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                                        BOAT SPECIFICATIONS  

In this section I will show the main Sailbboat specifications and some stability calculations done for Wayra. To start I recall some basic definitions used for Stability and Yacht design.
Sailboat '' Wayra''  main characteristics
LOA(Length Over All) 46'-10'' 14,28 m
LWL(load waterline length) 42'-0'' 12,80 m
Beam 13'- 01'' 3,99 m
BWL(bean waterline) 12' - 07'' 3,85 m
Draft 13'-1'' 1,80 m
Disp. Displacement or weight of the boat 40.000 lbs 18.143 kg
Ballast (Weight of the bilge or keel) 12.000 lbs 5.443 kg
Sail Area 1.200 square feet 111 Sq.mt.
Auxiliary Power 60 - 85 HP  
Displacement length Ratio 235  
Sail Area Displ. Ratio 16.5  
Diesel Oil 303 US gall. 1.150 liters
Fresh Water 224 US gall 850 liters
Ballast ratio 30%
AWP(waterplane area) 354 sq.feet 33 sq.m
AVS (angle of vanishing stability) 123,6 degrees  
     
Some draws and definitions has been gotten from: www.navaldesigner.com
Stability - Hydrostatic parameters & Definitions

Over All Length (LOA):  Overall lentgth from fore to aft

Beam Over All (BOA or B)

Maximum width of the vessel

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.

Beam Water Line (BWL):

Width of the hull at the level of the water plane.

It is a fundamental element of initial stability.

Draft: 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,

Immersed volumeVolume of the vessel below the water plane.

Waterplane Area:Horizontal area defined by the waterplane.

Displacement : 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 boat. 

* Cruising 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 24.

Formula:

   SA / D  = Sail Area / (Displacement in Cubic Feet )2/3

Displacement- Length  Ratio is the displacement to length ratio.   

 This indicates if the boat is a heavy cruiser (results greater than 325) or   a light displacement racing boat (results less than 200).

Formula:

  D / L = Displacement / ( 0.01 * LWL )3

     Displacement is in long tons

 

calculation moment of inertia of the waterline plane (Itwp):The 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 power. 

Sailboat "Wayra" stability, safety and performance

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 runing rigging.

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.

Initial stability: 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).

Reserve stability:Also 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 foot.

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 = 123,6 degrees.

This mean, that the Sailboat '' Wayra ''  only will capsize after has reached 123,6 degrees of heel. But taking in account the downflood angle criterial.

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 

 

Downflood angle: is the angle of heel at which water first can enter the boat, is an important part of the stability equation. Standard stability curves and calculations assume that a boat is all sealed up like submarine. In fact, once more boats over more than 110 degrees, water will start to enter the boat through ventilators and other small apertures.These leaks reduce stability by flooding the boat with water on the down side. Worse still, the water sloshes about, and sloshing action (known as '' free surface effect'' ) further reduces righting moment.Still if the hatches and windows are buttoned up tight and ventilators are off the side deck and well protected with water-trap boxes, this effect isn't too great.

Exacting stability analysis, however, assumes that any principal hatch - usually the companionway hatch or hatches - might be left open. Every so often freak wind gust or freak waves have rolled boat over very quickly, without warning, in otherwise moderate weather. A boat could have an AVS of 130 degrees, but downflood angle of only 112 degrees. If the hatch were open, water would flood the boat long before the theorical 130 degree AVS is reached, and the boat could capsize and sink.

Generally the downflood angle should be over 110 degrees for offshore boats.

Companionway hatch can reduce the downflood angle 20 degrees or more.

 
                       Sailboat " Wayra "  Hydrostatic and stability Calculations:

For the time being I have made some calculations at the DWL shown here in a PDF file.

Displacement at DWL, Prismatic coefficient, Block coefficient, Water plane area, Tons per centimeter of immersion, Longitudinal centre of buoyancy, Longitudinal centre of flotation, Longitudinal moment of inertia, Transversal moment of inertia, Transverse stability at small angles, Longitudinal stability at small angles.  

Click here to open calculations (PDF file)

 

Are you interested in more basic principles of ship stability and buoyancy?.

 I have gotten at Internet one good document from US Coast Guard. You can read or download it by clicking here " Ships Stability and Buoyancy booklet "
 

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All rights reseved to www.sailboatwayra.com   modified: 22-09-2009