Article published in Infomarine Magazine 
 
In this article, the process of generate a hullform using ShipGen module is explained, step by step.


 
 
 

 
Target
Lpp 108.5m
B 17.5m
H 9.75m
T 6.5m
Displ. 8866t
Lp 21.4m
Cp 0.707
Cb 0.701
Cm 0.991
KB 3.44m
LCB 54.76m
Lr 45.46m
The previous calculations to obtain the above target geometry and parameters are explained in the text of the right side.




ShipGen: a new system for automatic hull form generation. A practical example.
Let supose that the designer has estimated the main dimensions of the ship according with the requirements of the project and also adjusting a set of conditions with which the designer works. The estimated  refrigerated ship’s dimensions are:

Lpp= 108.5 m
B= 17.5 m
H= 9.75 m
T= 6.5 m
V= 17 kn
Displ.= 8866 t

Starting from these data the designer wishes to obtain more hydrodinamically adequate forms. At the same time he would like to estimate with more exactitude other kind of variables like capacities and the lightweight. And all that as quickly as possible.

The steps will be the following:

1. Select in ShipGen the most suitable type of template for the ship we are projecting.
2. Make with the system the necessary modifications so that the generated forms fulfil the requirements of the project.
3. Improve our project using our optimisation programs from an Excel work sheet, which is modified in each step.
4. Finally, print the lines drawing of the ship or export the forms to the CAD/CAM system of our technical office or shipyard.

To start with, the following coefficients are obtained from the estimated ship´s dimensions showed above:

Cb = 0.701
Fn = 0.260

These are between the usual margins for this relation L/B = 6.2 and that appear as determining factors in the project. Following we calculate the main frame's coefficient:

Cm = 1-2*Fn^4 = 0.991

If we know the Cb we can calculate the prismatic coefficient:

Cp = Cb/Cm = 0.707

Likewise, the floating coefficient is: Cwp = 0.18 + 0.86*Cp = 0.788

We are going to estimate the longitudinal position of the buoyancy centre. To do that we use the average values obtained from L.Troost, Van Lammeren, SNAME and MARIN methods. In this case the distance of the LCB to the stern's perpendicular is:  LCB = 54.76 m

On the other hand, the height of the buoyancy centre will be:

 KB = T*[1/(1+(Cb/Cf))] = 3.44 m

Likewise, according to the suggestions of Lindblad, the areas curve must have a shape so that the fore and aft bodies were calculated by the following formulae:

 Le/L = 1.975-2.27*Cb  ( Cb <0.75)
 Lr/L = 1.12- Cb

The obtained values are:

Le = 41.63 m   y   Lr = 45.46 m

So, the length of the parallel body must be Lp = 21.41 m
Finally, we can adjust several parameters of the bulb, as the length and the height of the protuberance, and the transversal area in the fore perpendicular. We calculate those parameters in the following way:

 

  • Protuberance's length = 3?Lpp = 3.25 m
  • The height in the maximum protuberance point (H) must have a non-dimensional value between the following: 0.35 < H/T <0.55
  • The non-dimensional transversal area,  is calculated by: 

  • Sa20 = 100*S20/S10
From Lpp/B= 6.2 and Cb= 0.701 we obtain S20=9.1 m2

We are able to obtain already the forms of the ship we want to design.

 


Figure 1: Body plan of the base ship that will be used to obtain the forms of the projecting ship.
The generated forms with ShipGen must satisfy the main dimensions and coefficients shown below:

Lpp = 108.5 m,  Cp = 0.707, B = 17.5 m,  Cb = 0.701
H = 9.75 m,  Cm = 0.991, T = 6.5  m,  KB = 3.44 m
V = 17 kn,   LCB= 54.76 m, Displ = 8866 t,  Lp = 21.4 m

ShipGen includes several templates of ships that, as it has been explained, are perfectly faired ships. We can make all kind of modifications from those.

In this case, we will use the template of a general cargo ship (Vid. Fig.1) with the following characteristics:

Lpp = 100 m,  Cp = 0.778, B = 19 m,  Cb = 0.772
H = 10 m,  Cm = 0.992, T = 6.6 m,  KB = 3.449 m
Displ = 9932 t,  LCB = 51.591 m, Lp = 27 m,  Lr = 37 m
 


Figure 2: New water lines obtained through modifying the main dimensions and the draft (in blue: new lines; in red: old lines)
First of all, we have to change the main dimensions of the base ship. This modification is made by ShipGen just indicating those dimensions. The values that we obtain after the modification are:

Lpp = 108.5 m,  Cp = 0.779, B = 17.5 m,  Cb = 0.773
H = 9.75 m,  Cm = 0.992, T = 6.435 m,  KB = 3.365 m
Displ = 9685 t,  LCB = 55.973 m, Lp = 29.295 m,  Lr = 40.145 m

