You Gotta’ Love a Smooth Bottom

Dragging You Down in Speed and Fuel

Interlux January 28, 2009

One of the ironies of boating is that the main ingredient necessary for such activity – water – is also the biggest cause of problems. Controlling marine growth has challenged mariners since the beginning of time, so properly choosing and using antifouling paint is critical for maximizing efficiency.


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Carefully applied bottom paint is essential for vessels kept in the water. Photo: Steve Dougherty

To be able to feed and grow properly, fouling must be attached to static objects. As they remain static most of the time, boats offer excellent feeding grounds for all types of marine growth.

The main biocide used in modern antifouling paint is cuprous oxide, commonly referred to as copper.

There are three major types of fouling (slime, weed, and shell) and each has its own unique characteristics. Each type of fouling also comes with a drag penalty that costs you speed, maneuverability, and fuel. Slime will have a one to two-percent drag penalty, weed will have up to a 10-percent drag penalty, and shell fouling can have up to a 40-percent drag penalty. Spending the time to properly select, prep and apply the correct bottom paint can have a major impact on the overall speed and fuel efficiency of your vessel.

Antifouling paint works by delivering biocide to the paint surface to deter the settlement of fouling. The main biocide used in modern antifouling paint is cuprous oxide, commonly referred to as copper. But there are other biocides, called booster biocides, which increase the effectiveness of copper. In addition to the two aforementioned biocides, Interlux, the industry leader in bottom paint technology, uses Biolux to control the way the biocides are released from the paint.

In simpler terms, there are two types of bottom paint: hard bottom paints which leach biocide out of the paint film, and those that disappear over time, such as Micron antifoulings, where the paint film wears away at a controlled rate to expose fresh biocide.

Hard bottom paints have been in use for a long time because they work well in most conditions and yield predictable performance. They work by leaching biocide out of the paint. There are various types of hard bottom paints and their resin types vary, as do the amount of biocide in the paint film. Hard bottom paints are easier to scrub or wet sand, if necessary, than those antifoulings that are meant to wear away. Hard bottom paint requires more surface preparation each year to maximize adhesion and limit paint build-up. Hard paint left to build-up on the surface will begin to crack and flake and will eventually need to be removed.

Many sportfish owners take advantage of a hard paint’s ability to be wet sanded by applying what is known as a Carolina Coat. For this system they will apply a hard, super-slick antifouling such as VC Offshore with Teflon to the planing surface of the boat and wet sand it until it is super smooth. Next, they apply a stronger antifouling such as Micron on the non-planing surfaces for increased fouling protection.

Bottom paints that disappear over time are loosely described as soft, ablative, eroding, copolymer, or self-polishing copolymers. You must understand that not all work the same way and it’s important to understand the rate at which the paint wears away and how the biocide is released. Conventional ablative bottom paints erode erratically, become rougher with time and release biocide unevenly. Micron antifoulings polish at an even rate and maintain a constant and effective biocide release. This results in a longer service life of the antifouling paint. Another benefit is that Micron antifoulings require less prep when it is time to recoat.

A smooth bottom is important because it not only helps avoid a drag penalty but it also helps deter fouling organisms, especially weed and slime fouling, since these organisms prefer to settle on rough, cracked and chipped surfaces. The yearly re-application of hard antifoulings results in the build-up of old coatings. As hard coatings age, they become weakened as a result of the water moving in and out of the paint film and cause the paint to detach from the hull. The roughness caused by chipped, cracked and loose paint increases drag, and slows down your boat.

To prepare the surface to reapply a hard antifouling, the surface must be sanded with 80-grit sandpaper – not only to improve adhesion but also to remove the honeycombed paint film and avoid paint build-up.

Micron antifoulings need to be applied to a clean, dry surface. Generally with Micron, a boatyard will power-wash the already painted surface, allow the boat to dry overnight, and then apply a new coat of paint. The only need to sand would be where there is loose and flaking paint or where the paint has been worn away.

Remember – the smoother the bottom the lower the drag, and the better the fuel efficiency. Proper surface preparation is the only way to ensure that the bottom is smooth. When it comes to selecting your bottom paint remember that in the long run, the least expensive paint may end up costing you the most!

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