Cleaning Your Boat Hull at the End of the Season
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Cleaning Your Boat Hull at the End of the Season

A dirty ring of lake scum has turned your boat’s usually white hull dark brown. Power washing doesn’t remove it completely. What to do?

Choosing a cleaner
I tested two popular hull cleaners on my boat to see which works best. My 24-foot Chaparral bowrider sits at the dock all summer on Lake George, in upstate New York. Sitting at the dock with only occasional use allows the nutrients and algae present to lock onto the hull bottom, and creates a tough barrier to clean in the post-season.  

For many years, I’ve used CRC MaryKate On & Off cleaner, which is a very popular muriatic acid-based liquid cleaner that’s mixed with water and applied to dirty boat hulls. This is a product to be used with extreme care; it will burn holes in clothing, cause eye and skin irritation, and ruin upholstery. Left to sizzle on a painted or galvanized steel trailer, it will burn quickly through the paint and cause rust to form. Protective garb, including safety glasses, protective clothing, and gloves should be worn when using On & Off.  

Then why use it? It cleans so quickly with so little effort, it’s tough to beat it.

I bought a leading biodegradable competitor called Slimy Grimy to try as well. This cleaner has also been around for many years; its claim to fame is eco-friendliness. Available in powder or liquid form, Slimy Grimy has none of the caustic properties of On & Off; it can be used without protective gear, although I recommend using eye and skin protection when using any cleaner.

The test
After mixing each cleaner according to the manufacturer specs, I applied On & Off to one side of the boat, Slimy Grimy to the other. Using a stopwatch, I timed how long it took for each cleaner to work through the crud on a section of the boat, without touching it with a scrub brush. I then timed how long it took to scrub the hull completely clean, with no traces of grime.

On & Off powered through the lake scum without breathing hard, removing all traces in about half the time it took Slimy Grimy. I even had a few stubborn stains that Slimy Grimy could not remove completely, but after rinsing off the residual Slimy Grimy, a dab of On & Off and some quick scrubbing wiped out those stains.

So the winner is? Drum roll, please.

Both. On & Off wins on cleaning power. However, it’s not eco-friendly and it’s difficult to use, and can cause injury and damage if not used very carefully. For those concerned with potential enviro-damage, and willing to scrub a bit harder, Slimy Grimy is a good choice.

When cleaning the exterior of your boat with chemicals:
  • Clean the boat on land, away from your lake or river, and take precautions to prevent groundwater contamination.
  • Use in an open, well-ventilated area as cleaner vapors can be harmful.
  • Wear protective gear including splash-proof goggles, protective clothing and latex or rubber gloves.

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