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Fuel Tank Anti Siphon Valve
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Author:  Flat_Bottum [ Fri Apr 03, 2015 10:12 pm ]
Post subject:  Fuel Tank Anti Siphon Valve
So, after having some top end issues with my boat, I was advised to check the anti siphon valve and sure enough that was my problem. Cleaned it all up and everything is good now...but I have some questions. First, I understand the purpose behind the valve but I am having trouble understanding why it could not be swapped out with a regular through fitting. To create and keep a siphon, you need the fluid exit to be lower than the reservoir fluid level, meaning if your sucking from a bucket, you need to keep the end of the hose lower than the buckets water level. If you lift the fluid exit up and over the reservoir fluid level, then you break the siphon. So with that in mind, it would be almost impossible to accidentally have your fuel siphon out of your fuel tank and into your boat. I have re installed my valve and do not plan to remove it but I would like to know if I am way off with what the valve actually does or if I am not. Thanks.
Author:  LouC [ Fri Apr 03, 2015 11:05 pm ]
Post subject:  Re: Fuel Tank Anti Siphon Valve
FEDERAL LAW

183.568 - Anti-siphon protection

Each fuel line from the fuel tank to the fuel inlet connection on the carburetor must:

(a) Be above the level of the tank top; or
(b) Have an anti-siphon device or an electrically operated fuel stop valve:

(1) At the tank withdrawal fitting; or
(2) Installed so the line from the fuel tank is above the top of the tank; or

(c) Provided that the fuel tank top is below the level of the carburetor inlet, be metallic fuel lines meeting the construction requirements of Sec. 183.538 or "USCG Type A1" hose, with one or two manual shutoff valves installed as follows:

(1) Directly at the fuel tank connection arranged to be readily accessible for operation from outside the compartment, and
(2) If the length of fuel line from the tank outlet to the engine inlet is greater than 12 feet, a manual shutoff valve shall be installed at the fuel inlet connection to the engine.


"Anti-siphon protection" is a term applied to the means of preventing the siphon action of permitting fuel to continue to flow out of the fuel tank in the event there is a break or rupture in a fuel distribution line, or if a fitting in the fuel line loosens, creating a leak.

"Anti-siphon protection" may be accomplished by one or more of the following methods:

(a) Keep all parts of the fuel line from the fuel tank to the fuel line connection at the carburetor above the level of the top of the fuel tank. The tank top level is determined with the boat in its "static floating position." Practically, the fuel pump and fuel filter(s) must also be above the tank top.

(b) Install an anti-siphon device at the tank withdrawal fitting. The fuel distribution line may then run below the level of the tank top. A filter may be installed between the fuel tank withdrawal fitting and the anti-siphon device.

The idea here is if any part of the fuel line drops below the top of the tank, it is possible for a leaking fuel line to siphon fuel out of the tank (keep in mind the fuel pick up in the tank goes almost all the way to the bottom of the tank). So it's different than the hose in bucket siphon scenario unless the hose goes all the way down to the bottom of the bucket and remains covered in liquid.

As an example during Hurricane Sandy we could not get gas, at least easily for about 2 weeks. I had about 35 gallons of gas for the generator (no electric for 2 weeks). After the first week I decided to siphon gas out of the tank of the boat, which I had filled up about a month before. I removed the rear seat, removed the hatch for the tank and then the cover for the gauge sending unit. I had a small hand pump hooked up to some fuel line. The gas can was on the ground, as soon as I got a siphon going with the line going to the bottom of the tank, I did not have to pump anymore and gas was siphoning faster than you'd think. I would up taking about 30 gallons out of the boat and that was enough to keep us going another week or so.
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