Rhino ship hull tutorial

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Ship Hull Creation Tutorial

Developed by Mark Pavkov for

The Center for Innovation in Ship Design, Intern

Program Naval Surface Warfare Center, Carderock Division

9500 MacArthur Blvd Building 1 West Bethesda, MD 20817-5700

2007©

This tutorial has been designed to give the reader an understanding of the

different methods that can be used to model a ship‟s hull in Rhino through a series of

pre-designed offsets These offsets are produced from when the hull form is portrayed

graphically by the lines plan (see: Introduction to Naval Architecture) This shows the

various curves of intersection between the hull and the three sets of orthogonal planes In merchant ships the transverse sections are numbered from aft to forward In Warships (U.S.) they are numbered from forward to aft The distances of the various intersection

points from the middle line plane are called offsets There are a various ways to complete

this modeling task and in no way does this tutorial purport to be the best way It will however introduce some techniques, developing a useable skill set in surface modeling

In this case, the body plane was scanned and offset points were created with a piece of

software called SHCP (Ship Hull Characteristic Program) This is a very fast, albeit inaccurate way to create a table of offsets Techniques will be introduced to help align these inaccuracies and solve the problem of the “sloppy offsets”

This tutorial will lead the reader through the following steps to complete the modeling of the ship‟s hull:

Introduction 2

1 Generating the Offsets 4

2 Generating a Rhino Points Cloud 6

3 Generating Station Curves 8

4 Refining the Curves 10

5 Introducing Special Features 15

6 Generating the Main Hull Body 17

7 Generating Waterlines 20

8 Generating Bow & Stern Surfaces 23

9 Mirror the Hull 31

Useful Commands:

Hide: Left click the icon and either select an object to hide or drag the selection box over a group of objects by depression and holding the left mouse button Then by hitting the Enter key or right clicking the mouse, the selected items will then be hidden By right clicking the icon, all hidden objects will reappear on the screen It should be noted that this applies only to active layers; hidden objects on inactive layer will remain hidden This command is quite useful when the need arises to work on an object that is obscured

by another or when there is simply too much clutter on the screen

Zoom window/ Zoom target: This icon will zoom onto selected areas Keep in mind that this command can be executed in the middle of another command, for example to pull the distant terminus of a line into view In the Perspective view, it has the added benefit of relocating the axis of rotation for all objects to the center of the selected area, which is very convenient when doing close up work on an object

Press ‘U’ in Middle of Command: This is an „undo‟ command to return to the previous

step in the overall command

Command Bar: It is useful to keep an eye on the „command bar‟ as this will indicate any

errors in the command and can be used for problem solving of incorrect modeling

Double Click on View Fields: You can „double click‟ on view fields such as front or

perspective to enlarge them to a full screen The double click can also be used to escape back to the original four window view

1 Generating the Offsets

AIM: To edit the SHCP table of offsets into a Rhino compatible form

METHOD: By editing the table in Excel

In order for Rhino to the use the offset data produced by the software package

SHCP it must first be put into the correct form Open the Excel file Containership lines

The format of the table of offsets is in a typical Cartesian system, with one anomaly: the x coordinates are given in a different scale then the y and z The x scale in this case is not a

scale at all, but a division of the ship called “stations.” As explained stations are a series

of transverse cuts at defined increments from the forward perpendicular (FP) aft (negative

x coordinates define the bow section forward of the FP)

There are usually 10, 20, or 40 stations in a typical table of offsets In this case there are

20 stations and several partial stations where the curvature is complex To make this table compatible with Rhino the following editing must be performed:

 Delete row 1, which is text and unreadable to Rhino

 Delete column D, to remove the redundant 88888 station breakers produced by

the SHCP program

Once this is done we must transform the scale of the stations The y column is in

meters and defines the point from the ship centerline (the transverse middle of the ship

running fore and aft), where as the z column defines the point from the ship baseline or

the bottom of the keel plate The spacing between stations is 9.875 meters

 multiply column A by 9.875 in order to get the correct longitudinal spacing of the

