, you will not need exit vents unless you want them for looks. The important thing with the extractor fan is to have it ducted with tubing or hose from the bottom of the bilge.

Rich

[jenko]

that the asymmetry of a single horn is doing my head in

Just got to laugh at that ocd I feel your pain

[Aussie]

Hi Glen,
I would go with the larger of the two vents from Glen.L.. i have seen both sizes and the smaller ones are just a bit small
For my liking.. as for placement i would get a hold of them and move around to try out what looks best...place them roughly.. leave a few days in between moves to see where they look right.....cant wqit to see some more pics..
Jamie

[trisailor]

I used two exhaust blowers, one for the forward compartment and one for the aft space, where the gas tank is located. There is a low (on the bilge floor) pick-up hose in each space, each connected to their respective blower intakes, and exhausted through the aft vents. I used one of the forward intake scoops for running a 3 1/2" hose back to the aft space. I threaded it along the side of the boat, above the copper exhaust pipe. I totally agree with Aussie. Set the chrome clamshells on the deck for a few days, move them around. Make sure you like the location, because there's only one shot at this. That reality really made me nervous. I had the same feeling when I drilled the shaft hole and the exhaust tips.

[Trackhappy]

Thanks for the helpful comments (and piccies to boot) as usual. Have instigated ordering the larger vents and will cogitate on placement when they arrive.
I have one blower (well, sucker really) with a nasty flexible hose running down under the engine, which is the lowest point and presumably where gases will accumulate. I am not happy with the flow of the single sucker, so may add a second, but my "OCD" (thanks Jamie.. ) means the outlet will have to be in the identical spot on the other side and I doubt I will have the room to run a pipe back under the tank due to my seriously oversize exhaust runs. I will check it out when she is back home.
Cheers,
Glenn.

[neel thompson]

Glenn-- I have two blowers also (following Trisailor"s design). They are both under the aft deck. I ran 2" PVC from under the motor to the area under the aft deck. I hooked one blower directly up to that pipe and aimed it out the exhaust one vent on the aft deck. The other blower picks up fumes under the fuel tank and exhausts it out the other vent. I have two intake vents directly over the motor. So, two up front and two aft. I don't think it would be a big deal if you positioned the front intake vents up forward as long as you have a way to get the fumes from low in the engine bay to some blower to take them out. I used the smaller vents from Glen-L and everything is cool (so to speak). There is plenty of air for the motor to run properly.

[Ozzieboat]

Glenn
The most important thing is to position the vents where they will have the least obstructed air flow, both the inlet and exhaust. The inlet vent is not a problem on the gentry but try to keep the exit vents outside of the those seated in the boat. The second thing is to ensure the air from these vents is being directed in a way to create the best scavenging effect in the bilge. Inlet vents discharge just under the deck and exhaust vents draw from the bilge. Remember the engine is going to inhale a lot of the ram air. So try to keep the engine intake out of the area to be ventilated. The engine deserves cold air not engine compartment hot air.
ozzieboat

[Joris]

Can anyone tell me why the inlets need to be just under the deck and exhausts need to draw from the bilge?
Hot air and fumes rise so i thought it would be better the other way around...

[jenko]

carbon monoxide, gas and petrol fumes like to sit at the bottom of the boat so to minimize suffocation and explosion risk the air should enter at the top and exit from the bottom to properly evacuate all dangerous gases

[Bill Edmundson]

Bilge "blowers" are really bilge suckers. The inlet side should be as low as possible in the area that might get fumes as gasoline fumes are heavier than air. Blowers are supposed to be run 5 minutes before starting and during refueling.

Bill

[Joris]

Thanks Jenko, i wasn't aware those fumes are heavier than air.

What about size? Is one 3" extractor (170cfm) sufficient for a boat like the Gentry?

[Ozzieboat]

Joris
For ventilation information do a search for "USCG regulations" then look up "compliance requirements" and then see Subpart K "Ventilation"
ozzieboat

[Joris]

Thanks Ozzieboat. Found it.
I wish CE-rules were as clear as that...

Joris