Contents
Sail Sizing
GENERAL...
This essay is based on an old essay, very old by internet standards,
written in Feb 1998. I'm changing it around a bit and adding things
that were learned over the past 13 years, but the basics are still the
same.
The question that always comes up is "How much sail area do I put on a
boat??" To me the secret here has nothing at all to do with placing a
pretty sail rig on the side view of the hull. That's about the last
thing you do. The secret is first to establish the stability of the
hull, which is determined almost totally by the cross sections of the
hull, their shape and width in particular. So one might ask, "How can
you put a Sunfish rig on an 8' Catbox when the Sunfish was twice as
long?" The answer is that the Catbox is wider and squarer than the
Sunfish and has walk around stability, something the narrower but
longer Sunfish doesn't have. In a way I'm thinking the length of the
hull has nothing to do with the stabiltiy of the thing except that long
hulls are heavier than short hulls and the weight figures directly into
the stability. Don't get me wrong - long hulls can be a lot faster than
short hulls, more seaworthy and roomier, so think long before you think
short. But if you are trying to adapt a certain sail plan to a new hull
you don't need to think about length hardly at all.
GETTING DOWN TO BUSINESS...
Here is the view of the hull under the force of sail that we will be
looking at:
Figure 1 shows a diagram of how to figure the righting moment of a boat
at a certain angle of heel. This is a static analysis which is to say
the boat is not accelerating. It can be moving, but all the forces on
the boat are in balance. If the boat is being pitched and rolled about
in angry seas, a much more complex analysis is required. Also, the
distribution of the ballast (as opposed to simply the location of its
center of gravity) becomes a factor in a dynamic analysis.
What we have here is the wind's pressure on the sail, up high, and a
balancing load on the keel or fin, conspiring to tip the boat over.
That "moment" or torque, is counteracted by the weight of the boat
pushing down and the buoyancy of the boat pushing up. These last two
forces are not in line with each other when the boat heels but are
separated by a distance called the "righting arm". If the weight of the
boat in is "pounds" and the length of the righting arm is "feet", the
righting moment is measured in "foot pounds". Note here that the
stability of the boat is a direct function of the weight. So heavier
boats are more stable than light boats and could carry more sail in a
given wind.
To figure the foot pounds of the righting moment, you need to know the
weight of the boat and the location of the center of that weight - the
"CG". Also you need to know the location of buoyancy of the heeled
boat. (The buoyancy itself is the same as the weight in a static
analysis.)
...FIGURING CG...
The CG is sort of the "average" location of all the weights. To get
stated figuring a CG location, you must have a reference line and in
these examples I will use the bottom of the boat as the reference.
We're only going to figure the vertical (up and down) CG in these
simple examples, but in a complex project you might also figure the
location laterally (side to side} and longitudinal (along the length)
locations.
Let's say our boat only had two elements, the hull and all its contents
weighing 500 pounds with that weight centered 2' above the bottom, and
let's say 250 pounds of internal ballast centered 3" (which is .25')
above the bottom. So the total weight is 750 pounds.
To find the CG, you multiply each weight by its vertical location, add
all those pieces, and then divide that sum by the total weight. So the
example calculation would be CG = (500x2)+(250x.25) all divided by
(500+250 and that equals (1000+62.5) / 750 = 1.42' above the base line.
So the effect of the 250 pounds of ballast was to lower the CG from 2'
to 1.42', a difference of 7".
In this calculation the makeup of the ballast is not a factor. The only
factors are the weight and location of the ballast. The only way the
ballast material could be a factor is if it were of such density that
it could be centered closer to the bottom. For example if the ballast
were water inside a rectangular tank 6" deep, 4' wide and 2' long, it
would amount to 4 cubic feet of water which is about 250 pounds and it
would center at 3" above the bottom and provide the above CG location
of 1.42". If we switched to lead which is 11 times denser than water,
we might have a plate only .54" thick. We could mount it centered .27"
(which is .0225') off the bottom instead of 3". The new CG would be
((500x2)+(250x.0225))/750=1.34'. So the CG of this lead ballasted boat
is lower by less than an inch.
...FIGURING BUOYANCY LOCATION...
This is very hard to do my hand. I'm going to use Hullform6S as a tool
to do this. To keep lots of variable from getting in the way, I'm going
to use the above pictured "boat" as the example. It is just a box, 16'
long, 4' wide, 2'deep. It weighs 500 pounds with its unballasted weight
centered on the top of the box, 2' above the bottom. I'm going to
ballast the box in four different ways, roll each example over at 10
degree intervals to 90 degrees, and record and plot the righting
moments as predicted by Hullform6S.
