3D orienting of ship blocks in Rhino

[Waldemar]

 

Persuaded by @scrubbyj427, who has been very supportive of my build log of the Sankt Georg 1627 3D model, I have prepared a short tutorial on positioning ship blocks in the 3D model space of a sailing ship. The methods shown are specific to the Rhinoceros CAD software.

 

1. Block preparation

Define your ship block as in the diagram below (single block in this sample). The three visible red lines are the most important, i.e.:

 

– the longitudinal axis of the block (red line segment),

– the rope/fall axis around the sheave (red circle),

– and the block’s strop axis around the block's shell (curve to match the shape of the shell).

 

It is most convenient (and correct at the same time) to design the block modularly, i.e. by taking the diameter of the fall as one unit, the diameter of the block's sheave, say, 5 units, and the diameter of the red circle acting as a fall axis – 6 units. The 3D shape of the block's shell itself, its sheave and the sheave's axis are already less important in this method, but it is also a good practice to design them according to the art.

Save this master block e.g. as "01. block single sheaved stropped", or similarly descriptive. In the diagram below, for illustrative purposes only, I have also drawn two green circles to symbolise the cross sections of the block's fall and its strop (these circles do not need to be saved in the block definition file).  

 

 

2. Strategy for using/inserting CAD blocks

It is best to insert blocks into the main file containing your 3D ship model as "Linked" and "Active" blocks, otherwise your main file will quickly grow to the size of Pacific and be unmanageable. Generally, you have two strategies to choose from here:

 

(a)    if you want to later count all the blocks used in your main file, taking into account their size, it is best to define a separate block for each combination of the block/rope type and size, and not just its type. This makes it very easy to do the model’s documentation later. In my case, six single block sizes came out, as follows:

 

 

It means, that in this very case, each block definition (type and size) must be saved in a separate file, e.g. "01. block single sheaved - fall 1 inch", "02. block single sheaved - fall 2 inches" and so on.

(b) if you only want to insert ship blocks for the show, then it is enough to define just one CAD block for a particular block type, without regard to its actual size. Later, after inserting it into the main file, you can scale it according to the thickness of the rope. In this variant, each instance of this block, regardless of the scaled size, will always have (in the “Block Manager”) the same name, e.g. "01. block single sheaved stropped".

 

3. Determining the initial run of the fall

Connect the fixing points to which the block pendants will be attached to with the lines, as below. These are usually on yards, sails, stays, shrouds, belaying pins etc.

 

 

4. Obstacle bypassing along the way (optional)

 

 

If there is an obstacle along the way of the rope/fall, the run of the rope needs to be adjusted accordingly. In this particular example, the cylindrical surface of the imaginary yard (grey cylinder) was extracted ("Extract surface") and then offset ("Offset Surface") by half the diameter of the rope/fall (here 5 units), resulting in a green cylinder. Green flat surface between points A and E was created, then "Object intersection" command was used to get a yellow circle. Next two line segments from points A and E were drawn tangent to this yellow circle. It was then trimmed (by "Trim" or "Split" command), and all these three yellow elements were joined ("Join") to get the corrected run of the fall:

 

 

5. Defining block’s pendant initial (approximate) length

In some cases, there is a need to define the block’s pendant length manually, especially for obtuse angles (but not only). In this example, we will only do it for the points C and D. For the other points, i.e. A, B and E, they will be determined automatically. For this, use the "Line: Bisector" command. For the point D I have given a length of 50 units, and for the point C – 150 units of length. These are the two rather short yellow lines in the diagram below:

 

 

6. Correction of the fall run

Connect the ends of these (temporary) pendants to the adjacent points (yellow lines in the below diagram):

 

 

And delete some of the initial lines that are not needed anymore:

 

 

7. Draw the circles

In this example, the diameter of the rope/fall is 10 units and the diameter of the rope axis around the block’s sheave (red circle in the block) is 60 units. So the circles with the diameter of 60 units were drawn using the command "Circle tangent to 3 curves", as below:

 

 

And fill these circles with surfaces using the "Surface from planar curves" command, as follows:

 

 

8. Insert your ship's block as a CAD block (optional: if you are using the second strategy described in point 2b, then after inserting, you may scale the block to the correct size)

 

Important: do not rotate the block after insertion otherwise the next command will not work properly.

 

 

9. Orienting blocks in the 3D space

 

Apply to the inserted block the "Orient objects on surface" command:

Base point: the center of the red circle in the block
Reference point for scaling and rotation: (the right) end of the block's longitudinal axis (red line segment in the block)
Surface to orient on: select one of your red circular surfaces 
"Rotation, Prompt" option in the command dialog window: ON
Point on surface to orient to: center of your red circular surfaces
Rotation angle: point your mouse cursor to the pendant's end or your fixing points (in this sample A, B, C, D, E)

Make sure that "Project objects snap" is OFF, and "Center object snap" is ON.

 

 

Delete the red circular surfaces that are no longer needed.

 
10. Trim all lines and circles (the latter drawn in point 7.) as below to get the final fall run. Delete temporary pendants drawn in point 5. (for better clarity, in the diagram below the blocks are temporarily hidden by applying command "Hide objects"):

 

 

And join all these lines and curves together:

 

 

11. Make a 3D rope/fall using command "Pipe: flat caps":

 

 

12. Make the axes of the block’s pendants by connecting the ends of the blocks’ red longitudinal axis with the fixation points:

 

 

13. Make 3D pendants and 3D block strops using command "Pipe: flat caps":

 

 

14. Enjoy

 

 

In practice, there can be more difficult cases, for example with double blocks, and then a bit of improvisation is needed, but the general principle is the same.

 

 

[Gregory]

That is really great work.

Would it be possible for you to make a .pdf available for this image?

This would be for my private use. 

 I can PM you if you would prefer to keep it out of the public area

 

I know you have invested a lot of time and resources, and I really appreciate it.

Gregory

 

 

[Waldemar]

 

Thank you very much Gregory. Unfortunately this plate is already copyrighted by my contractor (museum) and I cannot dispose of it too freely. There will be another one, in a commercial set, as I have retained the copyright to the design itself, but this will not happen very soon.

 

The best,
Waldemar

 

 

[scrubbyj427]

Excellent tutorial Waldemar! Apologies for being late to the party. I will try this on my Portland model. I individually rotated my deadeyes one at a time, in 3 separate planes and simply eyeballed it. This is incredible! I’ve also never managed to create lines(pipe) like that before, sometimes I can by sweeping but that doesn’t always work out well. Great function of rhino you have discovered!