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Old 03-01-2002
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Installing Depth Finders

I have a depth finder that also has sensors to measure speed and water temperature. The instructions say to drill a hole in the hull and screw in the sensor unit. Can you suggest an alternative way of installing the depth finder without damaging the hull?

Tom Wood responds:
If you have a steel, aluminum, wood, or ferrocement hull, get the hole saw out and drill that hole. The only other alternative on boats built of these hull materials is mounting the device on the transom an that's only if the transom of the boat is submerged. In that case you can use a powerboat transom-mount bracket, but remember, this is a great way to determine the depth of the bottom that you just passed over.

Fiberglass boats offer an advantage here in that the hull material acts very closely to water in the way that depth sounder frequencies penetrate it. If the hull is not cored with wood or foam in the area where you want to place the transducer, the signal can shoot through the hull with only a modest loss of signal quality. There are two ways you can take advantage of this:

1. The waterbox method involves building a fiberglass box inside the hull, placing the transducer on the top of the box. The box is then filled with oil or water, or a few small holes are bored in the hull allowing the box to fill with water from the sea (not recommended as it makes the unit unserviceable without major leakage). The transducer signals now go from water, through the fiberglass, and back into the sea with almost no diminution or disruption.

2. The glue method simply attaches the transducer directly to the inside of the hull with an adhesive sealant such as a polysulfide, polyurethane, or modified silicone sealant. I like this system better because itís easier to guarantee success than with a waterbox. Simply wire up the depth sounder and plug in the transducer cable. Select a spot to mount the transducer inside the hull (the boat must be in the water, of course) and unload half of a caulking gun full of sealant on that spot. Put another huge dollop of caulk onto the bottom of the transducer, ensuring there are no trapped air bubbles in the sealant. Mush the transducer into the glob of sealant on the hull, moving and turning it around to eliminate any trapped bubbles. Have someone watching the depth sounder readout while you are doing this and reporting to you when the signal appears to be solid and consistent. Once there, leave the transducer alone for five days to a week to allow the sealant to cure completely, and youíre done.

If you find that you canít get a good signal, and you have some time to wipe up all the sealant (what a mess), then try another spot. Signal failure in the case of a solid fiberglass hull (no coring, metal reinforcement, ballast, or other items encased in the hull) is usually due to a void, or air pocket, in the fiberglass. In this case, moving the transducer only a small distance should give you a clear signal. Itís also helpful to know whether or not your boat's builder used fillers for coring in the construction before you attempt this method.

If you do choose one of these methods, be aware that your temperature sensor will be very slow to respond. Fiberglass has almost no thermal insulation and changes temperature rapidly with a change in seawater temperature. The volume of sealant or water in a waterbox will take some time to respond, however, so donít go looking for the east wall of the Gulfstream with your temperature gaugeóyouíll be in it for a half an hour before the unit starts to register.

We also heard from another reader on this subject. Trey Sunderland wrote in to point out that boats with bottoms covered in a barrier coat such as West System Epoxy with aluminum powder in it, will not allow the transponder's signal to penetrate the hull. Mr. Sunderland suggested "sanding the epoxy down to the gelcoat directly under where you have mounted the sensor inside and then cover that small area with the same barrier coat without the metal additive."

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