Introduction to fairing
In discussing the fairing process, I am referring to the framework
of the Riviera (a 20 foot runabout), but the same process applies
to any cold molded hull. In this article we will cover the fairing
of the frame prior to adding the planking.
What is Fair?
A fair hull is one that has lines / curves that flow smoothly
in all directions with no humps, bumps, dips or sags. Having a
surface that is fair is NOT the same as one that is smooth. Smoothness
is a measure of surface roughness on a tiny, almost microscopic
level. Fairness is a measure of smoothness of the overall surface.
A rough comparison of smooth versus fair is that of a a newly
surfaced highway that has a smooth surface, but with ripples that
leave your car hobby horsing down the road. This is a surface
that is smooth, but not fair.
Fairing the frame
Staring at a complete frame you may ask: "How can I tell
what is fair? It is mostly air!"
Fairing is the process of changing your boat frame from being
a series of somewhat matched angular blocks of wood and making
it one sinuous, flowing framework.
For the engineers in the crowd, we start with a piece-wise linear
approximation of the hull shape and replace it with a series of
splined curves via an iterative process.
Piece-wise linear applies, since initially, the hull frame
can be visualized as a series of (relatively) flat rectangles
and triangles. The frames and battens define the edges of these
rectangles and triangles.
The reason they are "splined" curves is that we cannot
create a curve with one continuous pass of a long and perfectly
shaped tool. We will work on individual sections of the framework
at a time and blend each section into the other. The blending
process is similar to that of the spline fit.
The reason it is an iterative process is that it is not possible
to go from a rough frame to a fair frame in one pass or even
on one section at a time. In the process of fairing, the whole
frame is brought closer to a fair surface. The process is then
repeated, each repetition of the process, brings the entire
surface closer and closer to fair.
For the non-engineers, an iterative process is one where you
make many passes at approximating the solution to a problem.
Each pass gets you one step closer.
How do you tell the hull or frame is fair?
There are several tests:
By eye. Once the hull is completed, it will be covered by a
glossy paint or varnish finish, or a mixture of these two. Reflections
of objects off of this mirror-like surface will show imperfections.
We do not want a "fun house mirror" effect. Given
that at this stage, the hull is basically air, going by eye
By feel. Running your hand over the hull will reveal humps,
valleys and ripples. While this is very useful on a planked
hull, this one still does not have enough substance. Running
your hand over individual battens, chines, sheers or the length
of the keel can help, but going from one area to another, you
hand will "fall through the holes in the hull".
With a stick. Think of this a as a portable piece of hull.
By laying a flexible stick / ruler / batten over the framework
you can very closely approximate the shape of the finished hull.
Think of it as "draping" the batten over the framework.
When draped over the hull, the stick should contact every frame
member underneath WITHOUT having to be pushed down. With a limber
/ flexible batten only light pressure near the ends (~15-20%
of the way in from the ends) should be necessary to make it
conform to the shape of the frame and contact each frame it
Why fair now?
You may ask, "Why fair the hull now, when I will be adding
2, 3 or 4 layers of planking over the frame?". The answer
is that believe it or not, at this phase you are fairing mostly
air. By fairing the un-planked hull frame, which is maybe 5% wood
and 95 % air, you can "quickly and easily" achieve fairness.
If you wait until the hull is planked to start fairing, the hull
is 100% wood. Much more material will need to be removed. If the
initial frame fairing is not good enough, you may even need to
cut through one or more layers of the relatively expensive plywood
or have to put on large expanses of epoxy fairing compound (to
fill valleys). Throwing away the "air " you remove is
very inexpensive compared to plywood both financially and in effort
How good is "fair"?
How close you will fair the hull depends on the desired level
of craftsmanship you want to show off on the project, the degree
of gloss of the finish, and the size of the hull. For example
a matte camouflage painted duck boat does not require the same
degree of fairness as a glossy varnished runabout - for visual
appeal. Also the bottom of a 50' cruising yacht does not get the
same tolerance as measured in hundredths or thousandths of an
inch as a class D racer that will hit 100 mph. In all cases, the
fairer the hull, the faster the boat will go since water resistance
will be reduced.
Another attribute of a fair hull is good performance at speed.
A planing hull must have a dead flat bottom going forward from
the transom. If that area is curved or has bumps and dips, poor
handling, porpoising and other bad behaviors will occur.
For my boat I set a goal of 1/16" (~1.5mm) of error in fairing
the frame. There were a few areas that were slightly worse (mostly
dips) that were discovered while planking. For the finished hull
<1/32" of fairness was achieved over the vast majority
of the area. This is not the fairness of a show car, but surely
flat enough for a "see yourself" shine.
