I have a question for the list about the size of fuel supply and return manifolds. The factory installed stainless steel Supply Manifold on my boat is about 3 inch in diameter and 12 inches long with 5/8th copper piping coming from the tanks (about 2 inches above the bottom). This manifold has 6 tanks feeding it from the bottom and 2 outlets at the top going to 2 DD671ti 410hp engines, and 1 additional outlet feeding 2 8kw gens and a diesel heating system. The Return Manifold is just slightly smaller in diameter with 1/2 inch copper piping coming from the engines/gens and separate outlets going back to the top of each tank. With just the one supply and one return manifold serving all tanks and all engines, I am forced to use just one tank at a time, because there has been an overflow due to unequal suction and return from/to more than one tank. I am thinking about replacing the manifolds with separate port and starboard systems of 3 tanks each. In doing so we are considering using standard 4-way crosses of 1/2 inch bronze pipe fittings. This would provide all of the connections desired, but would not provide much of a reservoir of fuel to draw from. I am concerned about the DD engine sucking a lot of fuel and therefore not allowing any to get to the generator or heater. What size (volume?) of fuel manifold is needed under various conditions? If standard pipe fittings would be good enough, why don't the manufacturers use them? Any thoughts? p.s. Capn'Wil, thanks for your input on the Walbro pump. I had the very same question, and now know the detailed answer. Joel Leslie OA55 MKI

> I have a question for the list about the size of fuel supply and return > manifolds. Before you can do the proper design of the system you must determine the flow of each item and the total of the complete system. Getting that information from engine manufacturers is often difficult. If the system can be run as it is, you can to do the measurements yourself. That's not always easy. You can measure the return flow to the tanks from the engines at the dock. Run the engine up to top RPM for a short time and catch the return in a bucket for some measured time. The time should be fixed so easy measurement can be made. For example, if you get an amount that can be easily measured in five minutes, you would multiply that quantity by 20 to get the flow rate of the return per hour. Then you can add the actual burn rate for the engine to that for the total flow rate of that engine. The same procedure should be used for all engines in the system. Then you can generally get the flow rate of the heating system from its specifications. Now you have the total and individual flow in your system and you are ready to get to the really serious part of the design. The object of your design should be to keep pressure drop as low as possible where the various items take their fuel. The single most important thing, as you have already found out, is to make sure that all unburned fuel is always returned to the tank from which it came. Your available information is not enough to make many specific recommendations about the piping, except to say that suction lines can be too small, but they can't be too large. If you can get the necessary information and a drawing, I'll be glad to look at it and help with your design. The last time I did such a thing was with a boat in New Zealand. CaptnWil