|
|
|
| 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 |
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
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.
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)
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.
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.
- 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
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.
(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.
|
|
|
|
| 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.
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.
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.
