PIAS Manual  2024
Program for the Integral Approach of Shipdesign
Wind heeling moments
The effect of the wind on the stability is manifest in several places, and also the input has been spread over several modules. This is no peculiarity of PIAS, but is simply the nature of wind heeling moments. To give you still a proper idea of the coherence, this is discussed in this chapter.

Input data for the wind heeling moments computations

Part of the variety at wind heeling computations is caused by the fact that there can be multiple input data, for example several windage areas — like with various loadings — and several wind pressures, for example because another pressure is used at the stability criteria for intact stability than at the damage stability criteria. You can find the exact nature of the input data, and their location, in the table below:

Wind contoru

The wind contour belongs in principle to the ships' input data, and is therefore entered in Hulldef, see Wind contour. It is also possible to enter several wind contours, for example for multiple types of loading on deck.

Wind contour with containers

For standard cases, like for a stability book, several wind contours can be defined for various complete layers of containers. The trouble with this is that when a layer has not been entirely filled with containers, the windage of that entire layer is still taken into account, whereas the actual windage is obviously smaller. When a special loading module of Loading is used, like the container module (see Loading tools), for any loading condition exactly the actual wind contour is constructed by laying the ship's contour (without cargo, as entered in Hulldef, see above) and the loaded container on top of each other and taking the common windage of them.

Wind data

These are also given in Hulldef, see Wind data sets, and regard in particular the wind pressures. Several collections of these data can be recorded, so that all kinds of combinations are possible between wind contour and wind pressures at the various stability calculations

Specific wind pressure at a stability requirement

Although the wind contour and the wind data in principle provide sufficient data to be able to carry out further computations, some special input can take place at the stability requirements. One of them is the specific wind pressure, as discussed in Wind lever. That seems a bit redundant, because one can give any wind pressure on the wind data from Hulldef, so why then even here? The reason is that stability requirements may, for example at a damage stability criterion, use another wind pressure than the `standard' wind pressure which applies to intact stability. One should be very well aware of this aspect — a mistake is easily made — and even more so when creating standard criteria, because with this specific wind pressure it can directly be defined correctly. In short, this alternative, specific wind pressure is indeed redundant, but has been introduced to reduce the risk of errors and to enhance the user experience.

Multiplier on the wind heeling lever

In addition, a multiplier for the set wind heeling lever can be specified. That also seems a bit redundant, because why not in first instance already enter, in the wind data section, a wind pressure which has already been multiplied by this factor? This would indeed be an option — it would make the program internally even less complex — but occasional standard stability criteria exist where such multiplication factors are applied, and it is most clear to show and enter this factor in its most basic form. And by being clear the risk of mistakes is reduced.

Wind heeling levers entered by the user

It is by far the most practical to have the wind heeling levers calculated by PIAS. But only once in a while there are, however, wind heeling levers available from another source, for example from CFD calculations or model tests. In that case you can give these wind heeling levers yourself in Hulldef, at the last option of the wind data, see Wind data sets.

Where do wind moments exert their effects?

  • Tables of wind heeling levers can be printed with Hydrotables, see Wind heeling moment tables.
  • At the calculation of intact stability of a loading condition. The wind contour that is used there (as well for the calculation as possibly for the pictures) intrinsically belongs to that loading condition, and can (therefore) be entered in the upper bar of the weight items window of the loading condition, with the [Settings] option, see Settings per loading condition. The wind pressure that is used there intrinsically belongs to the stability requirement, and can(therefore) be entered at the Stability criteria, see Wind lever.
  • Tables and diagrams of maximum allowable KG in intact condition can be calculated for several wind contours, see Maximum VCG' intact tables. When you select N wind contours then there will be made N tables or diagrams, so you can take that table or diagram that applies to the future situation.
  • Tables and diagrams of maximum allowable VCG in damage condition function as for the wind similarly as the intact condition.

Recommended working sequence

There is no obligatory working or definition sequence with regard to the wind related issues. All input or options can be modified later on. But in the event of a project where nothing has been laid down yet, the clearest definition sequence is:

  1. Make one or more wind contours in Hulldef.
  2. Define the wind pressure(s) in Hulldef.
  3. If desired, a table of wind heeling moments can already be produced by Hydrotables, however, it is not required to do so, because this table is neither saved nor further used.
  4. Define stability criteria.
  5. Carry out the various computations.