Shaft Seal and Stern Tube Repairs Without Hauling Out
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Amanda McCallister, editor.
Propeller Removal & Repair | Hull & Bottom Cleaning Last Tuesday I was under the port shaft of a 58-foot Sea Ray at Bahia Mar. The PSS bellows had a pinhole leak, squirting a thin stream over the hose clamp. The owner had a haulout scheduled for Thursday, but he was flying in from Chicago that afternoon and wanted to fish Sunday. I had about twenty minutes of bottom time before I needed to surface and grab a new bellows from the truck. "We can do this on the buoy," I told the mate. He looked at me like I'd suggested changing an engine block under the same conditions. The truth is, shaft seal and stern tube work without hauling out is a regular part of my week. I've done it on everything from a 32-foot Center Console to a 100-foot Hatteras. It's not glamorous, but it's saved owners a pile of cash and a week of lost run time. The key is knowing what can be done underwater, what has to wait for a lift, and what the risks are when you cut a corner. I'm not saying it's always the right call. But it's not black magic either. Here's how I approach it.
What I Can Actually Fix Down There
A stuffing box is the old-school way: a bronze body with flax packing that you compress to stop water. You can re-tighten it underwater, but re-packing usually needs a haulout because you have to pull the shaft out of the way. Mechanical seals are a different story. A PSS uses a stainless rotor against a carbon face, with a rubber bellows that's held on with two hose clamps. That bellows wears out, and it's the single most common reason I get called. You can replace that bellows in the water, as long as you can keep the seal's water pressure from blowing it apart while you work. The trick is to jam a tapered wedge between the rotor and the carbon face to hold them apart. Then you slide the old bellows off, slide a new one on, torque the clamps to 35 in-lb, and pull the wedge. The spring loads the faces and you're done. For Tides Marine seals, it's more involved. They use a bellows with a built-in needle bearing and grease fitting. You can pump fresh grease onto the shaft to extend life, but swapping the whole unit usually means haulout because you're removing the bearing housing and the shaft log. I've done a field repair by fitting a new bellows section from a PSS kit, but that's a hack, not a how-to. Stick with what the manufacturer intends.
Replacing a PSS Bellows Underwater: The Steps
You'll start with the boat in neutral and the shaft locked. I use a small come-along tied to the rudder to stop rotation. The bottom usually has a couple feet of visibility, so I zip-tie a flashlight to my wrist. The job goes faster if you have a second diver holding the boat steady. First, you've got to get the old bellows off. Squeeze the clamps with a pair of pliers or a screwdriver, then slide them down the shaft. I use a plastic head hammer to tap the carbon face forward, loading the spring. Then slide a stainless steel wedge (I use a putty knife with the edges ground flat) between the rotor and carbon face. That compresses the spring so the bellows is slack. Then just slide the bellows off the shaft. It's that simple. But here's the catch: the shaft doesn't care about your schedule. If the shaft has a rough spot or scale, the new bellows will leak on an edge. So you clean the shaft with a stainless wire brush until it's smooth, and you rub a thin coat of PSS lubricant on the shaft before sliding the new bellows on. Install the two clamps but don't tighten them yet. Position the bellows so it has about 3/4 inch of free travel. Then tighten the clamps to 35 inch-pounds. I've seen guys torque them by feel, but a torque wrench is ten times more reliable. Pull the wedge out. The spring will snap the carbon face into contact with the rotor. You'll see a small dribble of water come in—that's the face seal self-adjusting. Check it after a minute. If it's dripping more than a few drops a minute, you've got a contaminant on the face. I've had to pull the wedge back in, wipe the face with a rag, and reseat a couple times. The whole job takes about forty-five minutes to an hour underwater. On a good day. On a bad day, you're fighting a current and the visibility drops to six inches. For that you need patience, not force.
When I Tell You It's a Haulout
If the shaft itself is scored, pitted, or bent, new bellows won't help. You can feel a scrape with your fingernail from a 1/8" groove. That groove will churn out the new seal in a week. Same if the shaft has corrosion pits near the seal. The only real fix is pulling the shaft, replacing it or machining it in place, and that's a yard job. Cutless bearings generally can't be replaced in the water. They're pressed into the stern tube, and the shaft has to be withdrawn to press them out. No way around that. And if the stern tube itself is leaking—as in, the fiberglass tube is cracked—you're not fixing that underwater. You need to dry the boat out, grind, and lay up resin. The sealing compounds I carry are emergency stopgaps, not repairs.
Gear That Gets the Job Done
My go-to bag for a seal job has a 1/2" ratchet and a socket set, a pair of 6" stainless scissors, a putty knife re-ground into a wedge, a roll of 1/2" Kevlar tape, and at least two spare clamps per clamp size. I also carry a contour gauge to measure the shaft for scoring. And a small submersible video light is essential because you can't see the seal if you're fighting for light. I once had a green sea turtle swim up and bump my elbow while I was torquing a clamp. I looked over and saw a school of amberjack trailing him through the marina. The turtle didn't seem to care that I was on the bottom of a million-dollar boat. It was a good reminder to keep your movements smooth down there—sudden moves don't help you or the wildlife. You need to know your limits. If the boat is pitching and rolling, if the current is pushing you off the shaft, if you're tired, abort. A free-diver can stay down for 2-3 minutes at a time; with a hookah setup I can stay all day. But you still need a surface tender. No exceptions.
Frequently Asked Questions
How long does an underwater shaft seal replacement take? Typically forty-five minutes to an hour of bottom time for a PSS bellows swap, but it depends on the boat, visibility, and current. I've done it in twenty minutes on a small boat in calm marina water, and I've had to abort entirely when the current picked up. Can I replace a Tides or Volvo seal underwater? A Tides unit usually means a haulout because the bearing housing comes out of the shaft log. Volvo's dripless system uses a face seal that can sometimes be cleaned in place, but the manual says to reseal the faces out of the water. Stick with what the manufacturer allows. What's the cost difference between an underwater repair and a haulout? Underwater repair is always cheaper because you're not paying for a lift, stands, traveling, or a yard's shrink-wrap. Diving rates vary from one diver to the next, but you're paying for time and skill, not a dry dock invoice. How do I know if my shaft seal needs attention? If you see a steady drip inside the boat, or feel vibration at the shaft, or see corrosion around the bellows, it's time to have a diver look. Regular inspections—every six months or so—catch problems before they turn into emergency jobs.