KN values in Hydromax

[JustinD]

Hello,
here's my problem,
I am producing GZ curves in Hydromax from the loadcase windows and I then calculate the KN values from the displacements and vcg's and use these to get the new GZ in Excel (GZ = KN-KGsin()). I am wondering is it possible to combine the KN values in Hydromax to get GZ curves because I would like to use the Criteria available to see if the GZ curves are right without having to do it manually.
Thanks,
Justin

[SailDesign]

GZ2 = GZ1-(shift*sin(angle)) where "shift" = the change in VCG

[JustinD]

That's great Sail Design but it doesn't answer my question.
Thanks anyway.

[SailDesign]

That's great Sail Design but it doesn't answer my question.
Ah, well - can't blame a chap for trying :)
Obviously you have to know Hydromax to understand the question..... Hope someone gives you a good answer.
Steve

[Andrew Mason]

If you want to compare a GZ curve calculated with the Hydromax Large Angle Stability analysis mode with on derived from a set of KN curves, the easiest way is to copy the data into excel and graph the two curves.

Hydromax doesn't apply stability criteria to KNs, as if you know the displacement and VCG, you are always going to get a more accurate result using the Large Angle Stability analysis to calculate a specific GZ curve.

One of the reasons that the direct calculation of the GZ curve is more accurate is that it accurately calculates the effect of the height of the VCG on longitudinal trim. This effect is lost in the KN analysis as the VCG is assumed to be fixed when calculating the KN values. This effect is genrally insignificant in large ships, but can be important in smaller vessels.

[JustinD]

Thanks Andrew,
I am analysing stability with fishing vessels and therefore I will use the KN values as all the stability booklets I have seen for fishing vessels use the KN values.
Thanks,
Justin

[Andrew Mason]

Justin

That's the problem with stability books, if you want to calculate a GZ curve for a specific loading condition then ideally it should be calculated directly without using KNs as an intermediate step. For the vessel's master however, they need to be able to calculate a GZ curve for a non-standard loadcase, so the KN's are provided to allow him to do so with a minimum of effort. Trouble is, the two methods can produce different results, particularly for shorter, beamier vessels, due to the simplifying assumptions implicit in the KN calculation.

This is just one of the many contradictions that comes about due to the need to simplify calculations (using free surface moments to calculate corrections for VCGs is another example) so that they can be performed on board a vessel by a person with a pen, paper and simple calculator. Maybe one day stability books will be in the form of a simplified version of Hydromax running on a PocketPC, then we won't need to dumb down the calculations.

regards

Andrew

[MikeJohns]

Justin

Maybe one day stability books will be in the form of a simplified version of Hydromax running on a PocketPC, then we won't need to dumb down the calculations.

regards

Andrew

Andrew

That sounds like a really good idea.. , if they ran on a laptop would be ok since they are inevitably aboard these days.