Fuel Burn and Fouling: What a Dirty Hull Actually Costs You
By Tomas Iglesias, Yacht service diver, South Florida. Reviewed by Amanda McCallister, editor.
Hull & Bottom Cleaning | Marine Salvage & Recovery Last Tuesday, I was under a 78-foot Hatteras sportfish that had been sitting in the Lauderdale canal for two months. The bottom felt like sandpaper under my glove, and the props wore a garden of barnacles. I surfaced and the owner asked the usual question: what's the damage? I gave him the haul-out price for a full clean, and he paid it without flinching. Then I told him what the fouling was doing to his fuel bill. He didn't flinch anymore once we did the math. The point isn't to make you feel bad about your boat. It's that most owners don't see what I see. They see the speed readout and the fuel gauges, but they don't see the cost of a slimy hull. So let me walk you through it.
What Fouling Does Under the Waterline
Warm Florida water is the perfect soup for marine growth. Within a week, a clean hull picks up a slime coat of bacteria and microalgae. Give it two weeks, and you'll feel a thin film that slows you down noticeably. Give it a month, and barnacles and grass start attaching, especially around the prop edges, shaft, and rudder tabs. That growth adds drag. The prop becomes less efficient, so the engine has to work harder to spin it. The hull skin friction goes up. Your speed drops, and your fuel burn goes up. It's not linear either. A little fouling costs more than you'd think.
The Fuel Burn Math
Let's use a typical 70-foot yacht with twin 1,000-horsepower diesels. At a cruise speed of 22 knots, those engines burn somewhere around 60 gallons per hour combined. That's a ballpark number, depends on the plant and the load, but call it 60 GPH. Now the fouled hull. Over the years, I've seen fuel burn increase on a dirty bottom by anywhere from ten to thirty percent, depending on how bad it is. A fouled running gear sets you on the high end. A month of growth in the Lauderdale canals is usually good for a fifteen percent penalty. So that 60 GPH becomes 69 GPH. At today's diesel prices on the water, call it $4.50 a gallon. That's $40.50 an hour wasted, just in the extra fuel. Run the boat two hundred hours a year, the way a serious sportfisherman does, and that's $8,100 in fuel you're burning for nothing. And that's just the fuel. The engine is also working at higher load, which means more heat and more wear. That's downstream money, but the fuel number is the one that shows up on the card.
The Cost of Keeping Clean
Now let's talk about what it actually costs to fix it. Down here, a diver like me charges by the foot, usually between $4 and $6 per foot for a hull clean with running gear. On that 70-footer, that's $350 to $420 a clean. Most boats in this climate need it every four to six weeks. If you do it monthly, you're at around $4,200 a year, maybe $5,000 with a prop polish included. Compare that to the $8,100 you're throwing away in fuel. Even in the worst case you're saving $3,000 a year plus the wear and tear on the engines. That's a pretty good return for spending a half-hour on the dive flag. Now, some owners try to skip a clean in the winter, or when the boat isn't used. That's a mistake. In Florida, the water doesn't cool enough to slow growth, and once the barnacles set in a few weeks, the cost climbs. A clean that would have cost three hundred turns into a haul-out and bottom job that costs thousands.
The Hidden Costs I See From the Bottom
You'd think the fuel burn is the big one, but there's more. That extra load on the engine affects your shaft alignment, and I've seen excessive fouling bend prop tips and chew up cutlass bearings. I've also seen bottom paint like Petit Trinidad erode faster when it's dragged through the water, so you'll end up recoating a year early. And I've pulled up anodes that were eaten down to nothing because something else in the marina was leaking current. One afternoon I was cleaning a 54-foot Viking and fouled my line on a stringer of fish left hanging off the transom. The owner had been complaining about fuel consumption and speed loss. After I cleaned the hull, the speed came back, and we both laughed about the fish. The point is, you never know what you'll find under a boat, but the waterline doesn't lie about the fuel bill.
Frequently Asked Questions
How much extra fuel does a dirty hull burn? It depends on the level of fouling. A typical slime coat and barnacle growth on the running gear can add 10 to 30 percent to fuel burn, with 15 percent a reasonable middle for a boat left a month in South Florida. How often should I have my boat hull cleaned? In warm waters like Fort Lauderdale, plan on every four to six weeks. If you're running a lot, the prop wash might keep the running gear cleaner, but the hull still fouls. Monthly is best to avoid the hard growth. What does a bottom cleaning cost? Most divers charge by the foot, typically $4 to $6 per foot for a standard hull and running gear clean. On a 70-footer that's $350 to $420. Add a prop polish and it's a bit more, but still less than the fuel savings. Does a clean hull actually save fuel? Yes. Enthusiastic yes. After cleaning a fouled hull, you'll see the same speed at lower RPM, which directly reduces fuel consumption. The fuel savings usually cover the cleaning cost several times over.