delftship hull thickness???

Hello,
I am using Delftship to design the hull(developable hard chined hulls). I wonder what is usualy taken as hull shell thickness. Do you use a layer thickness and also use the hydrostatic calc. considering thickened hull surface? Or do you consider 0 thickness hull surface and with another program, by offseting exteriour surface to inside inorder to model bulkheads?
I hope you understand what I meant

 

In software like Delftship always designing a inner part of shell plating. And this part of plate is taken to hydrostatic calculation

 

But if you model the interiour hull surface, the hydrostatic calc. results will be wrong. I tried two cases. First as you mentioned I designed the interiour hull surface. The hyd. calc. were performed to this surface. However after adding a thickness to this surface (10mm hull thickness) the displacement increased 1 m3 and lcb, Cp changed dramaticaly.
So what should I do?

 

What you did is double the mass of your hull skin. Only one of the surfaces should reflect the mass as the other is simply the other side of the same mass

example: you draw your outside surface, and add an inside surface 10mm from the outside surface. If you assign a mass of X(kg)/m3 and a thickness of .01 m to the outside surface then it is assumed that there is an inside surface...but delftship won't show it. By you adding the inner surface yourself you are simply making that surface visible and you can draw your bulkheads to that surface accurately. The interior surface should have no mass or thickness in and of itself as these are already taken into account by the outer surface.

 

Lewis,
Delftship assumes the thickness added to outside not to inside. I know that because when I added a thickness and after hyd.calc. the displacement increased.
My problem is when I add the thickness to a hull layer in DS, the Cp value can not be controlled. For example befor adding thickness to layer the Cp is 0.56, and after adding thicknes then the Cp is 0.62 . And trying to reduce it distrupt the hull shape. I can not reduce it. So I wonder how people usually make this. Do they first design the outside of the hull and offseting as much as hull thicknes to inside and finally modelling the accurate bulkheads?

 

Yes...that is exactly the point...DS is a hull modeling program first and foremost. You model the hull first then do the interior.

edited to add... checked with one of my models...when I add weight and thickness to the skin the displacement did not change. The various gravities did though.

 

I confirm what Lewis said. When I change the material thickness in the layer properties dialog (FREE!ship+) the mass of the hull goes up but the displacement does not.

The only way this can be true is if the faces represent the outside of the hull skin and the thickness is added to the inside.

Nothing suggests that this is accounted for in the panel development drawings, reinforcing the need to put generous chamfers on each panel before assembly.

Metin, when you say "added a thickness" do you mean that you added a parallel face layer to represent the inside of the hull? Or did you assign a thickness value in the layers dialog?

 

I thınk delftshıp and freeshıp are dıfferent from eachother. Because after assıgnıng thıckness to the layer then the dıspacement ıncreases. you can see my fıle that ıs created from DS. I wıll download freeshıp. Does ıt have auto faırıng property?

 

there are differences between freeship and delftship. The freeship project forked into the closed source delftship (under the ownership of the original programmer) and the open source freeship+ a few years ago.

The file you provided wouldn't open in the latest version of freeship+.

In reality (in freeship+) when you stitch (or weld) the developed panels together, if the parts are not chamfered/beveled, the inside surfaces will be in the location defined on the model.

 

The basic hydrostatics in all three are the same.

I get no difference in displacement when I remove the thickness and density from the layer properties in the file that you sent me. Do you have DS free or pro and what version? I have pro and it is version 4.46.158

Where are you looking at your displacement figures? Is it in the Hydrostatics report under displacement?

 

I use last version of DS free. In the preferences or settings there should be something like 'use actual submerged body' and ' use layer thickness' they should be selected. Yes, I look at the hyd. report.

 

I can confirm the previous observations on Freeship. Adding the thickness influences only the mass of the vessel, not it's hydrostatic characteristics. So the thickness is added towards inside.

 

I usually model the boat in DS. Then save as .dxf. And I open it from Rhino evaluation version... So I should offset the hull surface through inside ok? But offset surface is not smooth.

 

There seems to be some confusion.
Setting a layer thickness in the layer window does not alter the hydrostatics.

Specifying a mean shell thickness on the hydrostatics tab of the project settings however will include the thickness in the hydrostatics. In later versions of delftship you can also opt. to use the actual layer thickness (as specified in the layer window) instead of the mean shell thickness at this same tab.

 

I use actual layer thickness. And It changes hyd. resuts.