Waterline Systems

Why Does My Boat Need Fairing?

Why Does My Boat Need Fairing?


When resins cure, they shrink. This is an inescapable fact of composite boatbuilding. Of the prevalent resin systems, epoxy shrinks the least, polyester the most, and vinylester is somewhere in the middle. The mold can be perfectly fair but as the resin shrinks within the matrix, some areas of the molded part end up low and others slightly high. This is not a manufacturing defect but simply an unavoidable property of the materials used in hull construction.

Smooth vs. Fair

Many sailors mistake a “smooth” surface for a “fair” surface. We see people at regattas compounding, buffing, and waxing their bottoms; the boat looks good and has a nice finish, but it’s not doing much for their overall performances since any highs or lows in the hull still remain. Fairness and smoothness are different things, with fairness being by far the more important in terms of speed.

Water flowing around a hull does not like to change direction; if your hull is not fair, that’s exactly what happens and it’s not good for your speed. Highs and lows in your hull surface force the water to make rapid changes in direction, which leads to a dramatic increase in drag. Julian Bethwaite, designer of the 49er, ran full-size, in-the-water tests between a faired 49er and a 49er fresh from the mold. Incredible as it seems, he found a 14% speed difference between the faired and as-molded hulls. He published his findings in Seahorse Magazine in 2007.

Sailors grossly over-estimate the value of smoothness. At the speeds we deal with on keelboats a 400 – 600 grit finish is all you need. It’s not until speeds roll into the 30- and 40-knot range that smoother finishes add any benefit.

Keels & Rudders

Keel and rudder fairing is substantially more complicated than bottom fairing. With the bottom, we’re only concerned with drag reduction. Keels and rudders both need to generate lift as well. Highs and lows are still vitally important, but now we add the need for a very precise shape. Choosing the proper foil shape is the science part of the equation. Waterline Systems works closely with some of the world’s foremost hydrodynamic engineers and Naval Architects to help us determine the correct shapes. We need to balance lift/drag, pressure distribution and the limitations imposed by class rules. With the science in hand, art takes over. Actually getting the foil to the designed shape is pure art. Our team will fair more keels in a year than most yards do in a lifetime. Even with that level of experience, company founder/President Randy Borges is still hands-on with each keel fairing project, ensuring a perfect finish.