Fairing sticks / battens - The battens are straight
grained flexible pieces of wood. I used both battens and plywood
scraps as battens. Probably my most used batten / faring stick
was a 2"x6'x1/8" aluminum ruler (Empire Tools). When
placed on edge it is stiff and suitable for the straight and slightly
curved sections. When placed flat, it is flexible enough to conform
to most of the curves When tilted at an angle, the degree of flex
can be varied. Sighting under the edge placed and with a light
source behind the batten, the gaps between the batten and the
frame indicate the humps and valleys.
Hand planes - Block plane, # 4 or 4 1/2 and
a #6 or #7. Other sizes are also helpful. I recommend metal (Stanley
Bailey style) over wood for this task. The intermittent cutting
action as you cross frames or battens quickly knocks the wedge
loose on a wooden plane. The planes shown (left to right) are:
block, #4, #4 1/2, #6, #7. This shows the relative size of each.
As a side note, only the #4 1/2 was purchased new. All the others
were bought used at flea markets, antique stores, rummage sales
or on E-bay.
- The flex of your sanding board should be just enough to conform
to the shape of your hull. I used 1/4" plywood. It is 4.5x33"
long. This holds three half-sheets of sandpaper. The sand paper
is held on with sanding disk adhesive, Scotch 77 spray adhesive
or double stick carpet tape.
Lots of Lights - Both overhead and to be placed
behind the battens. For my Riviera I used 8x 4' dual lamp fluorescent
lights. The walls and floor were also painted white to reflect
light and to be visible behind the batten when sighting along
Colored Markers - Black for outlining the frame
edges, red for high spots / humps, blue for valleys, purple for
chip-outs (or pick whatever colors you have on hand).
Colored chalk - This is rubbed thickly on the
wooden battens. When the batten is then moved over the frame,
the chalk will transfer to the frame high spots. Grab some of
your kids (or grand kids) "sidewalk chalk" it is big,
soft and brightly colored.
Cyano-acrylate Adhesive and Accelerator (a.k.a.
Super Glue) - Get a 2 ounce bottle of the "thick" variety.
This can be found in a hobby store (for model airplane and rocket
building) or a wood working store. A common brand is "Hot
Stuff". Do not get the stupid little metal squeeze tubes
of "super glue" that you can find at the grocery store
or hardware store. The glue hardens in the ABSENCE of air (anaerobic
reaction). This is why, if you have used the tubes, and you go
back to use them later they are always hard. When squeezing out
the glue you also squeeze out the air, causing it to harden in
the tube! You also want to use the "thickened" , high
viscosity type of glue, and the stuff in the little tubes is low
viscosity (runny). A 2 oz (50 gram) bottle is usually $6-10 vs
$1 for a 2 gram squeeze tube.
You will need this to glue back down chips at the edges of batten
and frame members that occur while fairing.
Power Plane - This is optional, but is greatly
speeds up the shaping of the chines and keel.
Rasps - I had never owned a rasp before. While struggling
through the clean-up of excess epoxy, sandpaper did not really
"cut-it". I bought a rasp and was amazed. Fun tool!
Works very quickly and provides remarkably smooth results. I tried
both name brand (Nicholson ~$20) and bargain bin (MIT ~$3). Nicholson
is better and sharper, but the cheaper one is certainly a useful
and satisfactory tool. In addition to the rasp, a double cut file
(12") is useful, and I actually like the cheaper one better!
A file card is a necessity for the file as the epoxy will fill
The fairing process is also the aerobic side of boat building
(at least so far). This is a strenuous activity that is best done
in short bursts. In addition, the pauses provide a break, and
upon returning, a lack of fairness seems more obvious. Is it that
perhaps when you are refreshed you look at the frame with a more
Marking the Frame
The edges of the frame are your reference. Each frame will be
beveled. The "high" edge should remain untouched. When
the battens were inserted, you should have set them such that
one edge of each frame is aligned with the batten and the other
is high. Starting at the widest / highest part of the boat and
working towards either end, the "high" edges should
be those closer to the ends of the boat. The reference edges are
those towards the wide part of the boat. Take the black marker
and heavily color the reference edges of the frames. The edge
to color in is the outside, where the planking will be attached.
When the fairing is complete, a thin black line should remain
along the reference edges of the frames. This is your only indication
left from the plans, that shows the proper shape of the hull.