I should note here that the original essay was steering towards the
effect of different sorts of ballast which is interesting in itself.
And although the test model was a simple box, programs like Hullform
can figure the hydrodynamics of about any shape you care to type into
it. If you have a very specific hull shape in mind you need to define
it to Hullform essentially by typing in offsets taken from your hull.
...EXAMPLE A...
So example A will be the above unballasted 500 pound box.
...EXAMPLE B...
Example B is shown below. It's the same as A exept it has a 250 pound
lead fin with the weight of the fin centered 2' below the bottom. So
the CG of this combination is .66' above the boat's bottom. But, the
250 pounds of lead displaces some water, right? Its volume amounts to
.36 cubic feet of lead displacing the same amount of water which is 22
pounds of water trying to float the lead back up. So the total ballast
effect of the lead while it is under water is actually about 228
pounds. If the boat heels to the point where the lead is totally out of
the water, it has 250 pounds of ballast effect.
..EXAMPLE C...
Example C has the same 250 pound fin configuration as example B except
a water filled tank, 6" wide, and 4' deep, is used instead oflead. That
amounts to 250 pounds of ballast water centered 2' below the hull so
the CG of the total boat/ballast combination is the same .66'above the
bottom. Again, the 250 pounds of external ballast displaces some water.
The water ballast displaces its own weight in water! So the ballast
weight is exactly balanced by the buoyancy of the displaced water. So
this underwater tank has no ballast effect as long as it is under
water. When the boat heels enough to raise it out of the water, it
becomes effective.
...EXAMPLE D...
Example D is really the same as C except the ballast tank has been
moved inside the hull. So now it is 6" deep, 4' wide and 2' long. The
ballast weight is centered 3" above the bottom of the hull and the
overall CG is at 1.42' above the bottom. We don't have a separate
external tank displacing water. But compared to example A the hull
sinks a bit deeper to float that extra weight so the statics of the
Hullforms analysis is a bit different.
...EXAMPLE E...
Example E has a 250 pound V shaped ballast tank on its bottom. Whether
it might be called internal ballast or external ballast is in the eye
of the beholder. I included it because I thought it might represent
some water ballasted trailer sailers you can buy.
The results of the Hullform6S study are shown below:
Curve A is, I think, pretty typical of a light flat boat with no
ballast. The maximum righting moment of about 450 foot pounds is
reached quite quickly although I think a real boat would have the peak
at about 20 degrees. The boat looks to capsize at about 45 degrees
heel. My experiences with Jinni, about this size and weight, were
similar. It capsized twice in the time I had it. Both time it went over
well before it shipped any water over the rail.
Curve B shows the how effective metal outside ballast can be. Not only
is the maximum righting moment about twice that of example A, it still
has substantial righting ability at 90 degrees of roll. It will tend to
roll upright at any angle of heel up to about 110 degrees.
Curve C, external water ballasted fin, is an interesting one. Until the
fin starts to exit the water as the boat rolls, it has no effect. When
fully out of the water (about 80 degrees of roll) it is as effective as
metal. In between its righting moment goes to about zero. If rolled to
about 50 degrees it might stay there until acted upon by an outside
force such as a wave or maybe the crew moving about. If rolled to a
greater heel angle it will try to return to 50 degrees. If rolled to a
lesser degree it will roll fully upright.
Curve D, internal ballast, has about 50% greater maximum righting power
than no ballast or the external water fin. It should easily outsail a
water fin boat up until it capsizes at about 65 degrees of heel. At
that point the water fin boat gets back on its feet while the internal
ballast boat flops over.
Curve E, V ballast tank, cuts across about everything as a compromise.
It doesn't have the initial stability of the internal ballast boat, but
it has positive stability at high roll angles. I've heard some water
ballasted production boats behave this way.
...CONCLUSIONS...
I can't see any obvious winner here. All have advantages and
disadvantages. However, if you had a particular type of boating in
mind, the chart may help you make a choice. For blue water sailors, the
metal fin seems the way to go. Lots of righting ability. For inshore
sailors where a rare capsize won't mean death, perhaps the internal
water ballast, with its simple trailering abilities due to light
unballasted weight and very low draft, will be to your advantage. The V
tanked boat might be best for someone who cares a bit more about
ultimate stability. A combination of all of the above might be in order
for some folks.
