Thru-Hulls and Transducers Replaced In the Water
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Amanda McCallister, editor.
Hull & Bottom Cleaning | Marine Salvage & Recovery Three weeks back I dropped a new Airmar B744V into a 41-foot Sea Ray tied in a Fort Lauderdale slip. Viz was maybe two feet, the tide was falling, and the owner was already yelling about his Bimini run at six a.m. I eased the housing through the hole from above, had my mate inside tighten the nut to 30 inch-pounds, and by the time I surfaced he had the fishfinder flashing. Don't get me wrong—hauling out is the cleaner, drier, easier road. But when you can't get a slot, or the yard wants a grand to block a 40-footer for a weekend, an underwater swap is a whole lot less painful. I've been doing underwater repairs on props, running gear, and hulls for over fifteen years in and around Port Everglades, and I can tell you the process is a specialized dance between you and a buddy. You just need to respect the hull and know your sealant.
First, Make Sure You Should Be Doing This
Not every hull is a candidate. If you have a solid fiberglass hull, no coring to speak of below the waterline, and a backing nut inside that you can reach, you can usually pull it off. If your boat has cored hull—Divinycell, balsa, even plywood—that core needs to be sealed completely dry before any fitting goes in. Underwater, forget it. You'll get water migrating into the core and then you've got bigger problems than a shaky depth reading. Also, check the actual fitting. A thru-hull transducer like the Airmar B744V has a threaded housing that bolts through the hull. You can swap the internal sensor from inside the boat without a diver. But if you need to change the housing itself, that's the full deal. Same for a bronze thru-hull like a Groco IBVF. It has an outside mushroom and an inside nut, so you need coordination. Ask the yard what's the haul-out window. Many South Florida yards are booked weeks out. If the guy can't wait, this is a legitimate fix.
Gear and Prep
The dive gear is simple: wetsuit, mask, a brush, and a pouch with a bunch of tapered wooden bungs. I always carry bungs from a half-inch to four inches. You'll also need two-part underwater epoxy like Philadelphia Resins Splash Zone. It cures wet and doesn't mind a little salt. Don't use 3M 5200; it's too thick and won't cure right under pressure. Use a product made for it. Inside the boat, your buddy needs the right tools. A socket or a wrench that fits the nut exactly, a torque wrench (most transducers are spec'd in inch-pounds, not foot-pounds), and a tub of rags. For the Airmar B744V, the manual calls for 30 in-lb, but all transducers are different. The spec is written on the box—read it. A quarter-inch nut doesn't get foot-pounds. Before we start, I clean the outside of the hull around the old fitting. I use a stainless scraper and a Scotch-Brite pad to knock off any growth and paint. You want bare, smooth gelcoat. If the old fitting is stuck with hardened sealant, you might need a hammer and a blunt chisel, but go slowly. You don't want to crack the hull.
The Swap: Step by Step
Here's how it works, start to finish. With the inside person on the nut, you (the diver) hold the outside of the old fitting firmly. The inside person breaks the nut loose and turns it all the way off, then the diver pulls the old fitting out from the outside. If there's any pressure, jam a bung in the hole immediately. That's why you have them. Then you clean the hole again. Check the bore for any obvious cracks or delamination. If you see anything iffy, this is your stop—announce it and shut it down. On a solid hull, you're okay. Mix the Splash Zone per the instructions, and apply a quarter-inch bead to the underside of the new housing's flange. The new fitting goes through from the outside. I apply the bead, then push the threaded tail through the hole while my mate inside catches it and spins the nut. Tightening hand-tight, then torque to spec. The sealant smooshes out all around—that's normal. We let it set for at least the pot life, which is usually twenty to thirty minutes. I don't leave the water until it's hardened enough to hold.
The Transducer Part (And the One That'll Drive You Nuts)
The actual transducer element itself—the part that pings—is often a separate insert that screws into the housing from inside. On an Airmar B744V, you can swap that insert with the boat in the water, no diver needed, as long as the housing is sound. You unscrew the old element with your hand or a yank, then thread in the new one, and torque the little retention ring. That's it. It takes five minutes. But if the entire housing is cracked, that's where my work comes in. You have to match the old opening, sometimes with an adapter ring. That's when you need exact measurements. I common on the docks to see a guy with a brand-new Garmin transducer sitting in an Airmar housing and struggling. Measure twice, order once. Oh, and mind the fairing block if you have one. Some high-speed boats use a foam fairing that angles the transducer perfectly. When you replace it, you have to duplicate that angle or you'll read turbulent water and get a ghost signal at speed. You can bed a new fairing with Splash Zone too, but it's more work.
When to Shut It Down
There are days I tell the owner no. If the hull is cored below the waterline, don't even try. If the old hole is oblong—you know, someone already patched it with god-awful silicone—no. If the current is ripping and you can't hold position, no. And if the inside access is a crawlspace full of wiring and you can't get a wrench on the nut, no. Underwater replacement is a craft, not a shortcut. It works for solid hulls, for clean holes, for fittings that were installed right in the first place. The rest of the time, bite the bullet, haul out, and do it dry. Your hull's not something to gamble with on a hunch.
Frequently Asked Questions
Can a thru-hull transducer be replaced while the boat is in the water? Yes. If the hull below the waterline is solid fiberglass, the transducer has a separate internal element, and the housing is wired and accessible from inside, you can swap the sensor out in ten minutes. For a full housing replacement, it takes a coordinated diver and inside person, but we do it regularly. Just use a proper underwater epoxy like Splash Zone. Is an underwater replacement as safe as a haul-out? It can be, if the above-water conditions are right. You're only as safe as your bungs and your crew. I never go down without a second set of hands topside. But a hacked-in sealant job can leak, and a dry haul-out will always give you a cleaner seal. If the boat is cored, the answer is no. What does it cost to replace a transducer under water versus hauling out? A standard transducer insert is a couple hundred bucks in parts, and a diver for a housing swap might run $150 to $300 depending on access. Haul-out fees alone often run $25 to $40 a foot, and you'll pay for the yard time on top. For a 40-footer, you're looking at $1,000 before labor. The water job can save you real money. What transducer brands work for underwater replacement? I've done Airmar, Garmin, and Furuno. Most Airmar thru-hulls use a removable insert, which is ideal for in-water work. The housing stays in place and you just unscrew the element. For others, you need to match the thread pattern and housing size. Always cross-reference the part number before you order.