Vents through the deck, topsides and hull. What would you use?

[Chotu]

I have some ventilation to do for my fuel tank compartment.

A wonderful, unadvertised feature of fuel tanks is they permeate fuel stink.

I need to place 2 vents (as is specified by USCG construction guides), creating a natural air draft ventilation for the fuel tank compartment, which is below decks.

What would be the best way to do this on a catamaran?

Fuel tanks are down in a hull, not at bridge deck level.

What type of vents would you use to ensure a nice, steady flow of air?

My hull and deck are about 1 1/4” to 1 1/2” thick. (30 mm or so)

The wind comes from the bow 95+ percent of the time.

Thanks!

[Chotu]

The deck is about 2/3 the width of this one.



And the fuel tanks are aft on the port side.

Tanks are located way down low, approximately below where the winch is above the number “42” in this pic.

[rslifkin]

I'd use some kind of louvered vents on the sides of the hull, just below the deck line (ducted down into the compartments). With a cat, that shouldn't be a downflooding concern. I'd face the louvers forward on one side of the hull, aft on the other. That way you'll get air forced through with the wind from the bow or from the side.


The fuel smell is an unfortunate feature of plastic gas tanks. Only way to avoid it is aluminum or stainless.

[Dogscout]

https://www.westmarine.com/buy/perko...38?recordNum=3

As high up on the inside or outside (topsides) of the hull as possible.

[Chotu]

Originally Posted by Dogscout

https://www.westmarine.com/buy/perko...38?recordNum=3

As high up on the inside or outside (topsides) of the hull as possible.

Ah. They need to have a dryer hose size ventilation. These are way, way too small.

But were you just pointing out the vent type?

I have one of those vents already of the right size but it has almost no reach to get through the hull.


The fuel tank itself, which would use these vents you are showing, is ventilated already. The fuel tank compartment, however, is not. And it STINKS

[Tusitala]

Search variations on these: https://www.westmarine.com/buy/white...Y-_-PDP-Mobile

I know they're available with or w/o the 90° bend

Pat

[JimmyDarci]

Can you go smaller with small solar fans to assist air flow?

[rslifkin]

To add to my earlier post, my boat uses a bunch of these (with collector boxes and 5 inch ducts) for bilge ventilation (can be found much cheaper elsewhere): https://www.westmarine.com/buy/attwo...0?recordNum=17

[Chotu]

Originally Posted by Tusitala

Search variations on these: https://www.westmarine.com/buy/white...Y-_-PDP-Mobile

I know they're available with or w/o the 90° bend

Pat

Nope. Think dryer vent. That’s the size hose.

[Chotu]

Originally Posted by rslifkin

To add to my earlier post, my boat uses a bunch of these (with collector boxes and 5 inch ducts) for bilge ventilation (can be found much cheaper elsewhere): https://www.westmarine.com/buy/attwo...0?recordNum=17

I can’t shake the fear of these offshore for some reason. I feel like I’m the worse possible seas, they’ll start flooding my bilges with water splashing down.

Could be an unfounded fear but.... very uneasy about big holes like that at or below deck level on a sailboat.

[rslifkin]

Originally Posted by Chotu

I can’t shake the fear of these offshore for some reason. I feel like I’m the worse possible seas, they’ll start flooding my bilges with water splashing down.

Could be an unfounded fear but.... very uneasy about big holes like that at or below deck level on a sailboat.

On my boat I did end up turning the forward facing pair (which are about 2/3 of the way forward on the hull) around to face aft. The boat had an issue with blowing spray down the vents in rough seas up on plane. Never took solid water down them or enough to be an issue. Since turning the vents around, I haven't taken a drop of water down them (they're about 3.5 feet above the waterline). And the 2 pairs further aft on the hull (around the mid-point and just forward of the transom) that face aft have never taken water. On a cat where there's no heeling, I'd expect it to be less of an issue. On a monohull sailboat, I wouldn't do it.

Well designed collector boxes would also mitigate the issue, either a spray deflector between the vent and the tube, or the vent tube entering the top of the collector, or at one end with a baffle. Mine are open collector boxes with the ducts coming out the bottom (poor factory design), so they're very easy to dump water down.

If turning the vents around didn't fix the issue, my plan was to build water separator boxes, basically a dorade with a vent tube going in and one going out and a water drain to a thru hull above the water line. Then unless the vent were completely submerged, any water that splashed in would just drain back out without ending up in the bilge.


You could also go for scoops above the gunwale like typical sailboat dorades would use. One forward and one aft facing for each compartment.

[rslifkin]

To add on to my last post, here are a couple of pictures showing the vent placement I was describing on my boat.

[Cheechako]

I would vent out the back up high, or (preferably to me) into the cockpit. Use a 3" vent with plastic hose (like used in bilge vents, kinda like dryer hose) from bottom up to the vent. That way the chimney effect will draw it out better. it's amazing how much chimney effect you get in 4-6 ft of head!

[GRIT]

I would use louvred vents and build dorade Boxes on the inside.

I would have the louvred vent mounted inside the Box rather than have a hose leading to the Box. Now you can attach any size hose, from the box to the tank, you want.

Build a baffle and drain, and you're good to go.


This isn't structural, so polyester should work fine.

This way, hull thickness isn't a consideration. Either silicone the vents to the hull, or through bolt, depending on the design.

Ps, make one side of your box removable with screws for cleaning, and wasp nest removal.

Cheers.
Paul.

[stormalong]

I have a hidden fuel vent for my diesel tank. An idea I saw on (I think) a Hinckley.

It is a 1/4 inch stainless diameter standpipe that goes up a few inches inside a vertical stern rail tube and three tiny holes drilled near the base of that stern rail far below the inside standpipe. That makes the vent invisible and water proof. I've had it this way for about 20 years with zero problems.

I don't know about tank odors needing venting. The purpose of the vent is to prevent a partial vacuum or pressurization of the tank when drawing fuel and allowing air pressure to equalize as the fuel expands or contracts by ambient temperature or from the warming from the excess fuel return from the injection pump.