A gasoline tank usually sits higher than part of the fuel line to the engine. If that line breaks below the level of the tank top, the tank empties into the bilge by siphon, engine off, nobody aboard. An anti-siphon valve at the tank withdrawal fitting stops it: it opens only when the fuel pump pulls.
What the rule asks
The US federal rule is 33 CFR 183.568. Each fuel line from the tank to the carburetor must do one of these:
- run above the level of the tank top;
- have an anti-siphon device or an electrically operated fuel stop valve, at the tank withdrawal fitting or placed so the line from the tank stays above the tank top;
- where the tank top is below the carburetor inlet, be metal fuel line or USCG Type A1 hose with one or two manual shutoff valves, one directly at the tank and reachable from outside the compartment.
For runs over 12 feet, the manual option adds a shutoff at the engine's fuel inlet.
How a surveyor checks it
Trace the line from the tank to the engine. Find the withdrawal fitting and see what's on it: an anti-siphon valve, an electric stop valve, a manual valve or a plain fitting. Compare the line's lowest point with the tank top. Look and smell for fuel at every joint, and read the hose marking; see fuel hose type and age.
A plain fitting where a valve should be is often an owner's fix for fuel starvation. Ask.
What decides the grade
Leaking fuel with no siphon protection is an A on a gasoline boat. Missing or bypassed protection with no leak is a B. A fitted valve that may be restricting flow is a C.