BOATING & TOWING- Off-Season Boat Bottom Cleaning


By: John Tiger

A dirty ring of waterline scum has turned your boat’s usually white hull a dark brown hue. Scrubbing, even power washing, doesn’t remove it completely. What to do?


We tested two popular hull cleaners on the same boat to see which works best. Our subject hull, a fiberglass 24’ bowrider, sits at the dock from May to October. Sitting at the dock with only occasional use all season allows the nutrients and algae present to lock onto the hull bottom and create a tough barrier to cleaning in the post-season.

For many years, we’ve used CRC Mary Kate On & Off cleaner, which is a very popular muriatic acid-based liquid cleaning solution designed to be mixed with water and applied with care to dirty boat hulls. This is a product to be used with 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. To try, we bought a leading biodegradable competitor called Slimy Grimy. 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 we recommend using eye and skin protection when using any cleaner.


After mixing to each manufacturer’s correct specifications, we applied On & Off to one side of the boat, Slimy Grimy to the other. Using a stopwatch, we 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. We then timed how long it took to scrub the hull completely clean, with no traces of grime.

The winner in all respects was On & Off. It powered through the lake scum without breathing hard, removing all traces in about half the time it took Slimy Grimy. We even had a few stubborn stains that the Slimy Grimy could not remove completely, but a dab of On & Off and some quick scrubbing made them clean again.

The appeal of On & Off is its 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.



CRC Industries



Slimy Grimy


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