"Capt. JG" wrote

No... didn't miss it. He's dealing with bio/water issues every day. I
think I'll trust his experience, although he didn't say to be obsessive
about it. He's advocating exactly that... minimizing the surface area.

But he's missing out on the frequent opportunity to have the sloshing of a
partially full fuel tank knock off some bio film sticking to the sides.
He's also increasing the average age of the fuel with its absorbed water
content and oxidation. I've been working with commercial boat operators for
nearly four decades and never heard of trying to keep fuel tanks topped up
until this newsgroup.

The yard manager I brought my non-running engine to when the boat arrived in
Portland had similar or more experience. He successfully diagnosed that all
it needed was a thorough cleaning after the Yamaha shop and dealer in MI
told me it needed two new injectors and possibly an injector pump. The
engine has been running perfectly for three seasons on those components.
That gives him a lot of street cred in my book. His position on fuel was
that you should use up as much as you can before refueling so that most of
what you are burning is as fresh as possible. Why do experienced people
differ on points like this? Because it doesn't make much difference.
Topping off might make sense for the once a month weekender but would be
crazy for the almost daily sailor such as myself whether it was going to the
fuel dock and standing in line, doing the docking and undocking, etc., for a
gallon or lugging a can all the way down the dock at the end of the sailing
day when tired guests want to get home. The fuel sitting in that can is as
likely to go bad as what is in the tank. Do you keep topping up the can
that you top up the tank with? Where does it end?

What micron sizing do you use for the primary and secondary?

The Racor primary is 2 Mu because the polishing system keeps much crud from
every reaching it and it is oversized for the engine so there is no downside
to the finer element. It's pressure fed by the electric fuel pump.

The secondary is a Yamaha engine mounted filter and probably 10 Mu. Since
only their elements fit, I have to buy whatever they supply for the engine
anyway.

The polishing filter is 10 Mu but, as Rich H points out, will take out
smaller stuff because the fuel goes through it many times and it is a depth
filter instead of a surface filter. It also gets finer as it loads up.

--
Roger Long