ROLL STABILITY CALCULATOR   Home     Free Download    Menu 

COPYRIGHT Tim Lovett June 2004 

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Kids: How Smart are you?

The Program

(Download here)

The Roll Stability Calculator can be used to study the stability of various hull cross-sections in static equilibrium. (Tilting slowly in flat water). This is one of the first things to be checked in a new design. Static roll stability can also be easily verified by building a scale model and tilting it over in the water.

 

Getting Started

The quickest way to see something is to press Integral, and then move the Roll slider. You are now doing what a naval architect would do on a new hull design - checking the roll stability. The most stable vessels will have the highest curves (relatively speaking). The next thing to is to adjust the other two sliders in the buoyancy panel, KG/B (the height of the vessel's mass center) and Density. Keep exploring until you begin to get a feel for these design parameters.  If it crashes it simply means I couldn't be bothered making the program robust on peripheral parameter sets, but I might get round to it if its really annoying you.

 

 

So what are all these buttons?

They are predefined arks. 

'Gitt 38' stands for page 38 of the Werner Gitt booklet (currently in German, see Ref 1). The program is duplicating his results using a different calculation approach (see How it Works below).

The 'Gitt Custom' button goes to the next step allowing the user to define their own cross-section and then plot the same set of buoyancy curves. The numerical approach used in this program does not limit the analysis to a pure rectangle, as was the case in papers by Morris, Collins, Hong and Gitt. 

The second row of buttons labeled 0 to 12 relate to the Korean study Safety of Noah's Ark in a Seaway. In this study 12 hull variations were compared to the Biblical Ark (No 0) and tested by a team of naval architects and structural engineers from the world class Kriso facility. This program reproduces the first (and simplest) of the three major parameters - stability, strength and seakeeping, but can also work with a general hull shape rather than a purely rectangular transverse cross section. 

Note: If you are attempting to duplicate the Korean data you need to use a high number of intervals (1000-5000) to improve the accuracy.  Safety of Noah's Ark in a Seaway, Section 6.2, Table 3, column AR (m.rad). This column represents the amount of energy it takes to tip the ship over to its top corner. (Well, if you multiply by buoyancy force or weight you get Joules, and all these 'arks' are the same weight). You should be able to do better than 0.1% of the Korean researcher's 1994 TJ results.        

How Smart are You?

The most famous paper in the world about the safety of Noah's Ark is this one. A team of ship scientists (naval architects) wrote it. What they found out was that if you change the ark to a different shape (longer or fatter or taller etc) it is not as good as the Ark from the Bible. For example, by making it taller it will be stronger but might start to capsize. See Ark Safety Paper for dummies (Not that you are a dummy of course...)

Now, let's see if you can check those Korean scientists.

Look at Table 3. There are 4 columns. 

Column 1 is the different ark shapes

Column 2 (flim) is how far you can tilt (roll) each ark before the roof dips into the water

Column 3 (AR) is how much work it takes to do this tilt

Column 4 is boring.

 

All you have to do is run the Roll Stability Program, then press the button for the ark shape from Table 3. Number "0" is the Biblical ark. Now look carefully at the writing underneath the graph and you should see something like this;  Hong_Limit_GZ_integral_trapezoidal= .804847648645595m.RAD. This is supposed to be the same as the Column 3 number AR.   

Talk about a lot of decimal places!  Bit silly really, it is not this accurate because the number of intervals is only 100. You will notice the computer gets slower when you increase the intervals, and if you put too many it might refuse to do it !  That's because I was too lazy to make the program properly. 

 

Questions

1. The ark with the biggest number in Column 3 (Ar) is the most stable design. Which one is this? Why do you think it is hard to tip over?

2. The Biblical Ark is number "0". Where did it come in the stability contest? 

3. Why do you think the Ark "0" is still the best design, despite not having the highest stability?

 

Exercise

Assuming you have a fast enough computer, try 10 000 intervals. You should get pretty close to the research paper. 

Work out  how close you can get for each hull shape (as a percentage of the reading)

 

Answers

 

How it Works

The following principles are employed in the Roll Stability Calculator. (These were all developed by Archimedes before 200BC)

How it works

The program represents a slice of the hull. The output is GZ - the horizontal distance between hull mass centroid G and buoyancy centroid B.

 

INPUT: 

Define the hull shape, density and gravity center. Apply a roll angle

CALCULATION: 

Calculate area of hull to determine mass (using density)

Calculate current buoyancy area based on new roll angle

Check vertical force equilibrium. If unbalanced adjust vertical position and re-calculate buoyancy force

Find centroid of submerged area and measure GZ 

Repeat for successive roll angles and sum the values to give the integral curve.

 

Looks impressive? Not really. Let's take a closer look.

