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From: Candy Chapman and Gary Bell (no email)
Date: Mon Mar 03 2008 - 12:15:31 EST
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
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