Be sure that you do not go too far when fairing! If you accidentally
plane off the remaining line, mark it again but in a different
color. The different color will show where you may likely have
to shim later.
Prior to each planing session, run your hands along the frames
and battens looking for staples, crews, nails or other bits of
metal. Remove them PRIOR to starting to plane. Each time I put
it off, thinking I would remember later, I added a new nick to
the plane blade. Remember, sharp plane irons and embedded metal
seem to have an attraction for each other.
Start each session with a freshly sharpened plane iron. I normally
sharpen my plane irons at the end of each session, so I am ready
for the next day.
Most of the planing is done with a #6 or #7 jointer plane. This
is approximately 18-24 inches long. First plane the frames with
the plane at a 30-45 degree angle and the tail resting on the
adjacent batten(s) -- see photo at rig ht.. Take care that
the plywood gussets are not chipped in this process. Chipping
is greatly reduced if the outer edges of the gussets are first
planed at a 45 degree bevel approximately 1/8 inch deep, cutting
from the face of the ply towards the edge- not from the edge first!.
To fair the battens, chines and sheer, the plane is most commonly
used with the tail resting on the batten adjacent to the one that
is currently being faired. Where the batten is between 2 or more
others, cuts should be alternated with the tail on the batten
to the right and left (above and below) the batten being faired.
The plane is moved along the length of the batten, NOT along the
length of the plane. This is known as a shearing cut. A
shearing cut also requires less force to cut and leaves a smoother
finish than a straight cut with the plane.
The power plane is a big help for roughing in the chines and
sheers. After doing the chines, it looked like there had been
a sawdust snowstorm in the shop. Chips and sawdust everywhere.
I had trouble using it for final fairing, and switched to the
hand planes. The power plane has a tendency to dig in at
the start and end of the cut.
The silica filler is hard on the plane irons (not to mention
the odd staple and screw). I had to sharpen every 1.5 hrs on average.
The chip out would increase, shavings would get splintery, and
gussets would chip as the blade dulled. If you don't ding the
blade, it only takes a few minutes to sharpen (literally 3 to
get to hair shaving sharpness). Rather than struggle with a dull
or marginally sharp tool, take a break, crack open a beer and
sharpen the blade.
#6 plane shown in the proper orientation for planing along the
Note the narrow shaving being taken on the right side of the
Tear outs and chip outs
When planing, sometimes the trailing side of a piece of wood
will tear out or the plywood for the gussets will chip out. To
minimize tear out, bevel the trailing sides of the pieces with
the block plane or rasp at about a 45 degree angle. On the plywood,
cut though ~1.5 layers. When you make the next pass, the plywood
should not chip out. DOn't worry the bevels will be hidden later
under epoxy fillets after planking.
If you have a chip out on the plywood, stop. Any additional passes
will further fracture the chip or break it off completely. Now
is the time to pull out the super glue. Put the glue on the chip
and press it back in place. Chile holding it, spray on some accelerator.
Hold for ~30-60 seconds and it is glued in place. It will take
24 hrs for a full cure. If you are careful, you can bevel the
edge immediately and go back to planing.
Alternatively, If chipping occurs, use a marker on the outer
edge to note the damage and move on to another piece. At the end
of the session glue the chips back down with epoxy and clamp with
a clamp which has its faces covered with duct tape to prevent
sticking. The difficulty with waiting is the chips may get
Tear outs along the length of a batten are usually not important.
This is the surface that will be adhered to the planking and is
not visible. The real downside of tear outs on the battens is
that it interrupts your plane stroke. This may result in bumps
or valleys depending on how bad the interruption is and how you
re-start the stroke. If you are getting constant tear out on a
batten, reverse you planing direction, as you are probably planing
against the grain. Tear out in localized sections is usually due
to knots or swirls in the grain pattern. This is part of the reason
to save the "best" and most straight grained stock for
One step at a time
Remember this is an iterative process!!! Fair lightly overall.
Fair overall again, repeat .... If you try to do it all
in once pass, starting at one end and then proceeding to the other
the surface will be as ripply as a potato chip! The current fairing
effort relies on the surrounding area being close. Each individual
incremental section is only very little better (and can be much
worse if you are too aggressive) than the surrounding territory.
Be sure to keep alternating the sides of the boat as symmetry
of the hull is very important!. By comparing progress on both
sides, you will find it easier to see any errors in hull shape
that may start to develop.
Several times during each session, check your progress with the
battens. When sighting with the battens your line of sight is
parallel to the hull surface (get down on your knees). Having
a lamp or a light colored wall or piece of cardboard behind the
batten will help show the gaps under the edge.
