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- Junk
Sail Tutorial
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- Y2K is mainly gone and here are some
illustrations - at long last - of various authors'
geometries. More coming.
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A junk, probably photographed in the 1870s or
so, with bamboo-matting sails, and an array of
assorted canvas kites. The mat sails are the
original form of Chinese junk rig. Later cotton
became common, and today boaters use Dacron,
cotton, or polytarp.
Photo
courtesy Mark Anderson and his mighty
stereopticon.
- Contents:
- Sail
Geometries
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- A Traditional Junk
Sail by Vincent Reddish
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- Junk Rig Shapes
- These sketches show different approaches
to shaping the sail. I was working on showing
them all to the same scale so the size and
shape could be directly compared, but have
not had time to play with my CAD program.
See also my page about sampans: Cranks
With Planks
- Hasler
- Colvin
- Van Loan
- Yang Tse Pelican
- Traditional:
Vietnamese
- Bolger Gaffer
- Traditional
- Traditional
- Tiny Junk Sail on a 12-ft
Peero
- Etc
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- The Rigging
- The Sheeting
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Different
Sail Geometries
- Everyone who writes about junk rigs seems to
have a favorite sail geometry. Blondie Hasler's may
be the best-known and most-used because of his book
(Practical Junk Rig, by Hasler and MacLeod)
and its following in the UK. Tom Colvin and Derek
Van Loan are the two best-known American junk-rig
champions.
Traditional Chinese rig geometries varied widely
but two sources covering them are G.R.G.
Worcester's Junks and Sampans of the Yangtse
and the section on "Nautics" from Joseph Needham's
immense Science and Civilization in China.
Vincent Reddish analyzed photos in several
books in order to come up with his formula for a
traditional South China sail. Tom Colvin's
preferred sail shape is much the same as Reddish's.
Reddish has provided a very simple "cookbook"
approach to making the proper sized sail.
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Vincent
Reddish: South China Junk Sail
Reddish decided that the Chinese
must have used simple proportions to lay out a
sail, perhaps employing a rope which they could
fold into different lengths. He compared photos of
seagoing junk sails to get the following relations.
He settled on the boom as the unit and all other
measurements are fractions of the boom length.
Vietnamese and Thai balanced lug sails are
shaped similarly in many cases.
Reddish used two bamboo poles for each batten,
lashed (taped, actually) together in the middle.
The skinny ends of the poles met in the middle of
the sail while the larger ends were at luff and
leech. His yard and boom were of pine. He described
two sails. The larger one had a 17-ft boom.
His approach is nice and cheap.
His sails were made from polytarp, the battens were
bamboo and the yard and boom were pine. Boltropes
were polyester.
Laying Out the Sail
So the only dimension needed is the desired boom
length. Ten units is a good sample length; we'll use
feet. That way, you can simply use these numbers as
percentages. (Think of the Yard length; divide it by
10; 6.67 ft becomes 0.667 or 66%).
You can think of the center of effort as lying
above a point halfway along the boom. Reddish also
says that "only 8%" of the sail area is typically in
front of the mast. This would mean something a little
over one foot of our 10-ft boom ahead of the mast: 7ft
x 1.5ft is 10.5 sq ft so the boom must cross at about
15 inches.
Boom:
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10 ft
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Sail Area:
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111 sq ft (Aspect ratio is
about 1.1)
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Spars:
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Yard:
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6.67 ft
(66% of Boom)
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70 deg angle to Luff
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Yard Offset
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2.25 ft
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Distance from peak of Yard to the Luff
perpendicular
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Battens:
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5
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10 ft
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45 deg (approx)
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4
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"
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30 deg
"
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3
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"
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20 deg
"
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2
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"
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10 deg
"
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1
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"
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05 deg
"
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Boom
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10 ft
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< 5 deg
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Panels:
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Luff Overall:
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6.67 ft
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Each Panel
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1.11 ft
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Each Panel's Luff
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Head
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6.67 ft
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Panel 6
(Attached to Yard)
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Upper Roach
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5 ft
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Panel 6 (Yard Peak to
the end of Batten 5)
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Leech "L"
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3.33 ft
(50% of Yard)
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Batten 4 - 5
Panel 5
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Leech Dimension for Panel:
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4
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2.9 ft
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Batten 3 - 4
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3
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2.5 ft
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Batten 2 - 3
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2
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2.0 ft
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Batten 1 - 2
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1
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1.67 ft
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Panel 1
(Boom Panel)
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This information is from Practical Boat
Owner 1/92, "Junk Rig Decoded". Many thanks to
Peter Berrie for sending me a copy of the article.
