Permeability (Effectiveness)

In naval architecture, permeability is the percentage of a given space that could be occupied by water if flooded. The calculation of effective volumes are typically simplified by accounting for internal structure, piping, equipment, and cargo with a permeability factor which is often expressed as:

     Permeability = Effective volume / Total volume.

For non-cargo spaces a fixed value is generally assumed. In a cargo hold, the value typically varies with the amount and type of cargo. In many cases, stability regulations prescribe permeability values to be applied.

GHS uses three classes of parts: Displacer Parts, Tank Parts, and Sail Parts. The classic definition for permeability only applies to tank class parts. Effectiveness is synonymous to permeability, but is a more general term which is more intuitive in the context of displacing parts and sails. For the rest of this document, the term effectiveness will be used unless specifically referring to tanks.

The effectiveness/permeability factor is applied to waterplane area, lateral plane area, and free surface moment as well as volume. Being a simple factor, effectiveness does not recognize the distribution of elements which reduce effective volume. The assumption of uniformity, although convenient, is not without potential consequences.

For an extreme example, consider a cargo hold which is half full of bulk cargo but three-quarters full of water. The waterplane area in GHS will reflect uniformly distributed cargo, i.e., waterplane area x permeability. However in reality, the waterplane will not be reduced because the cargo is underwater. In this scenario the uniform model will underestimate the GM loss due to free surface effect.

Permeability in Geometry Files

In GHS, the Effectiveness/Permeability factor is an attribute of components. If not assigned a value between -1.0 and 1.0, the default for Permeability is 0.985 and the default for Effectiveness is 1.0.

An example of a displacer where Effectiveness could be less than 1.0 would be a mass of logs strapped firmly to the deck. It is takes less effort to model and calculate the volume of the logs with an envelope and an effectiveness than to model each log individually as a cylinder.

A part may be composed of several components, some of which can be deducting components. Unless there is a specific reason to do otherwise, deducting components should have an effectiveness equal to the inverse of the contributing components. This is because effectiveness is applied to the components before the volumes/weights are summed. The tables below provide two example cases. In the first, the engine room part is intended to have permeability of 0.85 so, the deducting component is set to -0.85. The second example show whats happens if the deducting component has a different permeability.

Part Maker can set a value using the PERMeability statement in either a CREATE or MODIFY command. If you are creating multiple components of a single part, any additional components created will inherit the permeability or effectiveness of the initial component unless otherwise designated. See HELP PM and HELP PM_TM for further details.

     INBOARD 3
     TOP 10
     BOTTOM 2
     PERM .95

Section Editor can also modify the geometry file by selecting the desired component and using the Edit command: Edit Effectiveness {value}. See HELP SE_REF for more information.

     Example for changing a component's permeability to .9:
     E E .9

Permeability values assigned by PM and SE may not exceed 1.0, but depending on the conversion type and included data, some geometry files imported from other sources may have greater values. These can be corrected by editing in SE, PM, or in the main program using the FIXUP command (see HELP FIXUP).

Permeability in GHS

The main program has the ability to override geometry-defined permeabilities (even allowing values above 1.0). This can be done using either the COMP /PERM:value (see HELP COMP) or PERM command (see HELP PERM). The COMP /PERM:value affects a component whereas PERM affects the whole tank.

If the FLOOD parameter is included in the PERM command, then the override permeability is only in effect when the tank TYPE is set to FLOODED. The original permeabilities from the geometry file can be restored using the PERM RESTORE command.

The PERM REPORT command lists each selected tank's average permeability, with any overridden value marked with an asterisk.


     PERM (ENGRM.C) 0.50
     PERM (*) REPORT

The example above overrides the geometry file values by specifying a 0.50 permeability for the tank ENGRM.C, then lists the permeabilities for all tanks.

Permeability in the Modules

Certain optional modules can also apply temporary permeability values while producing special-purpose reports. The FL command produces Floodable Length tables and permeability plots (see HELP FL and HELP FLMANUAL). The OUTFLOW /PERM parameter specifies nominal permeabilities for oil tanks while producing outflow compliance reports (see HELP OUTFLOW).

Copyright (C) 2011 Creative Systems, Inc.