AutoCad Tutorials Representation of a Ship Hull Surface

Using AutoCAD to represent a Ship Hull Surface using its Offset Table

Step by Step Tutorials

Tutorial I

“Drawing 2D representation in the form of Lines Plan”

  1. Start from Scratch:
  • When you load ACAD2000 or any other previous version, a startup dialog appears waiting for your orders as shown in the next figure
  • Select “Start from Scratch” button and press Ok
  • Here you have told the ACAD to use the default settings in order to have a quick start
  1. Setting the drawing environment:
  1. First of all we’ll adjust the units by selecting Units Format>Units
  1. In the units dialog box be sure to select decimal in the length type, 0.000 in the precision, then press ok
  1. Notice the mouse counter at the most left bottom of the CAD screen, it’s now have the new precision

Hint: you can also display the Units dialog box by typing units in the command line

  1. Now let’s set our drawing limits by selecting Units Format>Drawing limits.
  1. A prompt at the command line asks you to specify the lower left corner, type 0,0 in the command line and then press ENTER.
  1. Another prompt for the upper right corner; here the maximum length we have to deal with for ship lines is about 150 m. and the height of the drawing is the sum of the ship’s depth and half breath plan in addition to some empty spaces so we type in 200, 90 as our answer to the command prompt.

Hints:

  1. You can also display the Drawing limits Command by typing limits in the command line
  1. We simply chosen 90 as height because if we plot our drawing using 1 drawing units = 10 mm this 90 would be equivalent to 90 cm “the width of the paper roll and so the drawing scale would be 1:100 – to see the boundaries of your drawing space draw a rectangle with lower left corner 0,0 and upper right one 200,90
  1. Finally in order to see your drawing limits, press the zoom all button in the zoom fly out located in the standard toolbar, or simply you can type zoom then all in the command line
  1. Start DRAWING:
  1. Once you have set up your environment for drawing, you can start by
  1. Drawing the molded base line of the profile using the line command and specifying the starting and ending points (let’s say 0, 50 & 200, 50
  1. Drawing the Center Line of the half breadth plan using the line command and specifying the starting and ending points (let’s say 0, 20 & 200, 20

Hints:

  1. As you sure noticed the two lines are drawn with the same line type and color, to deal with this problems we have to learn about layers

b. Display the Layer Properties Manager dialog box by pressing the layers button in the toolbar shown above, or type layer in the command line:

  • As you see there is only one layer named “O” with white color and continuous line type
  • By pressing the New button a new layer appears with its default name highlighted waiting for you to change it, try to make the name expressive; ship center for example.
  • By pressing enter AutoCAD accepts the new layer’s name and then give you a chance to make another action (either adding anew layer or changing a property of any other layer)
  • Since this layer represents a center line so it’s a common thing to change the line type to center; you can set the line type by clicking the word “Continuous” – a select line type dialog appears but unfortunately it contains only one type so we have to add the type we’re looking for by pressing the Load button – now search for the center line type in the load dialog box and double click it
  • Repeat the last step to add a Hidden Line Type.
  • Before you exit the select line type dialog box be sure that the center line type is Highlighted to assign it to the ship center layer
  • Press OK to exit the layers dialog box
  • Now select the center line you have drawn latest and assign it to the ship center layer by clicking on the O in the layers toolbar shown above and selecting the required layer then press Esc twice to clear your selection, notice the changes!
  • The type line has changed to center but it’s still has to be adjusted, to do this select the required center line and then type properties – in the properties dialog box set the line type scale to 0.4 – it’s adjusted now so close the properties dialog box
  1. Drawing the Center Line of the Body plan using the line command and specifying the starting and ending points (let’s say 100, 45 & 100, 70 – as we did before adjust the layer for this new line as well as the line type scale “I think 0.1 would be good this time”.