Making the Sail
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Spars are laid on the ground and staked in
position. A rope (heavy grey line) forms a
framework along the luff and leech and along the
boom and yard. Reddish used 5mm polyester rope
(about 3/16"). This frame of rope is
interesting, because it allows a very taut luff
without depending on the sailcloth to hold the
tension. It also spreads the stresses on the
leech directly to the battens.
The sail itself was made of a single polytarp
cut to the proper shape and "hemmed with a
thinner rope" (1/8" polyester would work; so
would nylon or polyester webbing).
He is not specific about lacings other than
to say that the sail was then "stretched tight"
and laced to the framework and the battens. I
imagine this means sewing loops around the light
hem rope and the boltropes with heavy sail
twine.
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Worcester depicts a double-roping system used
on the large sails, and if I can find the
material I will add a scan of his diagram. It
doesn't seem to be strictly necessary, but it
would certainly add strength. This traditional
approach includes a head batten laced to the
sail. The head batten is then tied to the yard.
From what I've read, I suspect this is because
the yard was rarely removed from a vessel; the
head batten is a surrogate yard which is used
when laying out and constructing the sail.
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COD & Bob Cavenagh
construct a 160 sq ft sail, per Reddish, in
about 4 hours.
That's Bob doing quality control on the
luff.
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Sail on Bob's canoe
yawl. The boom is swung toward the camera and
is longer than it looks. It did work, though
we had almost no wind, and not enough time to
work out all the sheetlet lengths to our
complete satisfaction.
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Camber?
Reddish also says that he picked batten sizes which
would give him about a 10% camber in normal sailing
conditions. On our 10-foot sail, the battens would
bend so that they were about 1 foot out of line near
the middle of the sail. (Make sure the boom is stiff
enough that it does not bend, however). Photos of
traditional Chinese sails show some bending,
especially when sailing to windward.
Finally Reddish says he made each panel somewhat
baggy simply by pushing it down in the middle as he
attached the battens. The corners wrinkled some, he
says, but that did not seem to cause any problems.
Update: 1998
This note comes from Bill Samson in Scotland. As it
turns out, he was a student of Dr. Reddish some years
back and was able to get an update for us. The main
differences seem to be returning to rigid battens and
adding some darts to the leech to give the panels a
little shape.
Dear Craig,
I had a long telephone conversation with Vincent
Reddish this evening.
I asked him how he fixed the battens to the sail
and he said he didn't - the sail is fixed to the
battens! The sail is sewed on, with whipping twine,
to the yard, boom and bolt-ropes. The battens are
double - bamboo on each side of the sail. He puts a
stitch in every six inches or so, using burgee
cord, so that the sail is sandwiched between the
battens.
He uses a 360 square foot polytarp sail, made in
this way, on his Laurent Giles designed VERTUE
cruiser. His original experiments were with a
Laser-17. He was at pains to point out that his
sail is quite different from Hasler/Macleod, being
based directly on those found in Chinese boats.
His main reservation about polytarp is its lack
of stretch compared with the Chinese material, so
he has put in a little broadseaming along the leach
- at each batten. [To give the panels some
camber]
He firmly believes in rigid battens with as
little bend as possible - that's what the Chinese
use. The fan arrangement of the battens means that
sail twist is converted into horizontal aerofoil
sections and the amount of lift is controlled by
how much twist you allow.
He mentioned something about the drive coming
from vortices - particularly near the top of the
sail. In fact the top panel of the sail behaves
very like a crab-claw sail.
Just to show that Vincent isn't all theory and
no practice, he took his VERTUE around Ireland last
year single handed (over 1000 miles and only had to
leave the cockpit once, to redo a knot that he'd
tied badly in the first place). Not bad for a
67-year-old! Although his boat has a Bermudan rig
available, he much prefers the convenience and
comfort of his home-made polytarp junk sail - What
he calls 'armchair sailing in a boat'!
For those on the list who've never heard of him,
Dr Vincent Reddish is an eminent astrophysicist who
was Astronomer Royal for Scotland, and Professor
and head of the department of astronomy in
Edinburgh University. Maybe his greatest claim to
fame is that he managed to supervise me to
successful completion of a PhD in 1971! <G>
He is currently leading a world-wide network of
scientists in a systematic investigation into the
physics of dowsing - a real but, as yet,
unexplained phenomenon.
