Friday, June 4, 2010

Maxsurf Tutorial

MaxSurf Introduction:
MaxSurf allows the user to construct numerical models of hullforms using NURBS surfaces. The NURBS representation can be used within MaxSurf to generate lines drawings and determine hydrostatics. Within the MaxSurf suite of software the NURBS representation can be used to develop resistance and powering predictions (HullSpeed), develop seakeeping estimates (SeaKeeper), investigate damaged stability (HydroMax), or perform structural analysis (WorkShop). Finally, it can be exported to programs like Esprit, Ideas, or Rhino to create the construction/milling files required to produce your ship on the shops NC mills.
When starting MaxSurf, initially 10 windows pop up. . The Plan, Profile, Body Plan and Perspective windows are used to visualize the NURBS surface. The other windows display calculations and enable the user to enter or edit text directly when constructing or modifying the NURBS surface. All MaxSurf windows can be manipulated (minimized, maximized, etc) like any other Windows display.
Surfaces are defined within MaxSurf by the position of a set of control points that collectively form a control point net. The shape of a surface cannot be directly manipulated, rather it is the movement of the control points (i.e. the intersections of rows and columns in the control point net) that allows you to modify a surface into a desired shape. Essentially it is if you are changing the shape of a balloon by gluing rubber bands to it and stretching the rubber band to each control point. Corners of a surface are defined exactly by the position of the corresponding corner of the net and are represented as pink squares. Edges are defined only by the control points on the corresponding edge of the net, while the internal points of the surface may be influenced by many or all of the control points in the net. Internal and edge control are represented as grey squares.
Multiple surfaces can be used to create hulls panels, transoms, decks, keels, etc. which in turn can be joined or intersected with each other within MaxSurf to create a single hull model.

Procedure for creating a hull in MaxSurf
The following steps are required to create a hullfrom from scratch in MaxSurf. Instructions written in “SmallCaps” refer to pull-down menu options within the MaxSurf. Many of these options have shortcut buttons on the default toolbars that can be used as you become more proficient.
Use File/New Design to initiate a new design Use Data/Units to set the desired unit (metric, imperial, etc.) Create a surface using Surfaces/Add Surface/Cylinder Open the Assembly window using View/Assembly Within the Assembly window rename the surface Stretch the surface to its approximate final size using Surfaces/Size Surfaces Before progressing much farther the coordinate system within MaxSurf should be initialized.
Data/Frame of Reference is used to set the design waterline height relative to the baseline and the location of the forward and aft perpendiculars. Click the Find Baseline button, then enter the appropriate waterline height in the DWL box. Next click the Set to DWL box. Note that distances measured aft of the zero point (currently amidships) are assigned negative X coordinates. The reference zero typically corresponds to the intersection of the forward perpendicular and the baseline. These values are set using Data/Zero Point. Also uncheck the Locked Zero Point button. ß Important Use Data/Grid Space/Add Sections to enter station definition. Either use the space command to evenly space the sections, or individually enter the longitudinal locations in the section table. Note: the section spacing should correspond to any offset data that you may later import. The shape of the desired hullform is determined by the shape of the control point net. The simpler you keep the net, the easier the surface is to manipulate and fair. In general, the net columns should be kept vertical and be concentrated more in the ends than in the middle. As a first cut 5 columns should be adequate to define a simple ship shape without a bulbous bow or faired in skeg. Net rows should follow the diagonals if the hull is round bilged or follow the chine lines if the hull is hard chined. If the hullform is a typical monohull, the forward most column and the bottom row will be at Y=0 to insure that the port and starboard sides meet at the bow and that there are no holes in the bottom. Two simple nets are shown below:
Hard Chined Hull Round Bilged Hull

If the hull has no transverse curvature, the net can be simplified by reducing the order of the surface to linear (2nd order) in the transverse direction, requiring only one net row to define a longitudinal chine. The hard chined example above was constructed this way. If transverse curvature is present then the number of net points one less than the order of the surface must be compacted (co-incident) to create a chine. The round bilged hull form above is 4th order so three net points lie on top of one another to create the chine at the transom, which fades away forward as the points move apart.
Modify your net to look more like the appropriate net shown above;
Insert net rows and columns using Controls/Add Column and Controls/Add Rows commands. Net columns can only be added from within the Plan or Profile windows, while net rows can only be added from within the Body Plan Window. After clicking Controls/Add Column from within the appropriate window simply click the net where you want the additional column inserted. To specify the order of the surface use Surfaces/Surface Properties and change the appropriate values. Set surface stiffness to 4 in longitudinal direction and transverse direction if round bilged – or 2 in the transverse direction if hard chined.
Editing the Control Point Net
Control points are moved individually by selecting one (clicking and holding the right mouse button down with the cursor overtop the appropriate control point) then dragging it to the desired location. When the button is released the control point will be dropped. Alternately, specific X, Y, and Z values for control points can be entered in the Control Points window. Multiple points can be manipulated at once be shift-clicking or dragging a window around multiple points. The allowable directions of motions can be limited to orthogonal directions by holding the shift key down during the dragging operation. Control points can be placed on top of one another by clicking on the point that is already in the desired location, shift clicking on a second net point, then using controls/compact to place the second point on the first. MaxSurf provides several utilities to assist in fairing and smoothing the control point net. controls/smooth controls and controls/straighten controls have multiple options but generally work by highlighting a range of control points then clicking the menu option to smooth or straighten the interior and edges of the selected control point range. The controls/snap to grid option is also helpful as is the ability to directly edit the control point XYZ locations in the Control Points window.
Advanced Editing
Bonding edges Intersecting Surfaces Bow knuckles
A design begins with one of the standard surfaces (or an existing design). You then modify the shape of the surface's four edges, followed by its interior. The longitudinal edges are best modeled before the transverse edges, and it is generally best to form the edges in the horizontal plane (the Plan view) prior to the vertical plane (the Profile view). Having defined the edges you may then go on to manipulate internal points in the net to create the required surface shape, this is best done in the Body Plan view.
Goal is to achieve a simple clean net, i.e. one that does not distort the parametric surface and has the minimum number of rows and columns to adequately define the surface.
Control box in the Body plan view shows the currently active net column and the currently displayed section. Data/Grid Spacing creates sections, waterlines, and buttocks.
Station lines, waterlines and buttocks are defined in the “Grid”
Set Surface flexibility (2 for hard chined, 3 for round bilged, 4or higher to fair better

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