THE PROPER FASTENER
>From "The Practical Sailor Library - Volume IV - Do-It-Yourself
Improvement Projects"
Screws
Bolts
Rivets
Clicking Here.
Do you remember Richard III's lament about losing a battle for lack of a
nail? By contrast, use a nail in a fiberglass boat and it is likely that
the boat
will be lost. Nails, as well as wood screws, have few applications on a
modern boat. The reason is simple: Almost any fastener will do a better
job than
either.
The most practical fastener material is stainless steel. Stain-less
steel is strong, corrosion-resistant, and galvanically the most passive
of fastener
materials with the exception of bronze. Best of all, stainless steel
fasteners are commonly available, even in areas where an anchor is
regarded as an
odd example of free-form sculpture.
Let's take a close look at the choice of fasteners available to the
do-it-yourself boatowner:
Screws
There is almost nothing aboard a boat that can't be fastened with the
self-tapping screw (also called a sheetmetal screw). Self-tapping screws
are, of course,
ideal for screwing into thin, hard metals or even moderately thick soft
metal such as aluminum. All it takes is a pilot hole drilled to the
optimum diameter.
Self-tapping screws work almost as well in most woods as they do in
metal. They come in a variety of head configurations in sizes up to #14,
and in lengths
from 1/4 inch to 3 inches. Just like a wood screw, a self-tapper can be
countersunk and. bunged, or set with the head flush.
Where self-tapping screws truly excel is in fastening into fiberglass
laminates, whether cored or solid. They should be used to carry light
loads only,
however; they are not a substitute for bolts for heavier loads. As with
any screw, the strength of the fastening itself is generally greater
than its holding
power (it will pull out before it will break off). As a rule of thumb,
the laminate into which the screw is driven should be at least equal in
thickness
to the diameter of the screw.
For all their virtues, self-tapping screws do have some notable
limitations and drawbacks. The number of times they can be removed and
redriven is finite;
sooner or later they wear out the hole, diminishing their holding power.
Holding power is also reduced if the pilot hole is oversized, and they
may be
impossible to drive if the pilot hole is undersized. Remember too that
the holding power of the screw is no greater than the strength of the
material into
which it is driven. Soft woods, thin laminates, and thin metals cannot
carry much of a load.
Conventional wood screws are superior for fastening wood joinerwork, but
they should never be used to fasten into fiberglass laminates.
Bolts
For heavier loads, for pulling two surfaces together, and for fastenings
that may be repetitively tightened and loosened, bolts are the answer.
These functions
are in direct contrast to those that screws perform best. Machine screws
come with flat, round, and oval heads, all slotted for use with a
screwdriver.
Other bolts are available with hexagonal and square heads for use with a
wrench; recessed (socket) heads for Allen wrenches; and rounded,
carriage heads.
Nuts for use with these bolts are hexagonal, square, wing (for hand
tightening), jam (self-locking, also called aircraft nuts), and cap
(acorn). All are
capable of taking a variety of washers underneath. Machine screws axe
often used in heavier metal without a nut by drilling and tapping the
hole. (To be
used in this manner, the metal should be at least as thick as the
diameter of the bolt.)
Nuts are intended to be tightened against a washer. The washer not only
spreads the load on the lower surface of the material being bolted, but
prevents
the nut from cutting into that surface. As many applications aboard a
boat involve heavy localized loading -- more load than a washer alone
can handle
-- bolts may need a hard backing block of aluminum or fiberglass in
addition to a washer to better spread that load. A good sealant should
always be used
with through-bolts in a hull or deck, as the bolt holes provide
passageways for water.
Rivets
Pop-Rivet is a brand name, but rightly or wrongly, the term is becoming
generic for the type of blind fastener that expands when the center pin
(mandrel)
is extracted and broken off. Such rivets are commonly used in
applications that would call for a bolt, but where location prevents
turning on a nut (in
attaching mast fittings, for example). Since pop rivets are also quicker
to install, they are frequently used instead of bolts in production
applications
(such as the hull-to-deck joints on smaller and cheaper boats).
Pop-rivets are available in both aluminum and stainless steel, with the
stainless steel ones use for heavier loads. The center pin is pulled,
expanding
the body of the rivet, until the pin breaks ("pops"). They may be set up
with either a hand tool or a hydraulic tool. A word of warning:
Squeezing the
hand tool is a macho exercise, difficult for the aluminum rivets,
herculean for the stainless steel.
Removing rivets entails drilling them out, a job that must be done
carefully to prevent enlarging the holes in which replace-ment rivets
must fit. In removing
rivets (for stripping a mast for painting, for instance), you may want
to plan from the outset to use the next larger diameter when you replace
the rivets.
* * *
Fasteners bought one by one, or even a dozen at a time, are outrageously
priced. Bought individually, or in a blister pack of five or six, a
one-inch, #10
sheetmetal screw will cost about 15 cents. Bought in bulk, typically 100
to a box, they run closer to 5 cents apiece. At $4 a box, the same money
that
buys three dozen individual screws will buy a box of a hundred. If you
don't want to pass on the extra screws as part of your estate, surprise
your fellow
sailors with the gift of a handful.
And finally, keep in mind that a well found yacht does not need an
infinite variety of fastener sizes and types. A typical 35-footer can be
built with just
a few basic fasteners: 3/16-inch and 1/4-inch flathead machine screws in
two or three different lengths, a box or two of #8 and #10 self-tapping
screws
in 3/4 and 1-1/4 inch lengths, and a couple boxes of #8 wood screws for
the joiner work. Where the underside of a bolt or screw is hidden, the
excess length
makes no difference except to a racing sailor for whom any extra weight
is a cause for fantods. Of course, don't leave the excess length exposed
where
it can cause an injury or snag a sail.
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