Over All Length (LOA):
Overall lentgth from fore to aft
Over All (BOA or B)
Maximum width of the
Length Water Line (LWL):
Length of the hull from the fore perpendicular to the aft
perpendicular at the waterplane level.
Used to estimate the maximum speed of the vessel. The
longer the LWL, the faster the boat.Also called Design Water line.
Water Line (BWL):
Width of the hull at
the level of the water plane.
It is a fundamental element of initial stability.
Depth of the hull below the water plane
Block Coefficient (Cb):
Coefficient of filling of a rectangle block of length
equal to LWL and width equal to maximum width at the waterplane.
This coefficient varies from 0.4 (sailboat) to 0.85 (barge).
It gives an idea of the "V" of the hull. It is mostly used as a criteria
for lag\rge vessels
Prismatic Coefficient (Cp):
Coefficient of filling of a "prism" of a length equal
to LWL and section identical to the maximum cross section.
This coefficient varies from 0.5 to 0.8 resoectively
for a thin and regular shape (sailboat) or a wide and square shape (barge)
Wetted Surface:Area of the hull in
water.Designers looks for the smallest area to reduce friction,
of the vessel below the water plane.
Waterplane Area:Horizontal area defined by
Immersed volume x specific weight of the liquid in which is the boat.
Displacement gives the overall load the vessel can
carry (deadweight plus payload).
With freash waterm displacement equals the immersed volume. With sea
water, displacement is higher than immersed volume.
Center of Buoyancy:
The center of the hull below the water plane is the point
where the Archimedis force applies
Sail Area Displacement Ratio is the
sail area/displacement ratio. This ratio indicates how fast the
boat is in light wind. The higher the number the faster the
Boats have ratios between 10 and 15.
* Cruiser-Racers have ratios between 16-20.
* Racers have ratios above 20.
* High-Performance Racers have ratios above
SA / D = Sail
Area / (Displacement in Cubic Feet )2/3
Displacement- Length Ratio is
the displacement to length ratio.
indicates if the boat is a heavy cruiser (results greater than 325) or
a light displacement racing boat (results less than 200).
D / L = Displacement / ( 0.01 * LWL )3
in long tons
calculation moment of inertia of the waterline
drow shows a typical waterplane with a rectangle around it that exactly
encloses it. The difference in area between the area of the waterplane (WPA)
and the full rectangle (DWL X BWL) is called the coefficient of the
waterline plane (CWP). The coefficient is pretty constant for
average sailboats, as follows:
(DWL or also called LWL)
-Light, fine-ended sailboats = 0,65
-Average modern sailboats = 0,66
-Wide-sterned downwind sleds = 0,68
-Heavy full-ended sailbaots = 0,69
The following formula gives a fairly
accurate estimate of the moment of inertia of the waterline plane (Itwp).
(The CWP for Wayra is: 0.67)
Coefficient of waterline plane (CWP) = Area
waterline plane / (LWL X BWL)
(CWP) = 354 sq.f./ (42 feet x 12.6 feet) = 0,67
It's important to note that the greater the moment of
inertia and the lower the boat's center of gravity, the greater the
initial stability will be. we can see from the formula for Itwp that
initial stability increase as the cub of the waterline beam (BWL),
so even small increase in beam create a big increase in sail carrying
Hull form ,keel shape,rudder location,height and type of rig, sail
material,winch package, all are important factor in performance and
safety, but the real key to performance and safety for any safety boat
is stability. As well as being a critical safety factor, stability
provides the power to carry sail; it arrangement of the standing and
Stability - or the need for it -
governs hull form and much about keel shape, and through this the
location and proportion of the rudder. In short, stability affects
directly or indirectly almost everything about a boat.
There are two aspects of stability: Initial
stability and reserve stability. Initial stability can be
thought of as a boat's sail -carrying power or performance
potential. Reserve stability is a boat's stability to right itself when
knocked down.These differente aspects of stability are ofen confused.In
fact,design characteristics that generate losts of initial stability can
reduce reserve stability and vice versa.
