Fuel Demand Valve, Antisiphon?

[Marty_B]

Hi everyone,
I have a 1974 Irwin 28 Mk III boat with an atomic 4. When I acquired the boat, the wiring and fuel lines were pretty much ripped out. I am redoing the wiring, engine, exhaust, and fuel system. The fuel tank is directly aft of the engine, and the top of the tank is about a foot to 18 inches higher than the inlet to the electric fuel pump. I have read that you must have an anti siphon valve on the fuel line due to the height difference. The fuel pump is on the engine and I don't understand how a fuel pump will work in this configuration. Won't it just suck air through the anti-siphon valve? Also I have read about fuel demand valves but are not sure how they work. I was thinking about some sort of fuel shut off valve at the tank that would open only when the fuel pump is energized and close any other time. Any ideas?

[Dave Neptune]

Marty, fuel demand valves are used on "non-vented" tanks that can develop pressure from heat mostly from the sun. They are getting popular on old carbureted outboard engines using the new EPA style fuel tanks that are not vented.

The tank on your boat should be vented and 18" of fuel gravity pressure is not enough to push the needle and seat open causing flooding. It would be a good idea to have a shut off valve either manual or electric. Both common for use in standard style fuel tanks.

I had that problem with a new Moeler on deck fuel tank and just bought a vented cap, problem solved in my 1986 Calibogie.

Dave Neptune

[joe_db]

I think he is referring to those valves that are essentially a spring-loaded check valve that uses the pump suction to open and then spring closed when the pump turns off. These are inline with the fuel hose, not like an air-injecting vent that is teed into the coolant line.
Facet also sells pumps with a valve in them that turn off when power is removed.

[Marty_B]

Thanks for your replies. My main concern is that if the fuel line become damaged or cut, and that the tank is higher than the fuel inlet, potentially, the fuel could siphon out of my tank into the bilge.
The tank is vented so no pressure should build up. As I mentioned, I don't think I can use an anti-siphon vent as the fuel pump sucks from the tank instead of pushing from the tank. Thats why I looked into the fuel demand valves to see if they are suitable. My concern with them is that they may introduce too much resistance to fuel flow.
As you mentioned, I simple fuel shut off valve might be sufficient. I will just have to remember to turn it off. What is the typical practice for situations like mine? Thanks again for your help.

[Dave Neptune]

Marty, there are fuel valves that can be actuated by your ignition switch and if mounted at the tank you should be good to go.

As far as a leaking fuel line you should inspect it regularly and only use good quality CG rated hose. It should last many years.

Dave Neptune

[Dave Neptune]

PT II

Also, the fill tube and vent line should be inspected regularly. Most of the fuel related leaks and fires are related to the fill and vent lines as most are not inspected regularly.

Any gasoline on a boat is only as safe as you and your inspection practices are!!!

There is no need for an anti-siphon on a fuel line nor should there be one.

Dave Neptune

[Marty_B]

Thanks again. I plan on adhering to all of your suggestions.

[Marty_B]

When discussing anti-siphon valves, I didn't really know what I was asking about, but Joe db was correct, That is what I needed and purchased, essentially a valve that requires suction to open, more suction than 3 feet of siphon head.
Thanks again for your help.

[Dave Neptune]

Those type of valves are just a check valve set to a specific value. Not a bad idea at all.

Dave Neptune

[joe_db]

Those type of valves are just a check valve set to a specific value. Not a bad idea at all.

Dave Neptune

If you dig through the old messages, those have on occasion caused issues. FYI
* probably a dying weak pump or air leak

[joe_db]

https://www.pegasusautoracing.com/productdetails.asp?RecID=32334

FYI - this pump shuts off fuel flow when not turned on.