 Now save this spread sheet as a common delaminated file (*.CSV), using the

Save As command and changing the file type in the drop-down menu Give it the

name Offsets for Rhino Two warnings may come up Just select Okay or yes for

both

This will create a copy of the file in the new format and can be opened or modified in for Notepad or Excel The Data file is now ready for Rhino to read

NEXT STEP: This Rhino compatible „Ship Hull Offsets‟ table can now be used to

generate a ship hull‟s „Points Cloud‟

2 Generating a Rhino Points Cloud

AIM: To input the edited SHCP table of offsets into Rhino and create a ship‟s hull „point

cloud‟

METHOD: By using Rhino‟s „File Import‟ and „Grid Extends‟ command

To begin a new Rhino file needs to be created Then the table of offsets can be imported to create a „points cloud‟ structure for the ship hull modeling

 Open Rhino and select the new file icon and when prompted, select the template:

meters.3dm Or go to Dimension > Dimension Properties > Units and switch to

meters

 Go to File > Import If the Files of type line in the window are not set to Point

File, then set it as such

 Selecting your cvs file, click open

 Now save the model

Although Rhino will automatically create an Autosave copy of the file, make sure

to save often and save copies of the models at different stages under different names

This allows the user to go back to an earlier model as there are only a finite number of

undo commands that can be executed

A complete „points cloud‟ for one half of the hull model has now been created Since ships are longitudinally symmetric hull modeling only needs to be concerned with one half of the ship

NEXT STEP: To create station curves from the „points cloud‟

3 Generating Station Curves

AIM: To generate ship hull station curves utilizing the „points cloud‟ previously created METHOD: By using the Rhino command „Interpolate Points‟

Before the station curves can be modeled, modeling „layers‟ will have to be

created This will enable the user to hide parts of the model he/she is not working on or allow the user to make only work on certain layers that are „active‟

 Click on the layer icon

 Double click on the text of the default layer Rename it as points

 Go to the next layer and rename it curves

 Check the points layer box making it the active layer

To enable the selection of individual points in the „points cloud‟ the cloud will be needed to be broken up This can be achieved by using the „Break‟ command

 Click on any point to highlight the „points cloud‟ (to highlight all in yellow)

 Click on the Break or Explode command

Now the Station curves can be modeled, these will be used to loft the hull surface

 Make the curves layer the active layer, by checking the curves box

 Click on the Osnap box at the bottom of the screen and check the point box

 From the dropdown menu Curves > Free-form or right click the Control point curve icon and select Interpolate Points curve

 Start at the forward end of the ship with the top point of the first set of vertical points and connect one to another by clicking on each point until the bottom point (keel) is reached, then right click

Use the Zoom window command to centre in on the first station curve (see intro)

NB: „Right Click‟ to escape command then „Right Click‟ to redo command

 Repeat this process for the rest of the points until are set of ribs (stations) are developed

It should look like this:

NEXT STEP: To Refine the Station Curves

NB: Save as new file, it is prudent to save the model as a new file after each section in

this tutorial, incase you require to return to an earlier stage in the modeling

4 Refining the Curves

AIM: To refine the curves to gain a more accurate representation of the overall hull

surface once it is rendered

METHOD: By manually deleting and re-drawing certain sections of the station curves

In order to accurately refine the station curves they need to be looked upon in closer detail

 Turn off the points layer (yellow bulb)

 From the drop-down menu, Analyze > Curve > click Curve graph on

This command will allow you to assess the fairness of the curve, this is the

smoothness with which the curve changes along its length

 Select all the curves, press enter

The curve graph greatly exaggerates the curve‟s slope and inflection points Where the graph line crosses the curve is an inflection point The curves in your model may not be as warped as the in above picture, it depends on what display scale setting is being used

Several inflection points on lines appear to be straight It will help with the overall rendering of the hull surface if these lines are smoothed out There are two ways to do this

1 Through tweaking the control points

2 By letting the program find the best fit of a new curve

We will use the latter, but you may want to turn on the control points and play with them, while the graphing function is on, to get a feel for what is going on

