FreeCAD-Ship s60 tutorial
|Time to complete|
In this tutorial we will work with Series 60 ship, from the Iowa University. The tutorial is aimed to show how to work with a symmetric monohull ship, but multihull or non symmetric ships can be performed with the same proceeding.
You can learn more about FreeCAD-Ship here
FreeCAD-Ship works over Ship entities, that must be created on top of provided geometry. Geometry must be a solid, or set of solids, following criteria must be taken into account:
- All hull geometry must be provided (including symmetric bodies).
- Starboard geometry must be included at negatives y domain.
- Origin (0,0,0) point is the Midship section (Midpoint between after and forward perpendicular) and base line intersection.
Loading Series 60 geometry
In order to help new users FreeCAD-Ship includes a geometries examples loader, with the following to choose from:
- Series 60 from Iowa University
- Wigley Canonical Ship
- Series 60 Catamaran
- Wigley Catamaran
Executing the tool (Ship design/Load an example ship geometry) a task dialogue will shown. Select Series 60 from Iowa University and press Accept. Tool loads new document with s60_IowaUniversity geometry.
Create ship instance
In order to create a Ship instance select s60 geometry and execute the ship creation tool (Ship design/Create a new ship).
Creating ship task dialogue and some annotations at 3D view will shown. The annotations will removed when you close Ship creation tool, so don't worry about this.
Most relevant ship data must be introduced (FreeCAD-Ship uses a progressive data introduction system, so basic operations can be performed knowing only basic ship data, more information is needed as the operations become more complex).
Main dimensions must be introduced here:
- Length: Length between perpendiculars, 25.5 m for this ship.
- Beam: Total ship beam, 3.389 m for this ship.
- Draft: Design draft, 1.0 m for this ship.
Usually the Length between perpendiculars depends on design draft, so if you don't know what is the length of your ship you can set draft, and fit length in order to get bow and draft intersection.
Same process is valid for Beam fit. Note that requested value is total beam, but annotation is only refered to starboard half ship.
When you press Accept button program creates your new Ship instance called Ship at Tags & Attributes dialog. We don't need geometry anymore, so you can hide it.
From here onwards, you must have Ship selected before you execute any FreeCAD-Ship tool.
FreeCAD-Ship provides a tool that makes it easy to obtain a Lines Plan from the ship lines drawing
Lines drawing is a set of lines from section cuts in all 3 axis, that will eventually show the hull geometry in a Lines Plan. We need to provide the lines for the 3 following views:
- Body Plan (using the Transversal Cuts)
- Sheer Plan (using the Longitudinals Cuts)
- Half-Breadth Plan (using the Waterlines Cuts)
Usually 21 transversal equidistant sections between perpendiculars must be performed. in order to do it FreeCAD provides an automatic tool in order to do it, simply select Transversal type of sections, go to Auto create box and set 21 sections, then press Create sections.
Sections table is filled, and sections preview called OutlineDraw shown. Usually more sections was added at bow and stern, where more complex curvatures are registered, in order to do it go to the end of the table, and do double click at empty item in order to edit it, pressing intro to confirm. Add following sections:
- X22 = -12.1125 m
- X23 = 12.1125 m
Depending hull geometry complexity, sections preview can take some time. In order to remove a section, just fill it with an empty text and press enter.
Two longitudinal cuts must be added, so select Longitudinal type of sections, go to Auto create box and set 2 sections, then press Create sections. Sections table is filled, and sections preview updated.
6 Waterlines between base line and design draft must be added, so select Waterlines type of sections, go to Auto create box and set 5 (Z = 0 m will not be considered, add it manually if you need it) sections, then press Create sections. Sections table is filled, and sections preview updated.
Several additional waterlines must be added:
- Z6 = 1.2 m
- Z7 = 1.4 m
- Z8 = 1.6 m
- Z9 = 1.8 m
- Z10 = 2.0 m
Select 1:100 scale and press Accept to let the tool to generate the 3D sections in a new object.
In order to plot these sections you can use the drawing workbench:
Transversal areas curve
One typical ship design hydrodynamic parameter is the transversal areas curve, that retrieves some indicators about the behaviour of the hull (towing resistance, seakeeping, ...). FreeCAD-Ship provides a simple tool in order to perform transversal areas curve.
When tool is executed a task dialog is shown, and free surface prewied is created at 3D view (Free surface preview will be removed when tool finished, so don't worry about them). Into task dialog input and output data is present.
Draft and trim (Hull y edge rotation angle, positive if stern draft may increase) must be provided. Several areas curves may be performed, depending on ship load situations, but two typical plot should be performed:
- Design transversal areas curve: Without triming angle and using design draft, 1.0 m in this case.
- Maximum draft transversal areas curve: Without trimin angle, and maximum draft allowed, 2.0 m in this case.
Some relevant data is shown at real time:
- L: Lenght between perpendiculars, value set at ship instance creation.
- B: Beam selected at ship creation.
- T: Actual draft amidships.
- Trim: Trim angle.
- TAP: After perpendicular draft.
- TFP: Forward perpendicular draft.
- Displacement: Ship displacement (salt water considered, divide by 1.025 in order to know displaced volume).
- XCB: Buoyancy centre point X coordinate (relative to midship section).
When Accept button is pressed a plot is performed (depending on geometry complexity can take some time, you can see progress on terminal, and stop the work pressing Ctrl+C). When the task has finished FreeCAD will generate a Plot (see the plot module documentation) and a SpreadSheet (see the Spreadsheet workbench documentation).
You can perform maximum draft transversal areas curve in order to see the differences (for instance you are noticing that areas curve is passing through length perpendiculars now).
Hydrostatics computation is a critical stage at ship design due to know principal stability hull parameters. Hydrostatics are mandatory data in order to classification societies certificates ship, and joined with loading condition data (weights and gravity position) provides essential data about ship stability. FreeCAD-Ship provides a tool to obtain main hydrostatics curves (GZ curves are considered in other tool).
When tool is executed a task dialog is shown. Usually Hydrostatics curves are presented for each trim angle, in this tutorial only upright trimming angle will considered (0º), with an interval around every loading condition draft. Since we don't know what loading conditions we can get, we will consider almost draft possibilities (Usually, in order to get as many resolution as possible, naval architects fits the interval to feasible drafts).
So we set following data:
- Trim = 0 deg
- Minimum Draft = 0.1 m
- Maximum Draft = 2.0 m
- Number of points = 39. A lot of points or really complex geometries imply long computation times, in this case around 1 minute can be expend.