Trim

Is is possible to move a specific ballast weight around on a hull in freeship and view trim result?

 

Not that I'm aware of.

I explained it to you before, the boat will trim out when the CEnter of gravity, is directly over the center of buoyancy.

If this conditionin is not met, the boat will pitch and roll until the immersed volume of the hull changes such that the center of buoyancy moves. The CoB will stop moving once it reaches the CoG, and thus arrives at a stable trim.

If you want to design a boat which floats level on her lines, then you must ensure that the weights of all things put into the boat, are located such that the sum of all their moments ( weight x distance) equals the same location as the CoB when the hull is level.

 

I think Saqa is currently not worried about the theory of ship and how to determine the equilibrium waterline.
I think what he wants to know is how "to move a specific ballast weight around on a hull in freeship and view trim result". That is, get the software to show the boat with the trim resulting from moving the ballast.

 

Thats right tansl, what I am working with are a pair of very narrow slender hulls aimed for speed. I want to see the trim after adding a pair of 100kg each outboards to the transoms. Then placing other weights like batteries to level it

Secondly I want to move 200kg which will represent the fisherman and deckie assiting around to see how will that affect trim. Then I can start compromising between speed and load carrying to finish the drawing and build a model for some testing

 

Is there any shareware or freeware around that I can import a drawing into that can display trim/ballast?

 

[Saqa], yes there is. Polycad is one of them. You can export from freeship as STL-model, and then import it into Polycad. There you can change VCG, LCG and TCG to see how the model behaves. And remodel, if you wish. And a lot more.

To see, where your models CG is, add "weight objects" in FreeShip, like a triangular surface (as a part, if you wish), to the place you want it, in it's own layer, with the thickness and density properties to get the weight right, then check from the design hydrostatics the location of your models CG. Use those numbers in Polycad, and have fun!

Polycad is available from Marcus Bole's website http://www.polycad.co.uk/ . It requires a registration, but it's free, and no spam will come from there. At the website, there's also a wealth of information about ship design.

 

Sorry, how can give hydrostatics position of center of gravity "G"?

 

TANSL, I meant the design hydrostatics calculation in FreeShip. All the layers modeled in the program can be addressed with density and thickness, which gives the weights of items on the layer (area x density x thickness). Individual surfaces can be modeled in their own layers. And in the design hydrostatics calculation, there's a layer properties calc, that gives you the centers in x, y and z directions relative to baseline. Both of each layer, and the sum of all layers. Use those numbers, if everything is modeled (at least as weight objects), else calculate the G position with spreadsheet or pen and paper. Of course you need to know the positions and weights of objects, either way.

 

Thanks Jarmo, I am only just starting to play with freeship so will explore those options and will try polycad

 

Jarmo, sorry but I do not see it clearly. Any software gives you the wetted surface of the hull to various drafts. Skillfully used, the wet surface, can be used to calculate the weight of the various areas of the hull shell. What I don't see is how to calculate the c. of g. for those surfaces.

 

Dear TANSL,

Attached pictures should give you a feeling, of what I meant. Wetted surface is only one of the many properties calculated by Freeship. In the pictures, different parts (surfaces) are in their own layers, and each layer has its own properties (density, thickness, display color...). These properties (plus of course the area of each part) are calculated in the Calculations>Design Hydrostatics Menu, for both per layer and per whole ensemble. COG of each part, and the whole ship, are displayed in Design Hydrostatics. And you can use these COG positions in any other programs (I suggested Polycad).

 

Thanks Jarmo. Now I know clearly what really calculates Freeship.
I wondered how it was possible to calculate the cog of underwater hull for each draft. Obviously this is not possible.
Thanks again.

 

Not so obviously, but it is possible. The center of gravity doesn't go anywhere when draft changes, but the center of buoyancy goes. See attached picture. Underwater properties, like wetted surface, LCG, area and volume coefficients all change. But the CG of the weight of the ship that has been modeled does not. So you need to find an equilibrium, by trial and error or by some software.

 

We are talking about very different things. What I wonder is how a calculation of hydrostatic values could give the cog of the submerged hull (not the complete hull). The cog full hull should be calculated differently, taking into account, for example, hull material and its thickness in each area. Once known that value, you do not need many studies to deduce that does not change with depth.

 

TANSL,
Yes, I think we are talking about different things here. Sorry, English is not my native language, Finnish is. So some misunderstanding on my behalf should be forgiven. I think it is the issue, that in Freeship, the calculations are under the menu "Design hydrostatics", even though there are also calculations of the areas and weights of ships individual parts. They clearly have nothing to do with hydrostatics (unless submerged). I think it's the title that misleads us. Should be something like "weight study" instead of "design hydrostatics".