Maxsurf Training
Tutorial 2: Fairing 1. Open your design from the previous exercise (with a design window topmost, go to File | Open Design), if you already have a design open, you will need to close this first (File | Close Design). 2. We now want to manipulate the design to increase the overall fairness of the hull shape. The easiest way to ensure a fair surface is to keep the control net as fair as possible, since this will effectively amplify any unfairness in the surface. Try to keep control point rows smooth, and in keeping with the shape of the buttocks and waterlines, and keep columns as straight and evenly spaced as possible. This should ensure that the surface remains relatively fair, and reduce the need for further manipulation. Check your design in all views after any major design changes.
3. Use smooth and straighten controls (Controls | Smooth Controls) to aid in fairing the net. Smooth Controls – Stiff will force the selected control points into a fairer curve, Flexible and Medium will allow the original shape more dominance. Use this to fair rows in the plan and profile views, and columns in the body plan view. Straighten controls will force the control points into a straight line. This can be conducted in the Current Plane Only, or in Three Dimensions. Only Straighten in Three Dimensions if you want to create a flat plate, e.g. the transom. Straighten in the Current Plane Only to straighten columns in the profile view.
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4. Surface Properties – Transverse flexibility should be fairly low (i.e. more flexible) probably 3 or 4 for this design, longitudinal flexibility should be higher (5 or 6). 5. Turn on the outside arrows toggle (from the Display | Outside Arrows menu), and check that the surface arrow is pointing outward. If it is not, click in the circle at the head of the arrow to reverse its direction. If you do not, Hydromax and Workshop will not be able to read the design properly.
Have as few control points along each spline as is necessary to form your required shape. Review your design, and see if any rows or columns can be removed without adversely affecting the shape of the hull. If they can, then remove them now, using the Controls | Delete Row (or Column) command. Place the Pac-Man over the end point of the row or column you wish to delete and click the left mouse button. Now we will look at the various visual fairing aids available within Maxsurf. 6. We will start with the compressed view, since this is the simplest and most powerful fairing aid within the programme. First, turn on the contours you wish to view, either the waterlines in the plan view, or the buttocks in the profile view. Then turn on the compressed view using the toolbar button ; this compresses the vessel longitudinally by a factor of 4. This will highlight any unfairness in the lines longitudinally, particularly any ‘bumps’ and points of inflection that were not obvious before. It is particularly useful for long, slender hulls.
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7. We will now look at curvature. In the body plan view, turn off the sections so that just one section at a time is highlighted (or alternatively leave all the sections on, but highlight one section at a time with the half view turned on), then click on the curvature button in the toolbar or go to Display | Curvature. This will bring up a porcupine plot for that section. When you choose Show Curvature, "Porcupines" are displayed perpendicular to the curve, and their lengths are inversely proportional to the square root of the radius of curvature at that point on the curve. Therefore the longer the porcupine line, the tighter the curve. Curvature may be displayed on all surface contours except intersection lines. To select a contour, click on it. It will be highlighted with a different line thickness and colour. To select more than one contour, shift-click on other contours.
Note: The value of the smallest radius (tightest bend) on a given curve is displayed numerically at the end of the porcupine located at the corresponding position on the curve. Where the porcupines ‘swap’ from one side of the line to the other (as in the picture below), there is a point of inflection in the line; if the shape of the porcupine is unfair (distinct, sharp changes in length, not smooth and gradual), it is highlighting sharp changes in curvature over the surface. A fair plot will generally guarantee smooth graduation of curvature over the surface.
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To hide the display of curvature porcupines, choose Hide Curvature from the Display menu. If the porcupines are off the screen, making it difficult to view them all at once with a sensible scale, then you can scale their relative size by going to View | Preferences | Curvature Porcupine Size. If they are too small, and you wish to see the variation more clearly, increase their relative size. 8. Shaded rendering is another good method of visualising fairness of the surface. Check that the outside arrows are pointing in the correct direction, then select Display | Render. Choose Gaussian Curvature, and set the brightness level to 5. The hull will be displayed through a spectrum from blue (convex), through green (neutral) to red (concave). All changes should be gradual, and in general, most of the hull should be blue or green. If you want a concave area on your hull surface, then ensure that it graduates smoothly back to the convex sections. This is a measure of developability, rather than a direct measure of fairness, but still a useful tool.
9. Longitudinal and transverse curvature will highlight irregularities in the longitudinal and transverse directions, respectively. If there are ‘creases’ in your curvature plot, this is most likely because the splines are too flexible, so increase the stiffness by one degree, and see what affect this has. If you have compacted points in your
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design, it could be because too many points have been compacted together (should be one less than the stiffness of the spline). 10. The curve of areas (Window | Curve of Areas) can be used to display the variation of underwater section area along the length of the design. Check that your design has a smooth curve; the maximum point would be expected to be somewhere between the aft perpendicular and amidships. The curve should slope down to zero at the bow, but may have a cut off at the stern if you have a partially submerged transom stern.
11. Now check the hydrostatics again (Data | Calculate Hydrostatics), to ensure that any changes you’ve made have not altered the overall dimensions too much.
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