-- Bill
Bill's Chebacco News: http://www.taynet.co.uk/users/wbs/chebacco.htm
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Some
Junk Sail Shapes:
Hasler
the well-known westernized
balanced lugsail from Hasler and
MacLeod. Note the lower panels are all
identical, having the same measurement at
both luff and leech; and the upper wedges are
identical.
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Colvin
from his "Gazelle"
junk-schooner design. He believes in a more
traditionally rounded shape than either Van
Loan or Hasler. The batten-to-batten spacing
is equal on the luff and the sail fans more
toward the leech, unlike Hasler
or Van Loan.
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Van
Loan
his ideal is a
flat-headed and narrow sail. P is the
batten-batten spacing.
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These two drawings were scaled to be cut out
for a model I was making and so nominally
refer to feet, but the ratios are
dimensionless. I was keeping the luff length
constant.
The gray area is the higher
aspect sail superimposed on the lower, but of
course the sails would likely have different
absolute dimensions.
Proportions are given as percent of "x", the
boom length.
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Yang
Tze Pelican
a junk rig on a small
day-cruising boat. The Great Pelican
(right) has a sail much like the canoe sails
of the late 1800s, but shapewise it's not
that far from the twakow sail shown
immediately below...
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Twakow from
William Maxwell Blake.
Sorry for the crummy scan.
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An artist's rendering of the
Yangtze Pelican version of the Great
Pelican hull.
From V. Sokolow.
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Traditional
Vietnamese
(as used on Tim Severin's
bamboo raft Hsu Fu; courtesy Nick
Burningham)
More on Rigging Hsu Fu.
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Phil
Bolger's
"Chinese Gaffer"
an interesting idea
where an aerodynamic shape is combined with
Chinese-style multiple sheeting points for
sail shape control and light sheet loading.
Bolger has sketched out at least three
approaches which have appeared in magazines,
and his book 103 Small-Boat Rigs
includes longer essays on three rigs which
incorporate the Chinese Gaffer. The idea is a
cruising sail which also has good upwind
performance.
This one (right) has a single
sheet to the yard, and another single sheet
to the battens. The hauling part of the sheet
starts at the top batten via pairs of
sheetlets and is the version which has been
tested in use.
The version below on the
"Micro Navigator" perhaps owes something to
the Florida sharpies of Commodore Munroe's
day, which had high-peaked battened gaff
sails with a sprit-boom but only a single
sheet. This sail is small and has 3 separate
sheets.
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Slocum:
Liberdade
Joshua Slocum built and
sailed a "dory-canoe" from Brazil to the USA
before he ever sailed alone around the world.
This is a great
book (it's online here).
One of the few extant photos:
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Audemard
His ten volume set of
monographs contains umpteen sketches of boats
and sails.
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Petrie
"Voiliers d'Indochine"
shows Vietnamese boats in detailed
sketches.
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A
Sail on a Peero
The framework for a
small Reddish-type sail for the Bolger
Sailing Peero.
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Etc...
English yacht Heathen
Chinee, dual centerboards and lugsails.
Note jib battens.
This boat was only about 24ft on the
waterline. Something on the boat was
published in Forest & Stream and
I'd like to find out more.
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Etc...
At one point I prepared some
drawings based on a 174-sq. ft. junk sail
described in a U.N. fishing report as
something which could be effectively rigged
by impoverished fishermen. Click on them for
the full-size GIFs. The drawings are
not really to scale.
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Etc...
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RiggingMaybe
it doesn't have to be as elaborate as this
lake-trawling sampan.
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- There are loads of ropes tied to a junk rig.
It's baffling. It was for me. Fortunately, most of
them are set up once and that's it. When you look
at each line by itself and understand its purpose
you see it's not such a mess at all. One benefit of
all those lines is that stresses on any one part of
the rig are very low. The highest stresses are
probably along the luff since you want a nice taut
luff on your sail.
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- Note: I've been working on a way to
present this information and it's a little more
complicated than I anticipated - the
presentation, not the actual knots and lines,
but we will get there.
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- Sheeting
- There are two basic approaches: a multipart
mainsheet with euphroes and multiple sheetlets, or
a single-part mainsheet with a single sheetlet to
each batten. Traditional Chinese sails, and Colvin
and Hasler's ideas, fit in with the first. Van Loan
and Phil Bolger subscribe to the second.
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- Note: this page isn't ready yet.
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To
Plywood
Boats or to The
Cheap Page or to The
Odd Sails or to the Top.
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