Creating a 3D Hull Model

In my first post I talked about the lack of any available blue prints for the Zodiac. Our ultimate goal is to reproduce them by creating a 3D CAD model of the entire boat. The most complex part from a CAD modeling perspective is the boats hull. It is mathematically a very complex surface but optically has very smooth lines and a nice flow to it. And most of the hull you can’t see because it is under water and that is a good thing because that is what keeps the boat afloat.

zodiac full side profilezodiac interiorlayout

To get an accurate model of the hull we will do a 3D hull laser scan when the Zodiac is in dry dock in March 2013. But I wanted to get a head start and create an approximated 3D hull CAD model which I can then use to 3D print little hull models. So I started to look for pictures of the Zodiac from which I can trace some rough lines of the hull above water and approximate lines under water based on pictures from the hull in dry dock. I found a nice side shot from the zodiac and a rough interior layout plan from the website which I loaded into the CAD system and placed them in the proper planes. In the CAD system I created some very basic dimensions of the boat which is 127′ long and 26′ wide and draws about 16′ below the waterline.  Then I scaled the pictures inside the CAD system until the hull measured from bow to stern 127′ which is the known LOA today. The picture below shows the initial setup in MOI3d.

creating the hull - step 1

I could now trace the visible hull lines along the picture and approximate the lines under water based on a few dry dock pictures. After that I have 2 three dimensional curves (B-splines) inside the CAD model which now allows me to loft the hull. The following 2 pictures show that result.

creating the hull - step 2

creating the hull - step 3 - Basic Hull Lofted

As you can see the result isn’t all that useful yet because the lofting function just didn’t have enough data based on the 2 curves to create a hull. At this point I remembered that I saw some hull lines of the Zodiac which were created a few years back. I took some pictures of these hull lines and used the same technique as above arranging each view and scaling the pictures to match the dimensions. Then I started tracing the hull lines and arranged them in the proper 3D position.

This process was actually quite time consuming  since it required a high decree of accuracy and I tried various methods for creating the B-splines. I remembered that using fewer control points on the B-splines will make the lofting easier and the resulting surface smoother. So for each hull curve I tried to use no more than 5 control points and dragged the control points until the curve matched the line in the picture. The tricky part here is do the lines in the correct sequence or you will get lost with all these lines.

creating the hull - step 3 - Detailed Hull Lines

This step is the most important one in this process because the CAD software will use these lines to loft the hull. Now with today’s tools such as CAD software you can create and design such a complex surface in a couple of days. Now imagine the type of effort that was necessary to create a complex hull such as the Zodiac’s 100 years ago when the schooner was actually build. They didn’t have computers or even calculators so creating actual drawings must have taken weeks if not month. Naval architects used scaled wood models which they shaped with a lot of elbow crease, chisels and sandpaper, until they had the shape they wanted. Then they cut the solid models into sections at set intervals which produced the actual hull lines they now could trace onto paper. After that they used mechanical tracing tools to scale the lines.

I was able to re-create the hull in the CAD system in a few days but that includes a lot of learning time and trying different methods. Now all of this you only have to do for one side of the hull because you can later mirror the one side to create a full hull. The next pictures show the selected (yellow) hull lines and the lofted hull as a result. The software provides a few options to play with to improve the result by basically reducing the constraints for the surface. It basically makes a loose fit which makes a smoother surface. It is like the computers version of elbow crease for all you non engineers.

creating the hull - step 5 - Lines Lofted

At this point we have a pretty good approximation of the hull but we  have a lot more details to take care of. We need to close up the stern and then thicken the actual hull surface because the wooden planks of the hull  are probably about 3″ thick. Then we mirror the hull to get a complete hull model. Next is to add the keel beam and rudder. Here is the result after all of that design work.

creating the hull - step 6 - Thickened Hull with Rudder

closeupofnewframes

Once you have the hull model you can now start building the actual skeleton of the ship which is what traditional boat builders started with. In modern CAD systems is now fairly simple to create the curved ribs because you have an exact hull you now can slice in sections and project the precise curves needed for the ribs. The following picture which shows a small section of the ribs I took from the Zodiacs website. I used that photo to get some basic dimensions for the width and thickness of each rib and the spacing between them. In this picture I believe we see a port side section that has been replaced as built by 6”x6”double sawn futtocks on 24” centers.

In the next post I’ll continue with building out the skeleton of the ship and show you the results of my 3D prints. So stay tuned.

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2 comments

  1. Pingback: 3D printing of the Zodiac’s hull model « Scanning - Designing - Printing - all in 3D

  2. Fantastic! I downloaded the model from GrabCad with the intent of machining a half-hull model on my CNC router.
    I am planning about one meter size. I might go with a dark wood for the keel and light wood for the rest. A narrow dark strip on the waterline would be a nice touch but it’s not easily defined. Would be amazing to sail on Zodiak someday.
    Cheers, Janne


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