Making a Tyvek Sprit Sail on a Sewing Machine
 by William R. Watt
    Ottawa, Canada
    June 2004
 Revised June 2005



This tells how I made a 30 square foot sprit sail out of
Tyvek(TM) builder's paper. The Tyvek came from a construction
site. I assumed builder's paper was 10 feet wide so I asked for
10 feet to get a 10x10 piece. This paper was only 4.5 feet wide.
I didn't say anything when the guy brought me a narrow 12 ft
strip of Tyvek. He wouldn't take any money for it. At home I
tried to figure out how a sail could be cut from the narrow shape
but gave up. The Tyvek was rolled up and put in the back of a
closet.

The home made light nylon sprit sail on the Dogskiff was getting
baggy after four years of use and losing its ability to point, so
I took another look at the Tyvek. I've come to prefer a sprit
sail with a mast sleeve after making one for the one-sheet
Loonie. The sprit is tied to the sail, not the mast, so the sail
can rotate freely around the mast. Ideally a rotating mast is
wanted for a sprit sail but masts don't always rotate well.
Hanging the sprit on the sail frees the sail from the mast so the
mast doesn't have to rotate. By taking two 18 inch slices off the
top of the Tyvek and folding them over to make a mast sleeve I
was able to draw a sail 5 ft wide and 9 ft tall. For the shape of
the sail I used the information I collected from various written
sources and put on my website, and got a sail of the recommended
proportions.

Before making the sail I cut an 8.5 ft, 1.75x1.75 inch mast out
of a used 12 foot spruce 2x4. The corners were roughly sawn off
with the blade set at 45 degrees to make an 8-sided mast and then
a 40 pound weight was suspended at the midpoint of the luff to
measure the mast bend. (The weight is supposed to be 50 pounds
but I had a 40 lb concrete block handy.) That's when the mast
broke. My plan to cut around some nail holes in the wood had not
worked out. I went to the public library, consulted Fred
Bingham's book, made an 8-sided spar gauge according to his
illustration, and cut a shorter mast out of an 8 foot spruce 2x4
which had no nail holes in it, avoiding the larger knots. The
mast has a 2 inch bend, the same as the 7.5 ft mast for the
existing sail which was also cut from a spruce 2x4. Recently I
read a different opinion that the weight for measuring the mast
bend should be half the sail area. I assume that's in pounds and
square feet. For a sprit I decided to use the one off the nylon
sail, a sectional spar made out of three discarded tubular broom
handles, by replacing the middle section with a shorter one cut
to suit the new sail. By interchanging the middle section the
sprit serves both sails.

All of the descriptions of Tyvek sails I found on the Internet
used double-sided outdoor carpet tape to join the seams. I wanted
to try sewing the sail on a sewing machine instead. From the
condition of the mould on the delaminating carrying case I'd say
my second hand portable sat in a damp unheated garage for a few
decades. I had to hold my nose and discard the case when I bought
the machine for $5 at a rummage sale. It was cleaned and oiled 
and put aside for a couple of years until I decided to make this
Tyvek sail. In lubricating the sewing machine again I got
sidetracked dismantling the machinery to see how it worked. It
has stitch length adjustment, tension adjustments, zigzag
adjustments and decorative stitch cams all cleverly built into a
small space. Eventually I put the sewing machine back together
again and started sewing the sail.

Cutting the Tyvek was easy. The scissors did not have to be
opened and closed, just pushed along the pencil line, cutting
like a knife. Sewing the Tyvek was a lot easier than sewing
cloth. (I had previously sewn the nylon sprit sail on a sewing
machine.) I read somewhere that a zigzag stitch should be used on
sails to allow for seam stretch so I set up the machine for a
zigzag stitch and tested it on some of the cutoff scraps. Sewing
hems in the edges was a cinch. Tyvek is like stiff paper. To make
a hem just fold the Tyvek over and crease it down and it stays.
Then run the hem through the machine. Tyvek is slippery and
doesn't get pulled through under the needle by the jaws on the
sewing machine very well so I helped feed it through by hand.
Being stiff and slick the Tyvek went through fast and clean. Very
easy to sew. The only problem was I set the jaw (foot) tension
tight to try and pull the slick Tyvek through the machine and the
jaws scored the underside of the Tyvek. I don't know if that
weakened the fabric very much. Another time I would not set the
foot tension so tight, checking for scoring on the test material
before starting to sew the sail. The machine was able to sew
triple thick reinforcement patches at the corners of the sail.
When sewing through four thicknesses of material at the ends of
some seams I turned the wheel by hand. Because of my lack of
sewing ability the seams are pretty ugly. I didn't want to take
the time to double stitch until I tried the sail to see if I
liked it. I found it helpful to push two tables together to
support more of the sail when sewing with the machine.

So far the sail works well. It's cut a bit too full but I'm not
fussy. It moves the boat along well upwind and down. I followed a
small pocket cruiser around one afternoon and was pointing as
high. The sail has just the right amount of power so a gust of
wind heels the boat without capsizing it. I like my sailing
relaxed without a lot of scrambling about inside the boat. The
front of the sail at the bottom (tack) is loose. I am thinking of
inventing a sprit cunningham to go up inside the sleeve and keep
the tack taught.

The things I don't like about Tyvek are the advertising printed
on one side; the way the surface wore away when I tried to erase
pencil lines (decided to leave the pencil lines on the sail); and
the noise. Tyvek is very stiff. It makes a loud noise when it
moves, I call it "thunder paper". The noise soon disappears when
the sail is broken in. One other disagreeable thing about Tyvek
is the force needed to push a needle or pin through by hand. Hand
sewing is more difficult than with fabric.

Here are a some details about this particular sail.  
 
1. The curves were measured back from the edges of the sail. That
may be obvious to most people but I had to think for a while
about how to do it. The sail was drawn straight-edged as large as
possible from the piece I had, the camber measured in from the
edges, and a batten was bent around three bricks (two endpoints
and point of maximum camber) to draw the curved edges.

2. The luff has two cambers, the normal one 1/3 up from the foot,
and the one for the mast bend 1/2 way up. The two curves were
drawn independently along the edge. A pair of dividers was used
to add the curves together at several points along the luff, and
the combined curve drawn freehand by joining the points.

3. The luff curve was drawn on the sail and the mast sleeve was
sewn on along the curve. That may also be obvious to most people
but it took me a while to figure out how to avoid cutting a
curved mast sleeve.

4. A triple-thick pocket was hand sewn into the top of the mast
sleeve at the front to receive the top of the mast. At first it
was too low and the sail did not set well along the top. When the
pocket was raised the top of the sail set fine. At first the top
of the mast would bind in the pocket and the sail did not rotate.
After the top of the mast was sanded smooth and coated with
polyester resin the mast still sometimes bound in the pocket and
the sail would not rotate. Finally an aluminum beverage can with
its top cut off was put in the pocket and the sail has rotated
since.

5. The bottom of the sprit ties to a loop of line sewn to the
front of the sail. The weight of the sprit is adjusted with a
line running up inside the sleeve to the pocket at the top. There
is a slice of plastic pipe sewn into the sleeve at the bottom of
the sprit which holds the sail and the bottom of the sprit to the
mast.

6. Someone wrote on the Internet that grommets do not hold up
well in Tyvek. One eighth braided nylon line was hand stitched
along the edges of the corner reinforcements leaving a loop at
the corners for tying on the sprit and mainsheet. The line was
crossed over itself at the corner so the strain is directed along
the line. 

                            - END -