Following we are going to make a modification of draft , not as trivial as the previous one. To do this ShipGen will make changes in the profile of prow (Vid. Fig. 2). The dimensions and the coeficients of the new forms are:

Lpp = 108.5, m  Cp = 0.78, B = 17.5 m,  Cb = 0.774
H = 9.75 m,  Cm = 0.992, T = 6.5 m,  KB = 3.401 m
Displ = 9797 t,  LCB = 55.93 m, Lp = 29.295 m,  Lr = 40.145 m
 


Figure 3: ShipGen creates the lines with the new length of the parallel body, and its exact position in the ship. On the screen we can see the variation of the longitudinals before (red) and after (blue) the modification.
The next step is to give to our template the value of the longitude of the parallel body obtained from the hydrodinamical recommended values of the aft and stern bodies. ShipGen can modify not only the length of the parallel body Lc, but also the length of run Ls, so we can obtain automatically the length of entrance Le. (Vid. Fig.3)

After the modifications the obtained new values  are as follows:

Lpp = 108.5 m, Cp = 0.757, B = 17.5 m,  Cb = 0.751
H = 9.75 m,  Cm = 0.992, T = 6.5 m,  KB = 3.419 m
Displ = 9507 t,  LCB = 56.443 m, Lp = 21.4 m,  Lr = 45.46 m

Next, we will reach the main dimensions by obtaining the coefficient of the frame of the ship we are designing. To do so, the program can modify the form and the radius of the bilge.
By taking  R= 1.5 m, we will obtain Cm= 0.991, which is the coefficient we are looking for.

After the modification the obtained values are as follows:

Lpp = 108.5 m,  Cp = 0.757, B = 17.5 m,  Cb = 0.75
H = 9.75 m,  Cm = 0.991, T = 6.5 m,  KB = 3.422 m
Displ = 9497 t,  LCB = 56.445 m, Lp = 21.4 m,  Lr = 45.46 m
 


Figure 4: Modification of the main parameters that define the bulb. We can also appreciate  the variation of the longitudinals before (red) and after (blue) the modification.
We can modify the form of the ship's bulb in order to obtain the desirable shape (Vid. Fig. 4). The values of the parameters we have to include are:
  • Length of the bulb: 3.25 m
  • Height: Do not change the height of the template because it is already within certain values.
  • Transversal area in the fore perpendicular: 10.26 m^2
So, the new coefficients are:

Lpp = 108.5 m,  Cp = 0.756, B = 17.5 m,  Cb = 0.749
H = 9.75 m,  Cm = 0.991, T = 6.5 m,  KB = 3.423 m
Displ = 9488 t,  LCB = 56.397 m, Lp = 21.4 m,  Lr = 45.46 m

 


Figure 5: To move aft the position of the centre of buoyancy, ShipGen moves the volume of the fore body and transfer it to the aft body, obtaining the  faired forms. The figure shows the variation of the frames before (red) and after (blue) the modification.
We simply need to change the displacement and the longitudinal position of the centre of buoyancy. To do so, in this case, the program will remove the volume of the fore body and transfer it to the aft body. This becomes apparent by viewing the form of the frames.
(Vid. Fig.5).

Figure 6 : The screen shows the hydrodinamical values of the new ship obtained from the ShipGen's template and a 3D drawing in which we can see its frames and waterlines.
Finally, the following geometrical and hydrodinamical values are obtained after we make all the modifications. As we can see, they hardly difer from the ship we want to design.
 
 
Target
Generated by ShipGen
Lpp 108.5m 108.5m
B 17.5m 17.5m
H 9.75m 9.75m
T 6.5m 6.5m
Displ. 8866t 8859t
Lp 21.4m 21.4m
Cp 0.707 0.706
Cb 0.701 0.700
Cm 0.991 0.991
KB 3.44m 3.441m
LCB 54.76m 54.806m
Lr 45.46m 45.46m
 

Figure 7 : In  few minutes we have generated the forms of the refrigerated ship, as we can see in the new body plan.
As we can see, we have obtained quickly the forms of a refrigerated ship from a ShipGen's template. The modifications will  be different depending on the kind of ship we want to obtain. (For example, in the case of a fishing ship, we can  change the dead rise, the trim and the half-width of the keel.

Figure 8: ShipGen has a dynamic link with the Excel work sheet, where project formulae  can be defined. Here we can see the curve of areas.
Linked with Excel.

The program incorporates a small naval architecture calculations integrated module and has a dynamic connection with the Excel spreadsheet to which transmits in every modification of the lines the hydrostatic data of the new vessel. This feature allows the designer to write in Excel the code of his own models of project optimisation and feed them dynamically using the data received from ShipGen, thus closing up the iterative designing process.
 


Cintranaval-Defcar S.L.
Lauroeta Etorbidea, 4 48180 Loiu, Bizkaia (Spain)
Tlf: +34 902158081
Fax: +34 944638552
info@cintranaval-defcar.com