* The Body Plan Curves

  1. Now we can start drawing the curves of the Body Plan:
  1. Using UCS (user coordinates system) command, set the location of the origin to the intersection between the body plan the molded base line:
  2. Type UCS in the command line then press Enter
  3. ACAD prompts you for a UCS option, type New
  4. Specify the new origin as 100,50,0
  5. Notice the changes to the mouse counter after what you did
  1. Now you’re going to work with a small area in your drawing “The right half of the body plan area to draw the forward station curves” so using the ZOOM command magnify this area to fill your ACAD screen:
  2. Type zoom in the command line then press Enter
  3. ACAD prompts you for a zoom option, type win
  4. Specify the first corner as –20, -7
  5. Specify the other corner as 20, 25

  1. Now preparing for the body plan curves to be drawn set a new layer named “Stations” & give it a specified color rather than white to be easily recognized. Also make itthe current layer.
  1. Start drawing the station curves one by one with the forward stations to the right of the body plan center line and the aft stations to the left:
  • Assume that we have the following data for one of the forward stations in the offset table:

WL / O / 0.5 / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9
Offset / .0956 / .4348 / .6 / .7217 / .7652 / .7913 / .8 / .8261 / .8609 / .9043 / .9739
  • Here the water lines location are presented as ratio of the water line spacing while the offset values are presented as a ratio to the half breadth of the vessel
  • In order to draw this curve in our ACAD drawing we have to rewrite this table in x-y coordinates that are represented in m. so the values of the water line heights should be multiplied by the WL spacing (1.58 m. for this example) and the offset values should be multiplied by B/2 (11.5 m. for this example)

Y (m.) / O / .79 / 1.58 / 3.16 / 4.74 / 6.32 / 7.9 / 9.48 / 11.06 / 12.64 / 14.22
X (m.) / 1.1 / 5.0 / 6.9 / 8.3 / 8.8 / 9.1 / 9.2 / 9.5 / 9.9 / 10.4 / 11.2
  1. Every thing is set up now for using the SPLINE Command:
  • Type SPLINE in the command line or click on the SPLINE button on the DRAW toolbar
  • ACAD asks you to specify your first point so you should enter 1.1,0 then 5.0, .79 for next point, … & so on till the points are finished press enter without entering any value to tell ACAD that you have no more points on this curve.
  • ACAD prompts you to specify the start tangent with your starting point located on this tangent so you can specify any point on the molded base line to the left of the body plan center line to ensure that your curve will end tangentially to it “as it should be”
  • ACAD prompts for the end tangent for this curve and since we haven’t got a specified one we can simply press enter to finish the SPLINE command
  1. Repeat Steps 4&5 for the rest of the stations with negative x-values for the aft stations to be drawn to the left of the body plan centerline and thus your body plan is completed.

* The Bow & Stern Contours

  1. Use the zoom all command to view your drawing limits.
  1. It’s time now to draw the aft and forward perpendiculars of your vessel (remember that your origin is still located amidships at the molded base line.
  • Set the a new layer for these line name it “perpendiculars” and adjust its line type and color
  • Using the Line command draw the two perpendiculars using the following data for start and end points:

Aft Perpendicular / Forward Perpendicular
Start from / To / Start from / To
-L/2, -50 / -L/2,40 / L/2, -50 / L/2, 40

  1. Assume that we have the following data measured from the aft perpendicular for the bow contour in the offset table:

WL / O / 0.5 / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9
Offset (m.) / 2.0 / 2.2 / 2.4 / 2.7 / 2.4 / 1.3 / -3.1 / -4.9 / -5.4 / -5.9 / -6.1
  • Here only the values of the WL heights is to be adjusted as the Y- coordinates because the offset values are already adjusted in meters
  • Same steps used in drawing any of the stations curves are to be repeated after adjusting the origin at the intersection point of the aft perpendicular and the molded base line using the UCS command
  1. Repeat the last step using the data of the bow contour

* The half breadth Plan Curves

  1. Start drawing the half breadth curves one by one all above the half breadth plan center line, after adjusting the origin at the intersection point of the aft perpendicular with the center line of the half breadth plan center line
  • Assume that we have the following data for one of a waterline in the offset table:

