Story And Photography By Steve D’Antonio
n Part I of our two-part series on seacocks, we discussed ABYC guidelines and the importance of
suitable design and material selection. Now we delve into the details of how to properly install this
component, one of the most critical aboard any cruising vessel.
The importance of proper seacock and through-hull installation cannot be
overemphasized. After all, the watertight integrity of your vessel relies on these components.
Properly selected materials that have been improperly installed may lull you into a false
sense of security. While the following information applies to experienced do-it-yourselfers
and professionals, a thorough understanding of correct seacock installation techniques will
benefit any boat owner who might depend on others to carry out this vital task.
Before beginning installation, ensure that safety gear is used: eye protection, rubber
gloves, and a dust mask and/or respirator. Although there are several common installation
methods, my preferred approach is as follows. For a new seacock installation in a solid,
un-cored hull, select a location and drill a 1/8-inch pilot hole. After confirming that the
installation location is correct, use a hole saw to drill a hole that is as close as possible to
the through-hull fitting’s outside diameter. A snug fit is desirable; a 1-1/2-inch hole for a
1-inch through-hull fitting is less than ideal, while a 1-1/8-inch hole for the same fitting
would be fine.
Replacing an existing seacock with a like unit requires no drilling, provided the existing
hole is correctly sized. However, the old through-hull and seacock must be removed, a task
that presents its own set of challenges. If the through-hull and seacock cannot be easily
separated, the through-hull’s “mushroom” may be ground off using an electric or
pneumatic grinder and a 36-grit sanding disc. If this is done carefully, without damaging the
gelcoat or fiberglass (heavily masking the area around the through-hull will help), the
process may be accomplished quickly and easily. Once the mushroom is gone, the seacock
may be withdrawn from inside the vessel.
A backing block, an essential component for any seacock installation in a fiberglass or
timber hull, should be fabricated from 3/4-inch AA marine fir plywood or prefabricated
fiberglass sheet (the latter is commonly referred to as GPO- 3). High-density polyethylene
materials such as King StarBoard should not be used as backing blocks because they are
slippery and therefore seal poorly. Solid timber, regardless of species, should never be used
as a seacock backing block because of its propensity to fracture under load, particularly
when used against irregularly shaped surfaces like the inside of a fiberglass hull.
The size of the backing block should be proportionate to the size of the seacock’s base.
In general, a round backing block that shows a minimum of 1 to 1-1/2 inches between its
outside perimeter and the seacock base is appropriate. Thus, a seacock whose base measures
4 inches in diameter should utilize a backing block with a diameter of 6 or 7 inches.