"JamesgangNC" wrote in message
hlink.net...


Normal fuel pickups do not
go to the bottom to avoid picking up the water and crud that eventually
accumulates.

The dip tube can't go all the way to the bottom, if it pressed against the
bottom it could seal the bottom of the tube and prevent fuel from flowing.
The bottom of the tube has to be high enough to allow the desired flow rates
and to account for any flexing of the walls of the tank.

The reason they use a dip tube, instead of a fitting off the bottom of the
tank, is because they are require to do so. A marine fuel tank is not
supposed to have any fittings anywhere except on the top.

As far as picking up water and crud, it would be far better if the pickup
was on the very bottom. You will always get a little water into the tank,
either from condensation, splashes in the vent line, or from the gas
station. Usually, the amount of water introduced into the tank at any one
time is very small. Water settles to the bottom, and if it was sucked up
right away it would either get trapped into the water separator (where
someone could drain it out) or actually consumed in the engine. A single
drop of water would barely make the engine hiccup.

If a significant amount of water was introduced at one time you would at
least have the advantage of finding out about it right away. The most
common source of significant fuel contamination is from the gas station, so
if the fuel was drawn from the very bottom you would be a short distance
away from the fuel dock when your engine died.

With the dip tube, any water that gets in will settle to the bottom and stay
there. The layer of fuel above it will prevent the water from ever
evaporating so it will just accumulate. Eventually you will accumulate
enough that some of it gets sucked up the dip tube. This could put you into
the same situation as if you had a feed from the bottom, that any additional
water introduced gets sucked up, so the filter/engine only has to deal with
the amount just introduced.

The problem is that the tank can get agitated. When you shake things up the
water doesn't all stay at the bottom, and suddenly you can be sucking up a
lot of water. Without a separator a few tablespoons of water sucked up at
once could shut down your engine. A reasonable sized tank could have a
gallon or so of depth below the pickup tube, and if this is mostly water
then shaking it up can introduce more water into the system than the
separator can handle.

The real bad part is that this doesn't happen when you are 100 yards from
the dock. This happens when you are being beat to death in rough seas and
you really, really don't want your engine to die!

So, in my opinion, the dip tube is not an advantage for avoiding fuel
contamination problems, it only makes it worse.

Rod