Mark the high spots with the red marker and the valleys with
the blue marker. Having to hit the high spots while fairing may
seem obvious, but it is also equally important to avoid hitting
the low spots.
When fairing the battens on the first pass, very narrow shavings
will be taken. On the final pass, shavings that are nearly the
full width of the battens will be taken.
The sanding board was made from a piece of 1/4 inch plywood,
4.5 x 33 inches . The surface was sanded quite smooth. The sandpaper
was cut into 1/2 sheets lengthwise (4.5 x 11 inches). 3 sheets
were then placed end to end on the plywood. The paper was attached
with 3M feathering disk adhesive. This is commonly available at
the hardware store. Spread on the plywood and then firmly press
the paper down immediately and then wait 3-=60 minutes before
using. Alternative adhesives are Scotch 77 spray adhesive,
contact cement, and double stick carpet tape.
Over time the surface will become fairer and fairer. Using the
sanding board with 40 grit paper will leave an even set of scratches.
Areas without scratches are either low or next to a high spot.
When the scratches are even overall, you are getting close.
As you get close to a fair shape, it will seem that the wood
is evenly scratched and it become harder to discern the high spots.
To help visualize the high spots sand lightly overall on one diagonal,
moving the sanding board predominantly along its length and taking
over-lapping strokes. Now flip the board 90 degrees and make another
pass light pass. Inspect the scratch pattern. The high points
will show scratches along the flipped diagonal.
The corners of the sandpaper will wear more quickly than the
center. Buy top quality sandpaper (I like 3M). You will want a
resin coated paper. The grains on the cheap no-name or house brand
stuff from the home center will very quickly peel off. I saw a
better than 4:1 improvement in how long the sandpaper would last
using top quality sand paper versus the house brand / no-name
sandpaper. The premium paper only costs 25-30% more. Even better
you may wan to get 6x48 stationary sander belts and slice them
open. These really last a long time.
The sanding board is applied on the diagonal, similarly to the
angle that the planking will be applied. Start sanding in whatever
direction is most comfortable (e.g. stern-keel to bow-chine as
shown). Be sure to alternate directions spinning the board roughly
90 degrees (stern-chine to bow-chine) periodically. The boat hull
must be fair in ALL directions.
The stroke direction with the sanding board can be random. In
some cases stroking along the length of the board can be done.
In other areas, moving it along the length of the boat (pushing
it at an angle) will be appropriate.
Remember this is an aerobic exercise! Use your legs (moving your
entire upper body) while sanding and planing. The motions are
smoother than simply using your arms. It is also much easier on
your back and arms.
Keep a face mask on while sanding. I prefer an older respirator
(cartridges don't need to be new, just acting as dust filters).
You don't need to deeply inhale the sawdust.
Sanding board top view.
The stern of the boat is to the left in this view. I am standing
on the port side of the hull
The board is held in the stern-keel to bow-chine angle. The stern-keel
end is in my left hand (wrist watch).
Sanding board bottom view
Alternating tools, techniques and directions.
Sometimes , when planing and sighting with the batten, it is
hard to find the high spots. Using the sanding board and then
looking for scratches will highlight the high spots. Switch back
to hand planing for further work. When sanding, small bits of
abrasive grit will be left embedded in the wood, and this will
cause the planes to dull more quickly.
As you move along the bottom towards the bow, the curves are
much more extreme. Switching to a shorter plane such as the #4
and possibly to a thinner sanding board (4mm vs 6mm) will be required.
In addition, since the curves vary in the degree of curvature
with direction, you may find it easier to use the thicker board
on one diagonal (such as stern-keel to bow chine) and switching
to the thinner board on the other diagonal (stern-chine to bow-keel).
On the sides, the convex shape (tumble home) becomes very pronounced
towards the stern / transom. Here a thinner board is helpful as
well as a thicker one can get very tiring to use due to the force
required to bend the board and the awkward angle of use, as you
are down near the floor.
Sometimes you may have an area that is just too low. This can
be due to uneven fairing or a part that just started out low.
Rip thin strips of wood and glue them over the low areas and extending
several inches past the ends of the low spots. These strips will
then be planed down once the glue hardens.
It took approximately 30 hours over the course of 2 weeks to
fair the frame. In the process of faring nearly a full size trash
can (~40 gallons) of chips and shavings was removed.
Keep your sanding board. It will find further use as you fair
each layer of planking. More fun to come!
|Next - Completed frame
Top of Page
©2000-2007 Mark Bronkalla