The greater the initial stability,the more sail area a boat can carry
upwind in heavier air, with a taller rig, and the faster the boat can
go.This is so important that initial stability is someties just termed
'' power '', and a boat with a lot of initial stability is to referred
as a '' powerful''. Initial stability is also termed '' stiffness'', and
a boat with a lot of initial stability is a '' stiff '' boat. A boat
with little initial stability is said to be '' tender.''
Initial stability is generated by the boat's
waterplane area, combined with how far above or below the waterplane
the boat's center of gravity is. All other hull form factors are
secundary and can be largely ignored.
The waterplane creates initial stability - broadly
speaking - by its resistance to being rotated about its
centerline.Designers evaluate this with a quantity called the moment of
inertial of the waterline plane (ItWP).
know as ultimate stability,is every bit as critical because it
determines how safe the boat is. Reserve stability is defined as the
number of degrees a boat can heel and still right itself. Beyond this
point, the boat will, of its own weight,continue on to capsize. The
angle at which capsize occurs is termed the angle of vanishing
stability, or AVS. Reserve stability is also called range of
positive stability. If, for example, a boat maintains positive
stability up to 120 degrees of heel, its AVS is 120 degrees and its
range of positive stability is 0 to 120 degrees of heel.
As a general rule, boats under 75
feet LOA engaged in offshore passagemaking should have a range of
positive stability of 120 degrees or greater. For boats over 75 feet, a
range of 110 degrees is acceptable. The greater the range of positive
stability, the less time a boat will spend upside down before it rights
itself. With an AVS of 120 degrees or more, the inverted time will
usually be 2 minutes or less.
Calculating the Wayra's AVS: the Wolfson Unit
of Southampont University developed a formula for estimating AVS for
contemporary standard keelboats of ordinary form and proportions.This is
fairly reasonable (note: centerboard boats and keel centerboarders are
not accurately represented by this formula, which can't give an AVS less
than 110 degrees.)
We know the hull draft (also called
draft canoe-body,DCB),in feet.Ballast in pounds; Displacement
in pounds.Beam overall in feet. The density of seawater is 64
pounds/cubic foot; desnsity of fresh water is 62.4 pounds/cubic
Hull draft(DCB): is the draft of
the hull without its keel.On a midship section view of the boat is drown
a vertical line on-eighth of the total beam out from the centerline and
measured down from the waterline to where this line intersect with the
hull bottom; as shown in the above draw.
We use the following values for Wayra in the
calculations: DCB=2,72 feet; ballast = 12.000 pounds; displacement =
40.000 pounds; beam overall = 13,08 feet.
BR(ballast ratio) = ballast / displacement =
12.000 / 40.000 = 0,30
Displacement in cubic feet (for sea water):
40.000 / 64 pounds/cubic feet = 625 cubic feet.
Then we find the screening value (SV)
AVS = 100
+ ( 400 / ( SV- 10) )
AVS = 100 + ( 400 / ( 24,54 - 10 ) )
AVS = 127,5 degrees.
Finally, we make an adjustement to reduce the standard
Wolfons AVS, as if We have found to be a little too high. We multiply
the AVS by O,97
AVS: 127,5 X 0,97 =
This mean, that the Sailboat '' Wayra ''
only will capsize after has reached 123,6 degrees of
heel. But taking in account the downflood angle
This formula does not fully take into
account the vertical position of the center of gravity (VCG). The VCG
can be lowered by a longer keel or by having more ballast (weight of the
keel) at the end of the keel. However, according to Adlard Coles' "Heavy
Weather Sailing" thirtieth anniversary edition, "The effects of large
movements of the VCG on the propensity to capsize was surprising small".
Nevertheless, a low VCG will greatly help the boat in righting itself
once it has capsized.
Thus, boats with a long
lead keel or a lead bulb at the end of the keel may have a higher angle
of vanishing stability than that predicted by the formula