Setting Offsets in ProBasic or ProSurf

ProBasic does not come with the utility programs (Offsets, Digitize, and Hull Draw) for typing-in or digitizing offsets. (If desired, these programs can be added to ProBasic for $250.) You can, however, create a boat and use a variety of methods for forcing the hull shape to match pre-defined offset points. This technique is also useful for ProSurf users if they do not have "too many" offsets to match.

Either way, matching an existing shape on the compuer can be a tedious and non-exact process. Imagine giving a table of offsets to five different loftsmen. Each would come up with a different full-size lofted shape. Presumably, the shapes would be close, but there is still a lot of room for interpretation in any table of offsets.

The reason that ProBasic and ProSurf can match up existing hulls relatively easily is that both programs use B-spline (NURB) surfaces where the defining points lie directly on the surface. That way, if you want a defining hull surface point to have a specific offset value, you can use the Edit-Set Point command to force it to that position. You can also use the Point-Add Point command (along with the Edit-Set Point command) to set target points or tick marks through which the hull must pass. Then with the dynamic hull dragging capability (Edit-Move Point), you can drag the hull to pass through the tick point.

This overall hull matching process follows these steps:

A. Use the File-Create Boat command to create a starting shape boat which has the same principal dimensions as the target boat. This simple hull shape has only three columns: one at the bow, one at amidships, and one at the stern. We now want to rough-up the hull shape by adding-in additional columns at each of the defining stations in the table of offsets. If the offsets that you have are scattered all over the hull, then you may wish to add the offsets as point entities and use them only as target points (tick marks).

B. Use the Surf-Add Row/Col-Add Row/Col Angle command to add in columns approximately at each station where you have offsets. Remember that it is important to add rows and columns such that they are evenly spread over the length of the hull. If rows or columns are added un-evenly, then it may become very difficult to shape and fair the boat.

Note 1: B-spline surfaces are made up of a rectangular-like grid of "rows" and "columns". For convenience (and clarity?), the tutorials will refer to the vertically oriented lines as columns and the horizontally (lengthwise) lines as rows. This orientation and labeling is not required by the program and is only used to make the explanation more clear (?). The program marks all commands with "Row/Col" type labels because the user need not care which are the rows and which are the columns. In our old DOS version of the program, we tried to enforce the orientation of direction of the rows and columns, but that was restrictive and unnecessary.

Note 2: If you want a new column to be located at a FIXED station position (X-value), then you need to use the Surf-Fix Row/Col-Vertical command on that column in the profile view of the boat. If you use the Surf-Add Row/Col-Add Row/Col Vertical command to fix the column when you enter it, the column will be fixed, but it might not be located in exactly the proper position because you pick the new column position using the cursor. You can, however, UN-FIX the column using the Surf-Fix Row/Col-Fix Row/Col Vertical command. It acts like a toggle to turn off the fixing of the column. You can then move the column and re-fix it if you wish. Generally, it's best to keep the columns which are in the middle of the boat fixed so that they represent stations. In the ends of the boat, however, you may need to angle the columns so that they make an even transition to the angle of the bow or stern.

C. After you have added in the evenly-spaced columns at the required offset station locations, you can use the Edit-Set Point command to alter the hull shape. (Remember to add in enough columns along the length of the hull to create a nice, even spread of columns, even if it means adding columns where you do not have offsets.) It is best to start by using the Edit-Set Point command to set the offsets on the sheer, chine(s), and hull profile. This will fix the boundary shape to known values.

D. Next, use the Edit-Move Point command to fix-up the rest of the hull to match the (now) exact offset values on the hull. Don't worry about detailed fairing yet, just try to rough up the general shape.

E. For the curved portions of each station (column), it may be best to use the Point-Add and the Edit-Set Point commands to create target "tick marks" thrrough which the hull must pass. This is because the defining hull points at the intersections of the rows and columns may not fall near your offset values. It is better, then, to add and set points as tick marks so that you can drag the hull to pass through the tick marks. This is not as bad as it sounds, since you already have the surface edges fixed exactly and boat is close to the desired shape.

If you are using ProBasic and you want to be able to enter offsets or to digitize a body plan, you can add-on the Offsets, the Digitize, and the Hull Draw modules (Station Definition Package) for only $250. This allows you to have the same matching and converting capability as ProSurf, without paying for the full ProSurf package. As always, you can upgrage to a higher package by just paying the difference in price of the packages.