I have used silicon twice in the past for "temporary" testing in this
situation. (Figuring I would go back and fix it later once I found the
best spot). One was 4 years ago in my dingy and the other was 3 years
ago in my sportfish. Neither hull is cored. Both are still working
near perfectly for shallow water. (200ft) and high speed. I just cut
the caulking tube tip off at the widest point, made a pile of silicon
(no bubbles) and pushed the transducer down into the pile making sure
it was level to the world. I let it dry and tried it out. Since it
worked, I have not gone to back to "FIX" it... i have heard the evils of
doing this but I go back to the "if it ain't broke, don't fix it". I am
sure I am getting some attenuation but for what I am using them for
(SHallow water) I don't need all the power anyway. BTW... I do NOT
use this for my two 1500 ft sounders, those are thru hull with fairing
blocks, etc.

Using epoxy like larry suggested is a better solution but you better get
it right the first time.

Another trick for angled bottom boats.

Cut a piece of 3" pvc (or 4" if you have a big transducer) the approx
angle of the hull (about 2-4" long) glue it in with silicon to the hull.

Once it drys, fill it with water, test your transducer in the water at
all speeds. If the spot works, dry it out, fill it with epoxy, place the
transducer in the epoxy and let it dry. This allows more epoxy to
"flatten the surface"






Larry wrote:

"DPFresh" wrote in news:1144040012.290616.216280
@i39g2000cwa.googlegroups.com:


Hello all, so as oppossed to drilling a hole in my hull for a depth
finder sensor, I should be able to set it up inside the hull as long as
the sensor is in some type of liquid right? IT will be able to tell
depths through the fiberglass hull right?




That depends on whether the hull is cored or not. Fiberglass conducts
sound energy just fine. Hull coring, like a Boston Whaler, is damned
near a perfect sound absorber in the foam core.

So, test it. Hook up the sonar, but leave the transducer loose so we can
move it around and find a hot spot, a spot where it sees through the hull
the best...away from the motor noise, if you please. Put a couple of
inches of water inside the hull so we can couple the sound from the
transducer to the hull inside the boat without mounting it. Take the
boat out into 50' of water.

Now, keep the transducer underwater any time the sonar is on......

Put the transducer, with the sound output surface pointing at the hull,
down we assume, under the water and hold it there against the hull. Turn
on the sonar and see what we gots. See the bottom? Move the transducer
around in this area and look for the best picture. Ok, we found a little
spot it works better than others. Have a second party start the boat and
drive full speed in a straight line while you watch the sonar. Can we
still see the bottom in 50' of water, or did the display go all crazy
because the engine noise and underwater bubbles or cavitation under the
hull right here got in the way? While underway, where we want the sonar
to work, of course, move the transducer around under the bilge water to
see if our hot spot is the "underway hot spot". Note the location of the
best place.

Take the boat home, pump it out and give our hotspot a good scrubbing in
Dawn, because Dawn "gets grease out of your way", like the commercial
says. Use Dawn to clean off the transducer, too. Ok, the hull is squeaky
clean at our hot spot. Dry out the bilge, fully and take a heat gun
(hair dryer) to our hotspot to make it perfectly dry and warm. Heat it
up but don't burn it. While it's still warm put a nice little blob of
epoxy on our hot spot and work the sound output surface, MAKE SURE IT'S
THE SOUND OUTPUT SURFACE, into the epoxy blob to work out all the air
bubbles in the epoxy. Notice I said EPOXY, not bathtub caulk or RTV or
Silicone sealer or any of those rubbery, sound absorbing, compounds like
3M 4200 or 5200 sealer or any of that stuff. EPOXY, 2-part, make-damned-
sure-you-get-it-where-you-want-it-before-it-hardens EPOXY. Epoxy hardens
into stone, a solid mass that transmits sound from transducer to hull.
It will also hold the transducer in place for life.

I had a Sea Rayder 16' jetboat that went about 60. The transom-mounted
transducer made the boat turn from its drag every time you dropped power
because it was jet powered, no rudder. So, I mounted my sonar just like
this and it's still working just fine since 1997.....(c;

Your sonar company, mine was Eagle, has a special transducer with a flat
face for mounting with epoxy. It even came with epoxy. This is the
transducer you want for this type of installation.....call 'em and ask
for it.