The transverse section is formed by straight lines (polygonal). Provided the points are in consecutive order around the polygon, the area can be calculated the same way regardless of the position or angle of the cross-section. The waterline is at Y=0. The area of the submerged (displaced) water is then calculated by adding up trapezoids using the simple expression; (In semi math/code form)

 

    For i = 1 To (pt - 1)
        subarea = subarea + (Xi - X i + 1) * (Yi + 1 + Yi) / 2
    Next i

 

Sweet eh?

 

Example.  

Default hull, Cubit = 500mm, KG/D=0.4, Relative Density=0.58. Angle or Roll=40 degrees. Output data includes the following trapezoidal areas;

Point 1 x= 4.821 y=-6.945 A= 26.159 

Point 2 x= 11.354 y=-1.063 A= .276 

Point 3 x= 11.872 y= 0 A= 0 

Point 4 x=-10.654 y= 0 A=-6.529 

Point 5 x=-14.396 y=-3.49 A= 53.294 

Point 6 x=-7.424 y=-11.798 A= 31.764 

Point 7 x=-4.878 y=-13.152 A= 35.357 

Point 8 x=-2.106 y=-12.357 A= 66.857

CSA= 382.7895sqm, SubArea= 222.017sqm. Checking this submerged (displaced) area ratio 222 / 383 = 0.58 which gives the correct desired.

 

 

 

Once the displaced area has been calculated, the hull is shifted up or down until the buoyancy force matches the gravity force. The adjustments are done using the Newton-Raphson method which works reliably on these rather gradual curves. (The NR method takes next estimate as the tangential zero. The idea is not strictly an Archimedes thing, Newton was somewhat later, however the ancient Greek did touch on some of the basic concepts of calculus). Very effective, only three or four iterations will give sufficient accuracy. 

     accuracy = 0.000000001   
    While Abs(F) > accuracy 
        dFdy = (F - lastF) / (Ys - LastYs)        
        LastYs = Ys
        Ys = LastYs - F / dFdy         
        Call yTransform: Call BuoyancyForce        
    Wend

The next step in the process is to determine the centroid (center of area) in a fashion similar to the trapezoidal area calculation. I found this on the net in some other language. This is the sweetest bit of code, especially for anyone who has ever had to calculate centroids by hand (students obviously).   


For i = 1 To (pt - 1)
    k = (XB(i) * YB(i + 1)) - (XB(i + 1) * YB(i))
    Xbc = Xbc + ((XB(i) + XB(i + 1)) * k)
    Ybc = Ybc + ((YB(i) + YB(i + 1)) * k)
Next i
Xbc = Xbc / (6 * subarea)
Ybc = Ybc / (6 * subarea)

Finally, the GZ values are integrated from 0 to 90 degrees using a simple trapeziodal approximation as described earlier. The result is compared with the Simpson rule (Parabolic segments) for any number of intervals up to 10000. Interestingly the Simpson rule can be less accurate on the more linear curves, but it serves as a nice check. 

 

No special geometric cases need to be considered; the method works for any roll angle and any polygonal shape.

 

A set of curves can be generated by repeating the calculation for hulls of different relative density.

Here is an example generated using the data from Prof Werner Gitt "Das sonderbarste Schiff der Weltgeschichte" 2001.

 

 

 

Since the hull section can be defined as a polygon, any shape (single loop) can be analyzed. For example, the default hull includes a 0.2B bilge radius and a 2 degree deadrise (V-keel). The effect is shown below - very little. Ignoring the absurdly lightweight hulls (less than 0.2), the general shape of the curves and location of maximum are hardly altered by the bilge radius.

 


References

1. Prof Werner Gitt "Das sonderbarste Schiff der Weltgeschichte" 2001. (The most amazing ship in the history of the World).

2. Buoyancy and Stability of Ships Vol 1 R.F. Scheltema De Heere & A.R. Bakker Harrap London 1970


Answers

Answers to Questions

 

1. The ark with the biggest number in Column 3 (Ar) is the most stable design. Which one is this?

Ark number 9 = 0.964 is the most stable.  

Why do you think it is hard to tip over?

Because it is wide. Ark 4 is the same width but not as tall so it dips into the water more easily. So Ark 9 is safest in roll. Ark 1 is obviously the worst because it is so tall and narrow.

 

2. The Biblical Ark is number "0". Where did it come in the stability contest? 

Fifth. The order is 9,10,6,5,0,3,7,11,4,2,8,12,1 where 0 is the Biblical ark.

 

3. Why do you think the Ark "0" is still the best design, despite not having the highest stability?

Good question!  The Hong paper compares 3 parameters - safety (roll), strength (hull bending) and seakeeping (accelerations). The Biblical ark is not too bad in any of these.

 

Want more info? Hong study comments

 

 

Exercise

Assuming you have a fast enough computer, try 10 000 intervals. You should get pretty close to the research paper. 

Work out  how close you can get for each hull shape (as a percentage of the reading). Should be less than 1%. See results here.