 Click Analyze > Curve > Curve Graph Off

The Interpolate curve command wants to make a constrained curve with smooth

transitions from point to point Upon closer inspection, in the right view port, you will

see that some of these lines dip below the x plane as the turn of the bilge (underside of the

hull) becomes “flat” bottom

We will need to:

1 Break these curves where they meet the x plane, as fairing the curve will only

serve to complicate this problem

2 Then fair the curves above the x plane and create a straight line were the points run parallel to the x plane

Working in the right view port,

 Turn off the curves layer and check the points on

 Highlight all the points not on the x-y plane by dragging the selection box from the outboard edge just above the y axis to the upper edge of the centerline

 Click Hide

You will need to zoom in and look down the x-y plane for any points you missed (there are probably only 2 or so) and hide those also

Using the layers window,

 Turn off the points and turn the hull curves back on

You now may either use one of the following two commands,

1 Trim command

2 Split command

Note: Using the trim command visually changes the line, but it remains mathematically unaltered, whereas spilt alters the equation of the line—this is important to remember

when exporting Rhino geometry into FEA programs

In this case we will be using the split command,

 select split and select all the curves and right click, then press enter

 Next, turn off the curves layer and turn back on the points layers

The hidden points will still be hidden:

 Select all the visible points as cutting objects

 Turn back on the curves layer and hit enter

 Turn off the points layer

 Highlight the top sections of the curve and hide them and

 Delete the remaining “flat” bottom section of the curves

 Right click „Hide‟ to un-hide the curves

 Go to Curve > Curve edit tools > Fair, select your curves, leave the tolerance at

1.0, and execute the command

 Click Analyze > Curve > Curve Graph On

 Select Curves

NB: Check right view to make sure curves have been faired correctly and that no curve

passes through the x-z plane

There is now a marked difference in the curvature graphs

 Click Curve Graph Off

Working in the curves layer,

 Using the Layer window turn the points back on

 Starting at the bow, reconstruct the first deleted line with a ployline connecting the points and the faired hull curves

The curve and the line at this point will exhibit positional continuity (G0) More advanced techniques (using tangency continuity (G1)) can be applied to correct this

To match the lines to make a complete station curve,

 Click Curve > Curve edit tools > Match

The command box will ask you to select open curve to change;

 Select the faired curve at this time, start at the bow (first station curve)

The command box will ask you to “select open curve to match;”

 Select the poly line

The continuity radio button should be set to tangency

 Check preserve other end and hit Okay

The knuckle in the line has now been smoothed out while preserving the flatness

of the bottom and with limited ill effect on the fairness of the line

 One curve at time, join curves

 Repeat this construction on the rest of the hull lines needed

NEXT STEP: Now the station curves have been created additional features of the ship

hull can be included such as the Bow and Stern lines

5 Introducing Special Features

AIM: To develop the skeleton of the model to include additional features usually present

upon a ship‟s hull

METHOD: By manually manipulating the station curves and by importing standard ship

features onto the skeleton

To begin the parts of the bow and stern tube of the ship need definition in the xz

plane We are going to import a set of points that will help to develop these surfaces The points were created from the profile view of the ship: specifically, the continuous line from the point were the hull meets the forecastle, down around the bulbous bow, to the keel and continuing aft

 Go to File > Import look in CISD Rhino Tutorial

 Change „files of type‟ to Rhino 3D models

 Select Bow and stern pts > Open

 Activate a new layer and name it keel

 Draw a Interpolate Curve through the points starting with the tip of the bow down

to the last point before the end of the 4th station were it meets the keel

 Make polyline starting from the end of the bow curve, through the keel ends of the stations up to and including the 21st station

 Use a curve from the 21st

station through the stern points to the tip of the stern tube

 Use the Curve > Curve Edit Tools > Match command again selecting the bow

curve first and the keel as the match line

 Match keel to stern line

 Make a fourth curve from the tip of the stern tube up to the lower point of the transom

 Finally, draw a line from the bottom point to of the transom to the floating point that represents the top of the transom

NB: several colors have been used to highlight different features

NEXT STEP: To generate the main hull body