Here's how to install an in-hull transducer on boats with a cored hull.
First check that the spot for the transducer is indeed cored. Many cored hulls have solid laminates in areas like around the keel. Might save some work, but not much.
If you are not confident with fiberglass
work, you better do this during a haul out
. The project
is simple however, and can be done in the water as long as you keep thinking ;-)
This is a fun project
and perfect to learn how to use fiberglass
First thing to do is get a foot of fiberglass pipe for exhaust
systems (Centek industries, from the waterlift mufflers), 3" internal diameter. Now, find a way to put the pipe in the chosen spot straight up, using a bubble level, laser or whatever. Measure the max. distance between the pipe's lower end and the hull. Find something, like a book or piece of wood, as thick or high as that distance plus, say 1/2". Put it on the hull against the pipe and transfer the deadrise angle onto the pipe with a felt tip marker this way.
Cut the pipe, sand the outside and inside of it and put it back. Don't worry too much about 100% fit as it will have to be fine tuned later on anyway. Now, draw the outline onto the hull and remove the pipe.
Now decide the height of the pipe. Look carefully at the pipe and the transducer plus thru-hull fitting. Figure 1/2" between emitting-surface of transducer and end of pipe. Cut the pipe straight at the desired length.
Now we need to close the top of the pipe: fun! Put some clean pastic wrap on a work surface and cut a piece of fiberglass cloth round with 5" diameter. Put the pipe onto it and use scissors to cut V's out of the cloth around the pipe so that the flaps can be folded onto the outer pipe wall. Take the pipe off and wet out the cloth using a spreader. Put the pipe on and use a brush to first wet out the outer surface of the pipe and next fold the flaps onto it.
Prepare the next layer on new plastic wrap and wet it out while waiting until the epoxy just starts gelling. Turn the pipe around and carefully remove the plastic wrap and put it on the next layer, making sure to stagger the V's cut into the fabric
If you are experienced a bit or always go boldly, you can prepare multiple layers on the plastic, wet them all out while stacking them. Put the pipe on top and fold the flaps, layer by layer. Use slow hardener if you need more time.
Build up the same thickness as the pipe walls. Finish with first 2" wide tape around the pipe (2 turns) to cover the flaps, followed by 4" wide. If almost cured but still sticky, use peanut butter consistency with colloidal silica filler to put a fillet on the inside of the pipe.
Let fully cure and install the thru-hull fitting (holesaw, LifeCaulk or 3M 101 sealant). Drill a 2nd hole and tap it for a plug
. This is to let air escape when putting the transducer in wile filled with mineral oil. You can also use it for pouring the oil in with fitted transducer.
Now the fun really starts. You need to cut through the inner fiberglass layer of the hull. You may touch the core
, but never the outer layer of glass. There are many ways to do this, I'll describe some:
- If the bilge is almost horizontal, you can use a holesaw mounted on a longer pilot bit. The bevel of the pipe must be less than the hull thickness, or you will start drilling/cutting the outer layer. I would prefer another method, especially if you never cut a hole in your hull like this before.
- Rotozip tool is perfect. If the deadrise angle is big, you need shims to keep the tool upright. But if you are handy you can even do this by just holding the tool straight up.
- Jig saw. Use medium metal blade for curved cuts and put something under it to limit the depth of the cut.
- Dremel tool with cut-off disc for plastic.
Now you need to remove the cut inner layer. The core
will break if you put pressure on. I always prefer to make additional cuts so I can remove smaller pieces. I put a flathead screwdriver in the cut to force the layer up, protecting the "good" glass with something so that the screwdriver doesn't crush it. This phase normally only takes minutes.
Now you need to remove all the core. Use whatever you like: wood chisel, Dremel etc. Sand the inner side of the outer layer, the sides of the core and the area around the hole.
Look at the cut away inner layer of fiberglass. It will probably be thinner than the wall thickness of the fiberglass pipe, but if not, you need to increase the wall thickness of the pipe so it matches. Use epoxy and wrap with 2" wide fiberglass tape. Use fast hardener when the epoxy gels. brush more on so that the weave of the tape is filled. Now, you need to enlarge the cut-out in the hull to make it fit again.
Fit the pipe. It must meet the outer layer of the hull everywhere. Use a file and sand paper to shape the pipe. Also, check that the edge of the inner layer doesn't interfere with the fit. File it away if so.
Now you are ready for assembly. Mix epoxy and wet out the cavity (hull, core and edge of inner layer) and the outside part of the pipe that goes in. Wait until this epoxy starts to gel (cigarette break). Make a new batch of epoxy with high density filler to make peanut butter consistency and coat the side of the core and just where the pipe meets the outer layer of the hull. Put it in place. Excess epoxy will squeeze out inside the pipe. Use it for a fillet, adding some mixture if needed. Make a fillet between outside of the pipe and inner layer of the hull too, with the same high density mixture.
Let cure and sand. Wet out the outer fillet and surrounding area and wrap 2" wide tape over the fillet and lower part of the pipe. Fill weave. Cut a piece of fiberglass cloth like 12" diameter round with a center hole that fits over the pipe. Drop it over the pipe so that it covers the fillet/tape and runs out into the bilge
. Wet out and fill weave.
Ready. This area of the hull is now stronger than before the job. Even if the outer layer of fiberglass would be penetrated, the pipe and lid and thru-hull fitting act like a 2nd layer.
Another advantage of having an in-hull transducer is that you don't get growth on it. As for temperature: the oil will be very close to water temperature.