Now, given the curve above, you can say that (assuming the height of
the sail rig is the same for all cases) in a given wind these boats
could carry sail area in proportion to the maximum righting moments
shown. A, C, and E should all have the same sail area since they all
have a peak righting moment of about 400 foot pounds. The effect of the
ballast here is to delay capsize to deeper roll angles. But D, with
internal ballast, could carry about 50 percent more sail, and B, with
the metal fin, could carry twice as much sail.
...THE NEXT STEP...
...is to relate the stability and wind speed to the sail area.
Contents
Mayfly16
SAILBOAT, 15' X 5.5', 350 POUNDS EMPTY
Mayfly16 is large enough to swallow up three men or maybe a family
with two kids. She has two benches that are 7' long and there should
be plenty of room for all. I would say that her fully loaded maximum
weight might be 900 pounds and her empty weight about 350 pounds,
leaving 550 pounds for the captain and crew and gear.
At the same time the Mayfly16 can easily be handled solo, although
with just the weight of her skipper she will not be as stable as when
heavily loaded. The boat also has two large chambers for
buoyancy/storage and I can see her used as a solo beach cruiser
because the floor space is large enough for a sleep spot. I've made
her deep with lots of freeboard.
Mary and George Fulk built the prototype and passed by here with the
prototype on their annual migration north for the summer and I had a
chance to see and sail in Mayfly16 for a short bit. Weather was hot
and the wind light and steady, perfect for testing. She sailed quite
well I thought and everything worked as planned. It certainly was
roomy and easy to rig and use.
The balanced lug rig sets on short spars and sails very well reefed,
in fact can be set up with jiffy reefing. The spars are all easily
made and stowed, the mast being but 14' long setting 91 square feet
of sail. In addition there are oar ports for those with lots of time
and little money and a motor well for those with lots of money and no
time. Two horsepower is all that a boat like this can absorb without
going crazy.
The motor well is an open self draining well that uses the full width
and depth of the stern. It will come in handy for storing wet muddy
things you don't want inside the boat, like boots and anchors. I've
suggested in the plans that the rudder can be offset to one side a
bit to give more room for the motor. We did not use George's little
Evinrude since the boat sailed easily in all directions, but George
says the sidebyside sharing on the stern of the motor and rudder
works fine. There was no interference with the rudder. (As with any
outboard on any sailboat, the motor has a desire to grab the sheet
with each tack so you usually have to tend the sheet a bit.)
Mayfly16 uses conventional nail and glue construction needing six
sheets of 1/4" plywood and two sheets of 1/2" ply.
Plans for Mayfly16 are $35.
Contents
Prototype News
Some of you may know that in addition to the one buck catalog
which now contains 20 "done" boats, I offer another catalog of 20
unbuilt prototypes. The buck catalog has on its last page a list
and brief description of the boats currently in the Catalog of
Prototypes. That catalog also contains some articles that I wrote
for Messing About In Boats and Boatbuilder magazines. The Catalog
of Prototypes costs $3. The both together amount to 50 pages for
$4, an offer you may have seen in Woodenboat ads. Payment must be
in US funds. The banks here won't accept anything else. (I've got
a little stash of foreign currency that I can admire but not
spend.) I'm way too small for credit cards.
I think David Hahn's Out West Picara is the winner of the Picara
race. Shown here on its first sail except there was no wind.
Hopefully more later. (Not sure if a polytarp sail is suitable
for a boat this heavy.
Here is a Musicbox2 out West.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the
"ketchooner" rig, featuring his own polytarp sails, that is shown
on the plans. Should have a sailing report soon.
And the Vole in New York is Garth Battista's of
www.breakawaybooks.com, printer of my book and Max's old outboard
book and many other fine sports books. Beautiful job! Garth is
using a small lug rig for sail, not the sharpie sprit sail shown
on the plans, so I will continue to carry the design as a
prototype boat. But he has used it extensively on his Bahamas
trip towed behind his Cormorant. Sort of like having a compact
car towed behind an RV.
And a Deansbox seen in Texas:
The prototype Twister gets a test sail with three grown men, a
big dog and and big motor with its lower unit down. Hmmmmm.....
Jackie and Mike Monies of Sail Oklahoma have two Catboxes
underway....
Contents
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED
BY DATE
SOME LINKS
Mother of All Boat
Links
Cheap
Pages
Duckworks
Magazine
The Boatbuilding
Community
Kilburn's Power
Skiff
Bruce Builds
Roar
Dave
Carnell
Rich builds
AF2
JB Builds
AF4
JB Builds
Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived
copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds
Toto
Table of
Contents
