BY OUR TECHNICAL EDITORS
Where Does The Gunk Come From?
In our ongoing new column, devoted to the technical challenges that frustrate all
boat owners, our top tech team is standing by with the answers
In my sailing catamaran, the brass pickup tube inside the
plastic fuel tank is severely fouled with a black substance. Once
dry, it can be scraped off with a fingernail and crumbles to dust
when rubbed between fingers. Fouling is both inside and outside
the tube, but much worse on the outside. The tank interior is
clean on the sides and bottom and doesn’t have any deposits. I
use diesel fuel with ValvTect Diesel Guard and ValvTect BioGuard
biocide. Regular fuel filter changes don’t evidence any fouling. Do
you have any idea what the fouling on the fuel pickups might be?
— Gregg Merkel
Green Cove Springs, FL
Don Casey: It’s probably some form of microbiological
growth that somehow finds that the brass provides a viable environment for growth despite your chemical treatment. You can
determine if it’s biological by spraying a scraping of the deposit
with chlorine bleach. If the substance turns white, it is biological.
It might also be asphaltene or some other tar- or varnish-like residue from the fuel itself. Here the usual test is to put a scraping on
a piece of glass and spray it with WD- 40. If the contamination is
oil based, bleach will have no effect but the WD- 40 will emulsify it.
A third possibility is corrosion forming on the dry part of the
tube as the tank level declines. When it’s dry, the corrosion formed
on brass should be a light green color. Examine the “powder”
carefully to see if it looks like corrosion. In all cases, if your tanks
are otherwise clean, simply clean the pickup tubes — inside and
out — and refit them, making a note to check on this again in a
few months. It might have been the one-time result of some earlier
condition with the tank or fuel.
Freewheeling Prop A Good Idea?
This past September, my wife and I purchased a Carver 405
aft cabin. Does it do any damage to a motor or transmission to
cruise at 8-10 knots using only one motor? Some people say yes,
some say no, some say put the motor not being used into gear to
keep the shaft from spinning. I always start both motors and use
them for docking, so fluid is pumped through both transmissions.
Don Casey: The correct answer depends on the type and construction of the transmission and should be directed to the transmission
manufacturer. That said, almost any transmission can safely freewheel for a couple of hours without significant consequences. I’m
guessing your transmissions are ZF Hurth, and if so, these boxes
can safely freewheel. There will be, of course, bearing wear, both in
the transmission and for the shaft, but it will be negligible. You will
need to know what kind of shaft seal you have because a dripless
seal lubricated with water from the engine will burn up if the shaft
spins without the engine running.
It may seem counterintuitive, but the drag of a freewheeling prop is significantly more than that of one locked in position
because the spinning blade develops lift — exactly like an autogyro. For this reason, you might want to lock the prop rather than
allow it to freewheel. In almost all transmissions, that means putting the transmission in reverse, never forward. In for ward gear, the
water flow over the prop causes the shaft to exert rotational forces
on the forward clutch, with clutch wear almost a certainty.
Water In The Box
I found water in the electronic box containing our Simrad
electronic compass. We don’t know how water got in there.
Should we relocate the new replacement, or is there a way to seal
it better than the original, so water won’t enter the compass again?
— John Cambra
John Adey: You must determine how the water got onto the unit
inside the boat. If you have a leak that’s damaging the compass,
He’s been one of
the most consulted
experts on boat care
and upgrading your
boat for 30 years,
and he has been on
our BoatU.S. “Ask The
Experts” website for
the past decade. He
and his wife cruise aboard their 30-footer part
of the year in the eastern Caribbean. His books
include Don Casey’s Complete Illustrated Sailboat
Maintenance Manual, and the recently updated
This Old Boat, the bible for do-it-yourself boaters.
The VP/Technical Director
for the American Boat
& Yacht Council (ABYC),
John grew up boating.
He’s been in the indus-
try since 1990, with
diverse experience from
a yacht brokerage and
boatyard to owning a
marine supply store. He and his family sail their
classic 1976 Irwin ketch, a boat he completely
restored, including the hands-on installation of
all systems. John is a trusted source for techni-
cal information for industry professionals.
He’s maintained, lived
aboard, and cruised long
distance on boats with
his wife and family for
most of his adult life.
He can take apart and
fix almost every system
aboard a boat, has writ-
ten two books, filmed
a two-set DVD on East Coast cruising, served in
numerous editorial and columnist positions for
top marine magazines, and has won seven first-
place awards from Boating Writers International
and many awards for his technical writing.