Why is holding tank contents more likely to run out of the tank with
the fittings towards the hull than towards the centerline? Let's say
the fittings are on the hull side on the port side (as in my case).
Then, at a certain heel angle to port, the level of liquids inside the
tank will reach the fitting and overflow may occur (yuck).

Yep...because stuff runs downhill...So when you're heeled to port, if
fittings are on the port side of the tank, contents can spill out the
vent or run back toward a head on the port side. But if the tank
fittings are toward the centerline, contents will run to the port side
of the tank when you're heeled to port, but have no place to go because
the lines are on the other side of the tank.

But if the
fittings were towards the centerline, why wouldn't the same occur when
the boat heels by exactly the same angle to starboard??

No, because stuff DOESN'T run UPhill...your head and vent fitting are
still on the port side...right? So when you're heeled to starboard, it's
an uphill run from the centerline to a head and vent thru-hull on the
port side of the boat.

Liquid in a tank is always gonna run toward the low side of the
boat...so it's always gonna run away from any fitting that's toward the
centerline on one tack... any hoses running across the tank from the
side toward the centerline are gonna be running uphill on the other tack.

If the tank is set on the centerline--for instance, a bow-shaped tank
under the v-berth--the fittings should ideally be on the top and in the
center of the tank. But even if they're on the end, tank contents will
run to one side or the other, never toward the hoses, when you're
heeled...the boat would have to be stood on its stern to create a
downhill run in either direction.


--
Peggie
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Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/html/get_ri...oat_odors.html