Hydrostatics results from features

[Jaime Saelices 0]

Hi folks,

 

I'm modelling a hull and imported into FFW in order to optimize the hull in several ways. My surprise comes when using two features defined in the application (Wetted Surface from Offsets and Displacement from Surface Groups). The values for wetted surface are completely different and am wondering the reason why. The wetted surface was calculated previously using Maxsurf and Rhinoceros with identical values.

The difference is really high reaching almost double value using the Displacement from Surface Groups feature.

 

Does anybody know the method used for calculations? Inspecting the functions in the feature does not allow me to make a guess.

 

Thanks.

 

Regards.

 

[Mr. Claus Abt 14]

Do you use both sides of the hull?

Claus

[Jaime Saelices 0]

Important annotation because by default I realize that FFW always mark the geometry as symmetrical. It justifies that values obtained using "Displacement from Surface Group" are exactly the double of correct ones. However it is quite enigmatic that disabling symmetric checkbox carries the same double values. On the other hand values obtained using "Wetted surface from Offsets" are quite odd.

[Mr. Claus Abt 14]

Hi Jaime,

For the hydrostatics from offsets the defaults are aligned to typical naval architecture nomenclature, i.e. modeling one side of the hull and being a monohull designed along the x-axis z upwards etc. while the displacement from surface groups is more "physical" because it integrates the "local pressure" on the triangulation of the surfaces.

 

Can you give a small example for the odd wetted surface results? Maybe you have found a bug ....

Thanks

Claus

[Jaime Saelices 0]

Hi Claus,

 

The model is a yacht hull with chines, the value obtained with "GetDisplacementFromSurfaceGroup" for wetted surface is 89.27 square meters that is almost twice the correct value. This is quite weird because you said that it calculates the values integrating along triangulated surface. The value corresponding to "OffsetGroupWettedSurface" is 29.48 square meters.

 

Once the geometry is imported I always disabled the symmetry checkbox because I have to work with the boat upright and heeled. May be this is a clue for what is going on.

 

Thanks.

[Mr. Claus Abt 14]

Hi Jaime,

First of all, the surface from surface groups seems ok. You need to set the fourth argument to false yourgroup.getDisplacement(draft, heel,sink,false) otherwise symmetry will be expected.

The feature does indeed not calculate the right values, someone needs to go through it and check if it is just a programming mistake or a bug. No time for that now ...

Cheers

Claus

[Mr. Claus Abt 14]

Actually the feature seems ok, too. Please see this example that I set up comparing both methods. There is a difference, but less than 1 percent and may result from resolutions ...

hydrostatics.fdb

 

Cheers

Claus