T&T: Day Tank Design, was Fuel Manifold

[Candy Chapman and Gary Bell]

To resolve the issues discussed regarding fuel manifolds I have designed 
a day tank system which also includes fuel polishing and an attached 
gizmo to eliminate fuel fill barfing from the fill fitting or the 
overside vent.  My boat is a PDQ 34 power catamaran, with two Yanmar 
75hp engines, one mounted low in each hull aft of course.   It has a 115 
gal. aluminum tank mounted in the center, spanning the bridge deck just 
above the engines.  The system already gravity feeds to the injection 
pumps without lift pumps.   Another 70 gal. tank is mounted forward, 
which gravity feeds the genset.  I have a bi-directional fuel transfer 
pump. 

The plan for my boat includes a single day tank mounted above the main 
tank, providing even better gravity feeding for both of my engines and  
with both engine return lines fed back into the day tank.  The day tank 
input will be provided by adding a single robust lift pump, followed by 
fuel polisher style filters.  The pump flow rate will be selected to 
optimally operate the turbine water separator in the active filter and 
will far exceed the fuel needs of the engines.  This provides 
significant overflow of fuel (now polished) returning to my main tank 
through a generous sized overflow hose, which will exit the day tank a 
few inches below the top.  This overflow is large enough to completely 
handle the lift/supply pump's output without the engines running, so the 
system can be used for polishing the fuel in the main tank when the The 
day tank filling pump will also be guarded by a float switch fitted just 
above the overflow hose exit.  The day tank will vent from its top into 
the top of this fill chamber (from above the overflow and safety float 
switch, so the vent serves as a third backup preventing the day tank 
from overfilling).  An air vent in the top connects to the top of the 
fuel fill chamber -- more on that in a moment.  A second float switch 
mounted somewhat lower than the return to the main tank will warn me if 
the lift pump isn't keeping the day tank full.   At 15 kts my boat only 
consumes 4 gal. per hour, so most of the fuel arriving from the 
polishing filters will return to the main tank through the overflow, and 
the only instances when the day tank level would drop to trip the second 
float switch will be when the supply pump fails or more likely the 
polishing filter needs to be switched to it's alternative twin.   I 
haven't measured the temperature of the fuel returning from an engine 
(bypassed fuel is chiefly used to carry heat away from the injector pump 
and injectors).  I will use some temperature measurements to guide me in 
sizing the day tank -- I don't want the day tank fuel to get too hot.  I 
am considering a tiny drain line between the bottom of the day tank and 
the fill tube heading for the main tank, to slowly empty the day tank 
when it is not in use.

My boat has a terrible problem with barfing sometimes a liter or more 
fuel up through the fill fitting at the end of filling.  Slowing the 
fill rate to a trickle at the magic time only lessens the mess.  So the 
spot I chose for the day tank will be shared by a smaller tank 
incorporated into the main fuel tank fill line and serving as a burp 
proof fill chamber.  The vent line for the main tank will also feed into 
the top of the fill chamber, so the fill chamber can catch any fuel 
escaping from the main tank.  The regular overboard vent line will exit 
from the top of this fill chamber, to relieve any air pressure 
differences in the system but with virtually no risk of fuel escaping 
through the outside vent.    I am looking at one of those clever new 
whistle gizmos to mount in the vent line from the main to the filler 
chamber, hoping to get an appropriate sound to tell me to stop fueling.

Construction will be in welded aluminum, like my fuel tanks, and hoses 
will likewise be CG approved flex with swaged ends. 

Comments, criticisms, whatever?

Gary Bell