The Churn of the Screw
How To Keep Flora And Fauna From Fouling Your Propellers.
Here’s a common, if unwelcome, sight for boaters: You go to pull your boat out of the water for winter lay-up or perhaps an overland haul on the trailer, and you notice that your boat’s propellers have morphed from the once-streamlined tip of your propulsion system’s spear into a shaggy, shell-laden mess. Maybe you noticed something was off with your props before this. The cumulative effects of slime and barnacle growth on a prop can cut down its efficiency anywhere from less than ten percent to, well, all of it. That’s right, get enough growth on your props and you will almost literally be spinning your wheels and going nowhere—and also burning valuable fuel in the process. What’s more, harboring maritime critters on your props can pose a serious threat to delicate ecosystems by spreading alien species.
Theoretically there’s a solution to these problems: special prop paint, replete with biocides designed to make propellers inhospitable to most any invasive sea organism. I say theoretically because there is some doubt as to the effectiveness and durability of such paint. As one hard-core boater told me while I was researching this story, “One year, we had antifouling paint on one of our twin props and left the other one bare. By the end of the season, they looked exactly the same as far as barnacle growth.”
When I asked Jim Seidel, assistant marketing manager at Interlux, the world’s largest marine paint company, about this anecdote he intimated that the frequency with which you use your boat influences your need for prop paint. Seidel says, “The more you use your boat, the less likely [your props] will collect shells. If you use it every week and scrape your props occasionally it may stay clean on its own. But if your boat sits unused more than ten days at a time, you’re going to need prop paint.”
If you just can’t find the time to take your boat out once a week to clean her props, you’re either going to have to don some work clothes and head down to your local ship’s store or take your vessel to your favorite boatyard. Either way, you’re looking at a three-step process that will ensure your screws won’t end up as barnacled as a humpback whale. First, your props need to either be sanded or sandblasted to remove any existing growth, as well as to create a “profile”—that is, tiny (about 50 microns deep) etchings that allow the epoxy resin primer to adhere to the prop’s metal surface. Of the two methods, Seidel recommends sandblasting because it’s quicker and produces a more consistent profile, although you’ll probably have to remove the props. (Profiling was done chemically, but that method has been discontinued because the active ingredient, zinc chromate, was found to be carcinogenic.)
Next, you or your yard will need to apply a coat of epoxy resin primer, such as Interlux’s Interprotect 2000. Somewhat ironically, the epoxy does not technically protect the prop from the antifouling paint that will be coated over it, it protects the paint from the prop. The resin forms a barrier layer between the typically bronze or stainless steel prop material and the copper biocide in the antifouling paint. Due to electrolysis, copper, which is lower on the Galvanic Scale than the other metals typically found on a boat, will quickly turn into a flaky powder if a primer is not applied. The primer also makes the prop “stickier,” ensuring that the friction from the water striking the blades while the prop is turning doesn’t erode your paint into oblivion.
The aforementioned antifouling paint will let copper contained in it seep out slowly, ensuring that no mussels or barnacles encamp on it. Seidel does note however that, particularly after rainstorms wash lawn fertilizer into the water, some algae may attach to the props regardless of antifouling treatment. However, that slime is not as damaging as fauna growth and can be cleaned off by the spinning of your propeller.
After the antifouling paint has been applied to your props, allow one night for it to dry before launching. Your boat can go into the water anytime after that, but take care not to let it sit out of water too long, as after 60 days the paint may begin to erode due to normal weathering.
Accounting for normal use, Seidel recommends the entire three-step process be repeated every year to 18 months (paint erodes slower when the props are wet, so the longer your boat is in the water, the less frequently a prop will need a fresh coat). Yards typically charge between $100 and $200 per prop, so getting your props painted is reasonably cost-effective, especially considering the fuel you can save. And believe this, once you pull your boat out of the water and admire your smooth, clean props, you’ll be glad you shelled out for paint.
This article originally appeared in the April 2011 issue of Power & Motoryacht magazine.