Hypothetical tank conversion vent question

[Muaddib1116]

My diesel tank (stainless steel) is located starboard amidships and is vented to the transom via a long 1inch hose run up along the underside of the hull/deck joint, along the back side of the transom and then out the vertical back high up, right below the deck. In the photo below, it is the port side stainless vent cowl. It doesn't have a loop above the vent outlet, probably depending on the location of the outlet, the long run of the vent hose, and the natural pitch of the shear line to prevent water from getting to the tank.



I am thinking of eventually converting my smaller water tank, also stainless steel and also starboard amidships, into a second diesel tank. The tank is currently vented via a hose up the side of the hull, along the deck into the top of the mast cabinet, right next to the mast partner.

If I convert the tank, I would have to add a vent to the outside of the boat, but given where I could put the vent outlet, I don't think I would have room for a loop above the outlet, and I don't think it would be possible to replicate what was done for my existing diesel tank vent.



Not a super useful picture, but the vent would have to go on the side of the cabin trunk/pilothouse, somewhere under the forward half of the starboard pilothouse windows.

My concern would be water ingress, if I don't have that loop. So what I was wondering was, could I tee the new vent into the existing vent, but add one way valves to each, so the vent from the mast cabinet allows air into the tank when being emptied, and the vent to the cabin trunk allows air out for filling/expansion?

[thinwater]

If you cannot manage a high loop, then install modern P-trap fuel vents, designed to prevent surge from pushing water in. This is too important to goof around. I would add this to BOTH vents, since your current vent does not sound well-protected (but that could just be my reading of the description).


What you describe can be done with spring loaded one-way valves, and is done in industry for special reasons, but it will not accomplish what you want. if there is water in the "out" line you can bet it will slowly gurgle past the valve and into the tank.



P-Trap Fuel Surge Protector : Attwood Marine


[IMG]http://old.attwoodmarine.com/UserFiles/Store/Product/Images/576/full/1684-FFVENT-PTRAP-RGB_**.jpg[/IMG]

[Muaddib1116]

Thanks, that looks really useful, there might be enough room for that. I think it could still be a good idea to have a one way valve behind that, to prevent intake suction, and use the old vent as an intake vent. Since the side of the pilothouse is not vertical, the flush mount p-trap might not drain out adequately, and I imagine that the baffles that prevent water ingress could become filled up and overflow from intake suction.

This is probably not a project that I'm going to do till next year at the earliest, if I do it at all.

As far as my existing vent, I will take a closer look at it, and if need be, see if I can add a trap.