Fuel tank air problem

Hi all, I would need some help for finding the best solution of fuel system air problems. After sailing in side wind the engine, Nanni Kubota 14 hp, started as usual but stopped after a couple of minutes. Manual bleeding did not help, and I had to be towed to the port (not fun). This happened twice. There was a lot of air in the water separator. All hoses and connections were checked and one of them replaced. Helped a bit.
My old small V-shaped metal tank under the fore deck was replaced by a 100 litres (22 US gallons) flat bottomed tank in the aft a few months ago. It lies abeam about 4" above the engine fuel intake. No problems ever with the old gravity fed system, and I think the main problem is air getting in the system from the tank when it is only half full. Works all right when motoring here on the European inland waters, but probably not when sailing on the Med next year.
Now, should I install a different tank in the same or another place, put a check valve or a ball valve in the line, or something else? And should the fuel return be moved to the tank? Would really appreciate your help.

 

Connecting the fuel return line to the top of the tank is the 1th thing to do. If any fitting isn't 100% airtight or the pickup tube finds some air due to sloshing in a nearly empty tank, the fuel return line is the only exit.

 

.....excellent example that shows why fuel tanks should be tall (ie "small footprint") and never have a flat bottom! Flat, low tanks let the fuel slosh around; the fuel will be supersaturated with air in the form of small bubbles. The smaller the bubble, the lower its rising velocity, hence more air travelling to the water separator.

So: reshape the tank bottom, make sure there are dividing walls in the tank, and take the fuel return all the way back to the tank. NOTE: the return shall end close to the tank bottom (even if it is taken through the tank top), and it should be arranged far from the outlet, so that there is no short-cut into the suction.

Alternatively, use the old tank as a daytank feeding the Engine, and the new one just feeding the daytank. Still, take the fuel return to the new tank.

 

Thank you CDK and baeckmo, problem solved! The installation job in this old Shipman 28 is a little tricky, so I haven't found any suitable standard tank. Will order a new tailor-made tank from a Dutch manufacturer. Then this crazy single-handed journey with my old dog from Finland to Italy can continue with one thing less to worry about
Dank u wel / Tack så mycket!

 

Not sure what the rules are in Europe but you should install an anti-siphon valve at the top draw of the tank. If you have a fire, and the hose melts, the fuel will not siphon into the bilge and feed the fire
This is a small spring loaded check valve installed in line with the draw line, maybe 10 euros

 

Thank you Barry, that's a good point. Don't know what the regulations say, but it's always safety first.

Regards
Riitta