PIAS Manual  2021
Program for the Integral Approach of Shipdesign
Change the shape of the SAC
Attention
This functionality is turned off by default. To enable it, configure the project for design using a target SAC in [File]→[Preferences...]→[Fairway Project Settings...], see General Fairway settings.

If main dimensions have been set, including target values for block coefficient \((C_b)\) and longitudinal center of buoyancy (LCB), see Main dimensions (design) & hull coefficients, Fairway enables you to design towards these targets by means of the target sectional area curve (target SAC). This action is for the construction and manipulation of a target SAC, and is started from [Hydrostatics]→[Generate/Modify Target SAC], keys <Alt><C><A> or a click on one of the points on the target SAC. The SAC can be viewed in a dedicated modelling view by selecting [Hydrostatics]→[Sectional Area Curve (SAC) Window], which also shows the hull lines as seen from below and from the side, for reference, shown in the figure below. The view also shows the actual SAC in gray, which cannot be edited and represents all buoyant solids and forms the basis for hydrostatics information; it can be updated from the hydrostatics window (see Hydrostatic Data). The SAC view is automatically brought forward when this action is started.

Once a target SAC has been created, it can be used to compare the submerged frame area during frame manipulation (see Show Target Frame Area). The target SAC, if present, is also used to base automated hullform transformation on, see [Shift Frames (Lackenby)] and [Inflate/Deflate Frames].

Note
The target SAC only applies to a single buoyant solid. Therefore, if more than one solid is marked as buoyant at startup, the user is asked to identify one particular solid and others are made non-buoyant automatically. Buoyancy can be managed by hand in Object management.

If a target SAC is not present yet when the action is started, the user is asked to create one by pressing one of two buttons:

  • [Create target SAC from current solid] derives the SAC from the current shape of existing frames in the buoyant solid.
  • [Create target SAC from Lap diagrams] generates the SAC based on Lap's diagrams, which are discussed in some more detail in The use of Lap diagrams. When selecting this option you have to specify additionally whether the vessel is single or twin propeller, because different diagrams exist for these respective types.

The above functionality is also available during manipulation of an existing target SAC, with the buttons [Regenerate from solid] and [Regenerate from Lap] seen in the figure below.

action_sac_shape_800.png
Manipulating the target sectional area curve.

The target SAC is represented by a polycurve fitted through a number of given area values, and can thus be manipulated by changing these values; much in the same way as points can be changed on ordinary polycurves. There are four manipulation modes: [Drag], [Knuckle], [Insert] and [Delete]; their operation is completely analogous to the point manipulation modes.

Fit

During manipulation of the target SAC, two gauge bars in the action panel show how well the SAC matches the target values as specified in [Main dimensions (design) & hull coefficients]. The [Fit] button here will transform the target SAC so it matches the target values, using the same algorithm as in [When designing with a target SAC].

The use of Lap diagrams

When commencing a ship design from scratch, with a target displacement or LCB, an initial SAC might be a proper tool to speed up this process. Unfortunately, not many methods are available to generate a SAC, although Lap (NSP, Wageningen, The Netherlands) did publish one. Those are the so-called Lap-diagrams, applicable on bulbless vessels with a conventional cruiser stern (so, no pram-type of aftship). So, at the ship extremes (for and aft) a conflict might arise between the sectional area according to these diagrams, and the desired stem or stern profile. Such a conflict may be solved iteratively, for instance in the following fashion:

  1. Start with the SAC generated from Lap diagrams.
  2. Design the hullform, making certain that in the `mid region' (that is, not near the stem and stern) the frame areas correspond to this SAC, see Show Target Frame Area.
  3. Generate the target SAC from this hullform.
  4. This SAC will now have incorrect values of \(C_b\) and LCB. So it must be transformed towards the correct parameter values.
  5. Adapt the hullform to this SAC.
  6. When necessary, repeat the process from step two.
Note
Lap diagrams offer a facility to ease the design process, which may prove to be useful for the ship designer. However, its use is certainly not obligatory, and it is not the core of Fairway at all.

The Lap diagrams are numerically contained in a separate file, kvslap.txt, which is modifyable so a user can use alternatives, if available. File format of diagrams for generation of a sectional area curve contains more information on the file format.