this article, the process of generate a hullform using ShipGen
module is explained, step by step.
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:
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:
in ShipGen the most suitable type of template for the ship we are projecting.
To start with, the following coefficients are obtained from the estimated ship´s dimensions showed above:
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:
= 1.975-2.27*Cb ( Cb <0.75)
The obtained values are:
Le = 41.63 m y Lr = 45.46 m
the length of the parallel body must be Lp = 21.41 m
Sa20 = 100*S20/S10
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.
generated forms with ShipGen must satisfy the main dimensions and coefficients
= 108.5 m, Cp = 0.707, B = 17.5 m, Cb = 0.701
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:
= 100 m, Cp = 0.778, B = 19 m, Cb = 0.772
Figure 2: New water lines obtained through modifying the main dimensions and the draft (in blue: new lines; in red: old lines)
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:
= 108.5 m, Cp = 0.779, B = 17.5 m, Cb = 0.773
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:
= 108.5, m Cp = 0.78, B = 17.5 m, Cb = 0.774
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.
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:
= 108.5 m, Cp = 0.757, B = 17.5 m, Cb = 0.751
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.
After the modification the obtained values are as follows:
= 108.5 m, Cp = 0.757, B = 17.5 m, Cb = 0.75
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.
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:
= 108.5 m, Cp = 0.756, B = 17.5 m, Cb = 0.749
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.
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.
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.
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.
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.
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
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