General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

Multi-hull Seakeeping
(Requires GHS version 17.06A or later with SK)

Unlike the Archimedean spiral, multi-hull seakeeping is something we seem to get asked about with increasing frequency. The good news is that you can analyze a multi-hull vessel in GHS, but it takes some creative thinking and knowledge of the theory to ensure your results are acceptable. To illustrate, consider the following SWATH hull:

In its native form, SEAKEEPING works on monohull vessels. So how do we make it run on a multi-hull? We simply make a SEAKEEPING call for each hull. In this way, you could run SEAKEEPING on a trimaran, too, or maybe even a pentamaran... but let's not get too carried away.

The process is rather easy. First, we must ensure each hull in our multi-hull is modeled as a separate component. In this example, we've modeled the SWATH with two components: HULL.P and HULL.S. We then pass each hull component to SEAKEEPING explicitly:


`...establish loading condition

vari (str) skpars="/head:135 /samp:50,0.1,2.0 /bf:24"

wave (spe) p2 3.5

report temp.$$$
sea /hull:swath\hull.s {skpars} /data:stbd\hy,fo
sea /hull:swath\hull.p {skpars} /data:port\hy,fo
report off | erase temp.$$$

In this example, we are writing the hydrodynamic and forcing data for each hull to a respective "STBD" or "PORT" subdirectory.

The next step requires some help from SK.LIB. The latest update to the SeaKeeping macro library includes two new macros: CombineHydroData and CombineForcingData. As their names suggest, we use these macros to combine the hydrodynamic and forcing data from each hull into a single pair of data files. For more information on how these macros work, type .sk.libinfo after running SK.LIB.

run sk.lib /call /quiet

.sk.combinehydrodata "SWATH-HYDROS.DAT" "port\SK-HYDROS.DAT"

.sk.combineforcingdata "SWATH-FORCING.DAT" "port\SK-FORCING.DAT"

report close /preview

The final step in the process involves feeding the combined hydrodynamic and forcing data back into SEAKEEPING by use of the new DATA mode, as shown above. This means SEAKEEPING will construct the equation of motion using the external data and return a complete report--just like a regular seakeeping run.

However, there is one important caveat to this approach: because the forces on each hull are considered independently, any hydrodynamic interaction between the hulls is not considered. For a SWATH, this is generally a non-issue; for other multi-hulls, the interaction is decreased with forward speed.

The geometry file and run file used to create this example may be found here. The latest SK.LIB may be found here. And here's a run file that plots an Archimedean spiral.

Questions, comments, or requests?
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