Transducer Thru-Hull Fittings

tilted element
A tilted element transducer self-levels for boats with steep deadrise

My buddy Charlie and his brother Bill, co-owners of a 23-foot 1977 Slickcraft SS235, were given a new fish-finder as a present. While the brothers already have an old Hummingbird model with a transducer mounted on the stern, they wanted to install the new one that came with the fish-finder, due to its larger color screen and added features; they had even purchased the exact size hole-saw they would need, but first they consulted with me about mounting the finder’s thru-hull transducer.

Few people are comfortable drilling holes in their boats, especially below the waterline, but new fish-finders, boat-speed or depth-finders come with a transducer that needs to be mounted so it can measure depth, speed, or find fish. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a transducer. Here are a few tips to guide you through the process of installing a thru-hull transducer.

Slickcraft SS235

Since Charlie and Bill’s boat hull is solid fiberglass, the mounting process is a little easier than for cored-hull construction (discussed a little later in this piece). The first step is to find the right location in their boat so that the thru-hull stayed submerged below the water line.

I asked them about the kind of thru-hull fitting they intended to mount. It’s an old style brass fitting with a high-speed fairing block that needs to be mounted level so it points straight down, even if requires installing a wedge to make it plumb. They knew about the stern-mount type fittings because they already had one. They seemed to be unaware of the tilted element type that mounts flush and self-levels—this type may be right for you if the deadrise (the angle from the keel to the turn of the bilge) is steep. Given the shallow deadrise and relatively flat bottom on their boat, their fitting would not be a problem. Most manufacturers can provide a different type of transducer to meet your installation needs/budget.

The next issue is to find a location to mount the thru-hull that not only stays in the water, but also gets minimal turbulence.

Thru-hull Fittings on Powerboats

thru-hull diagram
The transducer slides into a housing-the housing allows you to remove the transducer for cleaning / inspection.

A good location for placing transducers on powerboats is where turbulence is minimized and the bottom is relatively flat—about a foot to the side of the keel and roughly amidships.

Make sure you place them where you have decent access from above—you’ll need to get at them from time to time for cleaning. If you leave transducers in for any length of time, the tips may become fouled with growth. To prevent this, I like to keep painted dummy-plugs in my boat when I’m not aboard. Just unscrew the transducer, pull it out, and after a quick gush of water insert the dummy plug tipped with bottom paint into the transducer housing and screw on the cap. This will keep your bottom and transducer clean. I leave a sponge in the bilge right next to the transducer for quick clean up, and I challenge myself to keep the plug ready and let in as little water as possible.

Thru-hull Fittings on Sailboats

Most speed and depth transducers perform well when mounted just forward of the keel or centerboard. I like to see them placed a foot or two off centerline, depending on the size of the boat. There is usually a relatively flat spot there and this area will stay underwater and have less turbulence than behind the keel. See special flush-mounting options for racing sailboats, and also read about cored hull considerations below.

Installation tips:

Raymarine B744V thru-hull transducer
Raymarine bronze thru-hull transducer with high-speed fairing block
  • When your boat is on the hard or even on a trailer it is not always level, so don’t true the transducer to the boat – true it to vertical by using a plumb bob.
  • Use underwater caulk/sealant like 3M’s 5200. It won’t come off easily, but in this case you don’t want it to.
  • On cored boats, water intrusion into the core can be a problem even if it is not leaking inside the boat. Water in the core can freeze in the offseason and cause delamination in the hull. To prevent this, fill the area around the transducer with epoxy to seal the core. The way to do this is drill the hole for the transducer through the inner and outer skin. Now use a slightly larger hole saw to cut only the outer skin and the core material centered around the first hole. Clean out the core material around the whole with a sharp chisel. Now tape the bottom hole from the outside, fill with epoxy from inside. When it hardens redrill the original size hole for the transducer and install. It should be surrounded by a solid epoxy wall all around and the core is sealed.


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