Cavitation Cleaning Systems Explained
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Amanda McCallister, editor.
Hull & Bottom Cleaning | Marine Salvage & Recovery Last Tuesday I was hunched over a 90-foot Hatteras in the slip, scraping a year's worth of grass off the running gear. Forty minutes in, I'd cleared maybe two square feet. Then I hauled up the CaviJet from Moser, and that same bottom was clean enough to eat off in under two hours. That's the difference cavitation makes. Cavitation hull cleaning is the closest thing we have to a magic eraser for a boat's bottom. It uses a high-pressure water stream that creates tiny bubbles, and those bubbles collapse and scrub the surface without you having to muscle a brush back and forth. It's been around for a while, but the tech is getting better, and more divers in South Florida are making the switch. Here's the why, the how, and the when to leave it in the truck.
How It Works
The physics is simpler than people think. You push water out of a nozzle at enough pressure that it forms a low-pressure zone right where the jet hits the hull. That zone turns water into vapor, and when the vapor bubbles collapse a millisecond later, they produce a tiny shockwave. Multiply that shockwave by a thousand times a second, and you've got a scrubbing action that lifts slime and grass without grinding into the gelcoat. Call it micro-implosion, call it whatever you want—it works. The real money is in the head. The CaviJet rotates a set of jets around a disc, so the cavitation sweeps a pattern about the size of a dinner plate. You hold the wand a few inches off the hull and let it walk. Pressure for my rig sits around 3,000 psi, give or take, and that's enough for 90% of the hulls I clean. Barnacles are another story. For those, you're still busting out a scraper first, because cavitation can fire-polish the shell and make it harder to remove. And here's a digression: I had a captain one time swear his bottom was so hard he needed the pressure turned all the way up. I did what he asked and we spent the next afternoon filling a pinhole in his gelcoat. That boat used more fuel that season than ever. The lesson: don't let the tool become the hammer.
Gear and Setup
You don't need a fancy boat to run this system. The cavitation head fits on a standard high-pressure wand, and the hose hooks to a pressure washer or a pump on the dock. I use a portable unit that pushes about 4,000 psi and 12 gallons a minute, but the exact numbers depend on the system. More flow matters more than more pressure—you need the volume to keep the bubbles forming. The setup is straightforward but it's where most divers fail. You need a clear hose path with no kinks, and every coupling has to be tight. I've spent way too many dives chasing a leak that dropped the pressure to a point where the cavitation just gives out. Then you're just spraying a watered-down hose at the hull. And always test on a patch of old bottom paint before you start the job. That saves you from finding out too late that the cavitation eats through your soft ablative coating like a kid eating cake. Also—and this is important if you're a dive contractor—make sure the dock has a place to contain the runoff. Most marinas in Fort Lauderdale require a spill pad. The water that comes off a cavitation job is green and smelly, and it's full of biocide as the paint leaches. You don't want a fine for that.
The Technique
There's a skill to running this thing, and it's not just pointing it at the hull. You want to start at the waterline and work your way down. That way the loosened gunk falls off and doesn't redeposit lower down. Overlap your passes by about a third of the head's width, or you'll leave a pinstripe of slime that's a pain to catch later. Speed is the tricky part. If you sweep too fast, you're not giving the bubbles time to collapse and do their thing. If you're too slow, you can mar the paint. I tell new divers to move the wand at about the pace you'd use to polish a car by hand. It should feel steady and patient. Here's a list I keep in my head: slime, a foot per second; grass, half that; barnacles, switch to a scraper first. Pull the wand back and forth in lines, not circles, and don't try to dig into a fouled prop with the cavitation head. You'll round the edges of the blades if you're not careful.
When to Trust It and When to Skip It
This system shines for regular maintenance on a boat that sits in a wet slip. In South Florida, that means every two to three weeks during summer. You can knock down a 60-footer in an hour and a half, and the owner doesn't have to schedule a haul-out. It's the difference between staying on top of the bottom and playing catch-up. But it's not a miracle. If a hull's been neglected for a year or more, you're going to need scrapers, wire wheels, and a lot of elbow grease. Cavitation can't fix that. And for older, softer coatings, the bubble collapse can pull chunks of paint right off the gelcoat. I always ask what the bottom's painted with before I run it. If they don't know, I test a small spot at the stern and see what happens. On the gear side, there are half a dozen cavitation systems on the market now. I've seen divers swear by the CaviJet, I've seen others use homebuilt heads that work fine. My only advice is to look for something with replaceable nozzles and a head that won't spin off underwater. I replaced two heads last year that just plain wore out, and that cost me more than the system did.
Frequently Asked Questions
Does cavitation cleaning damage my hull paint? It can, if you hold the head in one spot too long or crank the pressure on a soft ablative paint. I always test a small area first. On hard epoxy and most antifouling paints, it's safe and effective. On older, soft coats, you might see the paint dissolve before your eyes. How often should I have the bottom cavitation cleaned? Depends on where you are and the water temp. In Fort Lauderdale, every two to three weeks in the summer, and every four to five weeks in winter. The goal is to keep the slime off before it turns into grass and barnacles. Can you clean a propeller with a cavitation system? I can, and I have, but it's not my first choice. The cavitation head can round the edges of a prop blade if you're not careful. Use a traditional brush for props and running gear, and save the cavitation for flat hull panels. Is cavitation cleaning safe for my through-hulls and transducers? It's safe if you're careful. I'll sweep the head right over them, but I always slow down and keep the wand moving. Don't park the cavitation on a fitting and expect nothing to happen. Better to switch to a sponge for those spots.