ST / O / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10
Offset / .3304 / .6957 / .8870 / .9739 / 1.0 / 1.0 / .9652 / .8272 / .5706 / .2957 / .0348
  • Here the stations location are presented as ratio of the stations spacing from the aft perpendicular while the offset values are presented as a ratio to the half breadth of the vessel
  • In order to draw this curve in our ACAD drawing we have to rewrite this table in x-y coordinates that are represented in m. so the location of any station should be multiplied by the ST spacing (12.55 m. for this example) and the offset values should be multiplied by B/2 (11.5 m. for this example)

Y (m.) / 3.8 / 8.0 / 10.2 / 11.2 / 11.5 / 11.5 / 11.1 / 9.513 / 6.562 / 3.4 / .4
X (m.) / 0 / 12.55 / 25.1 / 37.65 / 50.2 / 62.75 / 75.3 / 87.85 / 100.4 / 112.95 / 125.5

Hint: have to find out the end points of each water line using the stern and bow contours before drawing them.

  1. Now work on the Body Plan to find out the offset value of any desired buttock to be drawn in the profile the distance command will be helpful in this case
  1. Finally finish your drawing using leaders, text tools after adjusting the drawing boundaries. Prepare for plotting by adjusting the pen assignment for each color used in your drawing.

Your final result should look something like this

Tutorial II

“Drawing 3D representation for the hull surface”

  1. Pre-drawing preparation:
  • Prepare your offset table in the form xyz coordinates; x represents longitudinal direction, y represents the breadth direction and z represents the depth direction
  • Assume that we have the following data for the amidships station:

WL / O / 0.5 / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9
Offset / .0956 / .4348 / .6 / .7217 / .7652 / .7913 / .8 / .8261 / .8609 / .9043 / .9739
  • Then our prepared table for this station would be

Z (m.) / O / .79 / 1.58 / 3.16 / 4.74 / 6.32 / 7.9 / 9.48 / 11.06 / 12.64 / 14.22
Y (m.) / 1.1 / 5.0 / 6.9 / 8.3 / 8.8 / 9.1 / 9.2 / 9.5 / 9.9 / 10.4 / 11.2
X (m.) / L/2 / L/2 / L/2 / L/2 / L/2 / L/2 / L/2 / L/2 / L/2 / L/2 / L/2
  1. Start Modeling:
  • In order to represent the Hull surface of a ship we shall use here the 3dmesh command there’s so many techniques used in modeling but I’ll try here to discuss the simplest one.

* The BODY Plan and half-breadth plan Curves as wire frames

  • Using the same SPLINE command start to plot the body plan curves one by one as you did before with only one exception you’ll give your coordinates in xyz coordinates. Notice that for the same station curve the x coordinates is the same.
  • Repeat for the half breadth plan curves taking care that the z-coordinate this time is constant for the same curve
  • This net of curves is called wire-frames; it gives us an idea of how does the hull look like. But in order to have better effect, we have to deal with surfaces or 3d mesh command.
  • Working with surface modeling:
  • Using the 3d command start to load 3d objects in ACAD and prompts you for the type of objects you want to create; type mesh and then press Enter
  • Specify the following corners:

(O, O, O)(L, O, O)(L, O, D)(O, O, D)

  • ACAD asks for the Mesh size in the M direction > Type the no of waterlines you have in the offset table, also type the no of stations for the Mesh size in the N-direction press enter to end this command
  • To recognize the difference between surfaces and wire frames type Shade in the command and see what happens – the mesh you have created are now acting like a plate sheet while the wire frames didn’t change. Press undo to cancel this shading
  • So we now have a surface but it’s still a plane one while our hull surface is curved, to attain this curvature we have make some deformation to our mesh – this is done by selecting the mesh.
  • Once the mesh is selected its internal nodes are highlighted by small boxes when any of them is selected its color turns red and then this node ask you where to move – specify the right location for each point from your prepared offset table and when you are done the surface will take the same form as the hull surface for have of the ship.
  • By using 3dmirror command you can have a complete ship hull “without the bottom and the aft most BKHD” a little bit finishing and you should have something like this.


With my Best Wishes

1/10

Eng. Mohamed Salah Ebieda

March 2001