Why Your Centerboard Needs to be Perfect
One of the biggest performance improvements I ever made was when I
replaced an old centerboard in my first 505 with a new Lindsay gybing centerboard. The
difference was more than a new suit of sails. A great centerboard is also my
secret
weapon in my 22 year old and otherwise hopelessly uncompetitive Albacore.
The Waterat foils - and the Lindsay foils we used to use - are
normally shaped to a NACA 00 family section. These sections are much more forgiving of
nicks and dings - and our less than perfect steering - than the laminar flow sections.
None the less, all the performance improvement you get from a great centerboard is only
there if you keep the centerboard in near perfect condition. When was the last time you
checked you centerboard for nicks and dings? Any imperfections can increase turbulence and
drag. Apart from robbing you of speed and pointing, a damaged centerboard can absorb
water, weakening it and eventually leading to it breaking. Now that is a performance (and
dollar) robber!
How to Make a New Foil Better
Waterat does an excellent job building foils; I think Larry's blades
are the best available. The one area where I think they can be slightly improved by the
owner is the finish. Waterat spray paints the boards with a very high quality two part
polyurethane, which forms a very tough, shiny finish. I believe the blade surface can be
improved by careful wet sanding with 600 or smoother sandpaper. This removes the gloss and
any orange peel on the finish, resulting in the smoothest, fairest shape possible. I do
this on centerboards I am taking to a World Championship.
However, as Larry points out, wet sanding the foil removes that very
tough shiny finish, revealing a softer layer underneath. Once you wet sand a board, you
will have to keep wet sanding it, as it will scratch more easily. A wet sanded surface
also gets dirty more easily, and therefore requires more maintenance than the smooth shiny
finish. Eventually, the foil will have to be refinished, and you can start all over.
Waterat does NOT recommend sanding your hull!
If you are interested in more discussion of surface finish on foils,
check out C. A. Marchaj’s books - the first is Sailing Theory and Practice,
Dodd Mead & Company, 1964, the second Areo-Hydrodynamics of Sailing, Dodd
Mead & Company, 1979 - and Frank Bethwaite’s recent book,
High
Performance Sailing, McGraw-Hill, 1993. Long time Annapolis area 505 sailor
Bransford Eck also wrote two articles discussing 505 centerboard cross sections,
planforms, area, and finish, in the '70s. Many long time 505 sailors have copies. Much of
the discussion would be applicable to other centerboard classes as well, though the
resulting centerboard would be different.
What Damages a Foil
Centerboards are damaged and worn in several ways. The easiest is by
running aground. I sail on the Chesapeake Bay which has a (primarily) soft mud bottom.
Soft mud is a lot better than rocks, but the mud has the texture of 100 grit sandpaper,
and does ruin the tip. Another easy way to damage the board is to hit something in the
water. This seems to happen most frequently in spring, when lots of debris have washed
into rivers and then into the lakes, bays and oceans we sail on. Any grit or dirt in the
centerboard trunk will also scratch the foil as you raise and lower it. I also find that
the centerboard bolt sometimes makes the bolt hole bigger and exposes wood, so that water
can get in. Even if you never run aground, never hit anything in the water, and never have
dirt in your centerboard trunk, just raising and lowering the board will gradually scratch
the airfoil portion, and will wear away the widest part of the centerboard head. All of
these slow you down! Any damage that allows water to get into the spruce core is bad. The
board will be both heavier and weaker. Check your board from time to time, and anytime you
think you may have damaged it. Repair any areas with uncovered wood
immediately.
How to Repair the damage
Small scratches can be sanded smooth, or a soft filler can be
applied, and then sanded or filed smooth. Anytime you fill with a material harder to sand
than the polyurethane finish, you will have trouble sanding down to a smooth fair
shape.
Dents and dings in the leading edge are a little more challenging;
you want to end up with exactly the same shape on the leading edge - this is critical! Cut
or file away any broken fiberglass or wooden fibers, and then use epoxy and filler to both
seal the wood, and fill the dent. I find that West epoxy with microballoons mixed in works
well. It is strong, yet files or sands easily. Being softer than the polyurethane finish,
you should be able to sand or file it to fit, without taking too much paint off around the
damaged area. I usually shape the repaired area using a medium file. I find that if I do
not press too hard, the file does not scratch the polyurethane on the surrounding area,
but does easily remove the excess filler material sticking above the surface.
Trailing edge damage is easier to fix. Clean out up broken glass or
wood fiber, then find a piece of semi rigid plastic - a small piece of mylar is perfect.
Tape the plastic to one side of the blade, so that you are creating a mold for one side of
the damaged area. Then using an epoxy and filler mixture, fill in the damaged area.
Trailing edges are inherently weak, if the area you are repairing is much more than a
thumbnail in size, you should reinforce the epoxy with a little fiberglass cloth. Simply
lay the cloth in, and make sure it is saturated with epoxy. The low density fillers like
microballoons reduce the weight you are adding in the repair, but this is usually
insignificant for a small repair. Low density fillers do make the repair easier to sand
and file to shape. High density fillers like microfibers make the repair
stronger.
I use the same mylar approach to repairing a CB bolt hole. After
I’ve cleaned away all the broken material, I tape one side of the bolt hole closed
with the mylar, and then put epoxy and filler in the other side. Sometimes putting just
the right amount of filler and a second piece of mylar on top results in a repair that
needs minimal filling and sanding; you just have to drill the new bolt hole through the
filled area. The CB bolt hole in the centerboard should be lose enough that the board can
gybe from side to side without being constrained by the CB bolt; it should not be any
larger than it needs to be.
From time to time, I build back up the gybing head of the
centerboard. I clean up the worn area with sandpaper or a file, mix a batch of epoxy,
using a high density filler (I’ve used both microfibers and carbon powder for this),
and then applied it over the worn area. I shape it immediately with a clean trowel,
scraping off much of the filler, but leaving the filled area almost flush with both planes
of the gybing head (you should see a distinct ridge where the two planes meet). After the
filler has cured, I carefully file and then wet sand the repaired area.
Rudders
Though I believe rudders are less critical than centerboards, I
repair them the same way. They are built using the same materials as the centerboards. A
common problem with rudders is where the leading edge of the rudder blade is cut back to
form the rudder head, just where the bottom of the boat would intersect the rudder blade.
That portion of the rudder is typically damaged over time by pulling the rudder off - the
rudder hits the bottom transom gudgeon. This can easily be fixed using the same flexible
mylar used to repair the centerboard. This time, you force the mylar to wrap around the
leading edge of the rudder where it is damaged, making a mold shape for you to fill with
an epoxy and filler mixture. Remember to cut or file away any damaged glass or wood before
you start.
Sometimes You Need Professional Help
Though I had summer jobs working for boatbuilders and have done lots
of small repairs, I am not good enough to get the original shape and finish back on every
repair. Eventually, even a well-taken-care-of board or rudder needs to be properly
repaired and repainted. When that happens, I send the blade back to Waterat (they have the
skills, the templates, and the painting equipment). What returns a few weeks later is
absolutely indistinguishable from a new blade (see the section on how to make a new foil
better).
I have a 12 year old centerboard that is as light, stiff and fast as
a new one - it has probably been refinished two or three times.
Centerboard and rudder repairs can take time, but perfect foils give
you better speed and pointing; critical to racing success.
Ali Meller