How to define seams and butts

Seams and butts are the joints between different hull plates in the ship's hull. Defining seams and butts is a crucial step following the creation of basic construction elements in the shell expansion drawing. It ensures efficient use of materials while adhering to classification society rules and shipyard standards. A well-defined seam and butt layout is essential for the detailed design and generation of accurate hull plate expansions. 

In general, defining seams and butts involves strategically placing them to minimize their number while considering factors like: maximum allowable plate size, classification society rules and engineering standards, the ability to develop the plates.

Fun fact: The phrase "ship-shape and in good fashion" comes from the meticulous attention to detail in shipbuilding, especially when it comes to seams and butts! It originally referred to ships being built with perfectly aligned seams and well-staggered butts, making them durable and seaworthy. A ship built to these standards was considered "ship-shape", meaning it was in excellent condition, both structurally and in appearance. Even today, the phrase is used to describe something that’s neat, orderly, and ready for action!

Defining seams & butts in MasterShip

In this video is explained how to setup a shell expansion, and define seams and butts in MasterShip. Note: There are several ways of defining seams & butts. This video demonstrates the most convenient workflow. 

And here are some of the best practices from the engineering perspective in defining seams and butts with the applicable MasterShip and AutoCAD functionalities:

• Make plates as large as possible: Larger plates mean less welding is required. But, keep in mind the limitations of available plate sizes and forming equipment. While not specifically for determining the plate size, the functions SHIPLONGS2D3D and SHIPLONGS3D2D allow you to transfer seam and butt lines between the 2D shell expansion drawing and the 3D model. 

Although the 2D representation in the shell expansion of seams and butts is not the same as in the final plate expansion, it does give a good indication of the size of the plate.

• Consider classification rules: The thickness of the hull plate is determined by classification rules. The thickness of the plate is typically displayed in the shell expansion drawing. This can be done with for example the AutoCAD command MTEXT.
• Account for section division: If the ship is built in sections, consider how the section division will be. By definition, there will be a seam and/or butt on the place where two sections are joined. 
• Start at fixed points: Begin defining seams and butts at fixed points on the hull, such as hatches or doors. This makes these 'inserts' easier to cope with. 
• Ensure developability: The shape of a hull plate is preferably developable, meaning it can be formed in one direction without distortion. These single curved plates are easier to produce then double curved plates. 

SHIPSINGLECURVEDRULERS can check for the developability of a hull plate. Sometimes, it is hard to judge from the visual appearance in the 3D model if a hull plate is developable or not. Therefore, this function can come in handy. The function draws rulers. If they cross each other, it means the plate is not developable.

• Check for maximum plate stretching: The definition of the seams and butts determine the amount of stretch in a plate. Excessive stretching during shaping can lead to cracks in the hull plate. 

When the function SHIPDOUBLECURVEDEXPANSION is successfully executed, a file with stretch data about the specific plate is generated. See the article 'How to make hull plate expansions' for more information.

• Adjust seams based on existing seams: The command SHIPCREATESEAMTYPE gives you 7 different options to adjust seams. E.g. to account for knuckles, a pipe, overlapping plates, extending or offsets. 
• Avoid stress concentrations: Multiple welds that are close to each other and in roughly the same direction lead to high stress concentrations, and can cause the material to deform or crack. Here below are some typical pitfalls:
  • Avoid sharp crossings of seams and butts with primary and secondary scantlings.
  • Avoid multiple butt-seam connections within a distance of 100 mm from each other.
  • Avoid hull plates where one of the angles is very sharp. 
• Account for asymmetric structures: If the ship has asymmetric structures, create separate starboard and port side seam and butt layouts for the shell expansion drawing.

Typically used MasterShip commands

SHIPPOLYPROJECTION can be usefull to project a polyline in the 3D view onto the ship in any direction. Also, the AutoCAD function PROJECTGEOMETRY can be used for this purpose. Though, the advantage of SHIPPOLYPROJECTION is that it can project a line on a wireframe of polylines, while the AutoCAD function can only project on surfaces. 

the functions SHIPLONGS2D3D and SHIPLONGS3D2D allow you to transfer lines between the 2D shell expansion drawing and the 3D model. These functions works best with (3D)-polylines, and therefore he function SHIPCONVERTLINES can be used to convert lines into polylines.

Output and next steps

By following these best practices and using the relevant functionalities in MasterShip, you can create a seams and butts arrangement that optimizes plate usage, minimizes welding, and ensures the structural integrity of the ship.

The final output of this process is a finalized shell expansion drawing that depicts the arrangement of the seams and butts. This information is then used in the successive process steps of making hull plate expansions and creating the detailed construction design.