Mounting Depth Finder Transducers Inside the Hull
by Tom Neale
Manufacturers frequently recommend using epoxy or similar
substance if you decide to bond a transducer to the inside
of the hull. They want you to use something that will set
up without bubbles or voids, and that has no filler material.
We never use this epoxy. If you find that you've mounted the
transducer in a bad place in the hull (and you won't really
know this until you start running) you're going to have a
very difficult time moving it without damaging it if you've
We prefer to build a water box. It's relatively easy to do,
the "bond" from the transducer to the hull is much
better than it would be with epoxy, and you can move a water
box if needed. What you use and how you do it depends upon
the location and access. Usually we simply cut the bottom
out of a yogurt container (or something similar), contour
it, if necessary, to the hull, and seal it to the hull with
a product such silicone sealant. You must remove any paint
or grease that may be on the inside hull surface to obtain
a good seal in that spot. We then just fill it with water
and put the transducer inside. You want the transducer to
be pointing down with as little variation as possible, so
a container not much larger than the transducer, or the use
of some other method to stabilize it, will help. Water will
evaporate from the box eventually, requiring replacement.
It will last longer if you cut a slit for the transducer wire
in the top of the container and put the top back on the container.
This will also help to support the container walls.
Location is key. It's best if the transducer is reading through
a section of hull that is relatively thin and that has no
voids. Thicker areas are normally found around stringers and
joints, such as those where the transom and hulls join the
bottom. Avoid areas where there are build ups of fiberglass
tape laid around structure features. Flat planes usually give
the best results. Obviously, you can't know in advance whether
voids in the hull laminate exist. Tapping may give a clue
if the void is large, but small voids will also interfere
with your reading. They can occur in the best built boats.
It's also important that there be as little turbulence under
the transducer as possible. This can be caused by many different
structural or other characteristics of an outer hull surface,
and the effect of a particular characteristic on turbulence
can vary with speed and sea state. These are reasons why we
never epoxy our transducers to the hull, and prefer to not
mount them through the hull. We've found that we never really
know the best location for a transducer until after we've
used it under way in varying conditions.
Some manufacturers recommend against a material such as Life
Calk or silicone sealant for bonding the transducer to the
inside of the hull. There is greater likelihood of air pockets
as the sealant sets up and it isn't as good a transmitter
of the sound waves as is epoxy or water. However, we've used
this successfully for temporary tests of hull locations. If
you use these products, try to get as thin a layer as possible
between the transducer face and the hull, and carefully mash
out any air pockets. Beware that if performance isn't optimal
using these sealants for testing purposes, part of the problem
may stem from the use of the sealant itself. But the transducer
will usually work well enough with such a sealant to let you
know whether you've got a really bad area of hull. If you've
found a good area, they it's easy to remove the sealant and
install a water box over the spot.
See Tom's Tips in his cruising section on www.boatus.com
for information about why you might want to have a water box
inside the hull, instead of a thru-hull installation and for
important East Coast Alerts.