3D Ship Hull from a Linesplan with Rhino
Contents Introduction ......................................................................................................................................................................................................................................... 2 Conditions for a fair vessel hull surface ............................................................................................................................................................................................... 2 Reverse Engineering Illusions ............................................................................................................................................................................................................... 2 Illusion 1 – Loft ............................................................................................................................................................................................................................... 4 Illusion 2 – Sweep 2 Rails ................................................................................................................................................................................................................ 4 Illusion 3 – Surface from curve network ......................................................................................................................................................................................... 5 Conclusion ...................................................................................................................................................................................................................................... 5 Method for Reverse Engineering ......................................................................................................................................................................................................... 6 Step 1 - Prepare 2D......................................................................................................................................................................................................................... 6 Step 2 - Trace the desired lines....................................................................................................................................................................................................... 6 Step 3 - Reference Ribbons............................................................................................................................................................................................................. 7 Step 4 – Modeling and fairing the hull surface ............................................................................................................................................................................... 7
Gerard Petersen - gerard@rhinocentre.nl
Page 1 of 8
10-5-2017
Introduction A common demand in the maritime industry is to create a fair 3D vessel hull shape from an existing linesplan. Therefore, RhinoCentre developed a method to do this in Rhino. This method is used to do reverse engineering jobs for our clients and is also taught in our “Hull Design and Fairing Training�.
Conditions for a fair vessel hull surface First of all, there are several conditions for creating a fair vessel hull surface: 1. A fair vessel hull surface contains one or more surfaces. The reason for this is that when more than one surface is used, the surface continuity, between one surface to another, can be extremely challenging. Especially when a second surface is used for a bulbous bow, it is nearly impossible to create a fair transition to the rest of the hull. In the case of a multi chine hull, it is clever to divide surfaces at the chine. This will result in a polysurface. However, for round bilge hulls, it is desirable to create a hull with one surface. 2. When a vessel hull surface contains only a few control points, it is more easy to create a fair surface. Also editing the shape at a later time is more easy to accomplish. Therefore, try to avoid large amounts of control points. This makes it very easy to analyze any existing ship hull surface according the following criteria: 1. Does the vessel hull surface contain one or more surfaces? 2. Does the surface contain few or many control points?
Reverse Engineering Illusions The biggest illusion is that AutoCAD curves from a linesplan can be used for making a fair and high quality hull surface. Generally speaking there are three commands in Rhino to create freeform surfaces from existing curves: 1. Loft 2. Sweep two rails 3. Surface from curve network For a number of reasons it turns out that it is impossible to use existing linesplan curves for reverse engineering. This means that you cannot use existing station, waterline or buttock curves for making a single high quality surface with these commands. Maybe it is possible to create several surfaces for parts of the hull from existing linesplan curves. But again, the reason for the goal to create a single surface is that this is most easy to fair. When a ship hull is created with several surfaces, a fair surface continuity is very challenging and often leads to headache. In the case it is successfully done, the hull cannot be edited easily anymore. So what to do instead?
Page 2 of 8
10-5-2017
The only way to make high quality hull surfaces is using an existing linesplan only as a reference. This means that a new surface is created from scratch and shaped within a certain tolerance to the target linesplan. This results in a fair hull surface with a small amount of control points. In the training most trainees cannot accept this immediately. Therefore the illusions will be explained next.
Page 3 of 8
10-5-2017
Illusion 1 – Loft The loft command only accepts curves that do not touch or cross each other. This means that you can use only the stations from a linesplan, or the waterlines, or the buttocks, but never a combination. The problem then is that the stations are a good representation in the mid ship section, but do not show the roundness (tangency) of the stem for example. Also a bulbous bow is not possible when stations are lofted. Waterlines however do represent the tangency in the bow area, but do not represent a correct bilge radius. When using buttocks, it also turns out that a tangent stem and bilge radius is problematic. Another big issue is that for an accurate loft, the Loft style ‘normal’ has to be used. This style is very problematic in creating a fair and smooth surface. They often end up with wrinkles and other discontinuities. The last problem is that an accurate loft surface based on linesplan curves contains way too many control points to be able to fair the result manually.
Illusion 2 – Sweep 2 Rails The challenge with the command Sweep-2-Rails is to choose the rails that work best with the section curves. With a linesplan, it is a possibility to use the sheer line and keel line for the rails and station curves for the sections. The image shows that the bulbous area is a problem then. Another problem is that the surface is not fair and also contains way too much control points to be able to fair the surface at a later time.
Page 4 of 8
10-5-2017
Illusion 3 – Surface from curve network
This is the most misleading command for professional hull modeling. As this command is able to use curves that cross or touch each other, it looks promising. However, bulbous bow or aft bulbs are only possible with several surfaces. This again leads to challenging fairing of surface edges. Furthermore, the surface contains so many control points that it will never be totally fair or editable.
Conclusion It is only possible to use an existing linesplan as a reference for creating a clean loft with new loft curves. Over and over again I see how difficult it is for trainees to accept this. Even after four days they tend to try to use existing section curves as input for hull surfaces. This shows how brainwashed we are to use a linesplan as input.
Page 5 of 8
10-5-2017
Method for Reverse Engineering As mentioned earlier, an existing linesplan can only be used as a reference for creating a new hull surface from scratch. In order to do this, RhinoCentre developed a simple approach to do this within a certain tolerance. The basis for any reverse engineering job are the curves that represent the shape of the hull. In case of a 3D linesplan, the stations, waterlines and buttocks are already at the correct location. In case of a 2D lineplan, the curves already exist and only have to be oriented and positioned at the correct 3D location. When there’s only a scanned drawing like the example in this article, it is necessary to trace the curves in Rhino. In that case we have to prepare the 2D drawing in Rhino.
Step 1 - Prepare 2D This is the starting point. When the input is a drawing. The several decks and views have to be cut. Then the parts have to be oriented and positioned to the correct scale and location. This way a virtual 3D drawing is created.
Step 2 - Trace the desired lines It is possible to use the drawing directly as a reference during the modeling and fairing of the ship hull. However, most of the times the view becomes cluttered. Therefore it is wise to trace the desired waterlines, stations or other curves and then create the orange curves. The orange curves don’t have to be fair as they’re never used as input for the 3D hull surface.
Page 6 of 8
10-5-2017
Step 3 - Reference Ribbons The next step is to make so called ‘reference ribs’ or ‘refribs’ with an orange outside direction and a green inside direction. The depth of the ribs is an arbitrary value. The smaller, the more accurate and more difficult to create a fair hull. Most of the times a total rib height of 2% of the maximum breadth is a good starting point. This means that both the orange and green rib heights will be 1% of the breadth.
Step 4 – Modeling and fairing the hull surface In a number of steps, the hull surface is modeled and faired within the tolerance of the ref-ribs. Most important to notice is that the total skin of the hull surface is within the 2% accuracy. Also notice the small amount of control points. This makes the hull easy editable if necessary. The bulbous bow area shows the value of working with reference ribs. It is easy to see that the hull surface is within the 2% accuracy.
Page 7 of 8
10-5-2017
Another analysis is to plot the deviation. The dark blue ‘hairs’ show an accuracy of less than 1% to the ref-ribs.
The end result shows a very fair hull surface. It even turns out that the glossy display of the hull can show irregularities that are not visible in the linesplan derived from this hull surface.
The linesplan is derived from the hull surface.
Page 8 of 8
10-5-2017