How can I setup my fuel system for my Single outboard Catamaran with dual tanks?

Hello, I'm new to this forum and I am designing a catamaran with a single outboard. I have one tank for each hull which makes it a dual tank. How can I set this fuel system? What kind of valves can I use to make the fuel flow simultaneously to the single outboard motor?

 

We have a similar arrangement on our mono hull work skiffs and we use a t connector going to double ball valves for both suction and return (if you have a return). We balance the valves by hand over time to find stasis and then remove the handle and cap the stem. I've tried a t port ball valve and found it to be slightly more finicky to trim.

 

Another option is to use a 3 way ball valve. You can plumb it somewhere near the engine where the lines from the two tanks converge. You just swap between tanks every so often depending on use to keep the fuel from going stale, or when one tank is drawn down when on a long trip. If you have return lines (which I don't think you would on an outboard) you simply run them in the same arrangement. The only issue is you have to make sure they are set on the same tank or you will transfer all the fuel over to the second one!

 

The bigger modern outboards have fuel through and return lines.

While I don't understand the physics of why, but we had a heck of a time balancing flow with a 3 way valve. We tried both t and l type valves and could only get it to work some of the time. A crewman who was an engineering grad student at the time suggested we weld a manifold and put separate ball valves connected to what amounted to a y fitting. Then we adjusted the valves for in small increments until we got a balance. It tuned very quickly, and when loading conditions allowed we could tune it fairly rapidly to adjust for list balance.

I'm not up on the science but two separate valves open at 70 and 80 percent respectively were much more effective across a range of time and load ranges than a 3 way valve. Last point was meter from open not from close, so clam the valves are mostly open with varying degrees of restriction.

I've since setup 3x diesel boats and one more outboard boat with split tanks on a similar method.

 

This is from Anderson Brass but there are others such as Groco. They switch draw and return simultaneously with no chance to draw from one tank and return to another. Most important to size fuel line size to horsepower/flow rate requirements. You will see the bottom return ports are a bit smaller.

 

You cannot.

Modern fuel tanks have an anti-siphon valve and require suction to work, so you can only run one tank at a time. Otherwise, the ASV on both tanks will not open and you’ll be going nowhere once the system reaches stasis after the filters are pumped out.

The fuel pump cannot apply enough pressure to both ASVs to open them.

 

Should add, the other guys are not wrong, but the tanks they are talking about probably do not have modern anti-siphon valves..