FreeCAD-Ship s60 tutorial (II)/ro

Înainte de a începe acest tutorial, vă rugăm să vă asigurați că ați parcurs deja the first part.

You can learn more about FreeCAD-Ship here

Introducere
În acest tutorial vom lucra cu încărcări și compartimente pentru a calcula curba GZ (brațul cuplului de redresare), cel mai important parametru de stabilitate hidrostatică. GZ este momentul static generat atunci când nava ia un unghi de rotire, desigur, deoarece brațul GZ este pozitiv, nava are un moment pozitiv și va încerca să recupereze poziția verticală, dar când GZ se aprinde pe numerele negative nava nu mai are stabilitate, atingând o situație critică.

IMO (International Maritime Organization) set following criteria:
 * GM >= 0.15 m. GM (metacentric height) is the initial tangent of the GZ curve.
 * Maximum GZ value must be placed over 30 degrees of roll angle.
 * With 30 degrees roll angle, GZ value must be 0.2 m at least.
 * Area involved by GZ curve up to 40 degrees of roll angle must be at least 0.090 m · rad.
 * Area involved by GZ curve up to 30 degrees of roll angle must be at least 0.055 m · rad.
 * Area involved by GZ curve from 30 to 40 degrees of roll angle must be at least 0.030 m · rad.

In this tutorial we will set weights and tanks for our series 60 ship, in a unreal situation.

Încărcarea navei
Pentru a putea calcula curba GZ trebuie să cunoaștem greutățile navei și poziția lor la fiecare unghi de rotire, astfel încât greutățile vor fi împărțite în două categorii:
 * Greutăți fixe, care sunt pe deplin legate de mișcările navei.
 * Rezervoare, în cazul în care forma fluide se schimbă cu unghiul, necesitând un centru de greutate calcul în fiecare poziție.

FreeCAD-Ship provides two different tools to generate each instance.

Weights definition tool icon.

Weights definition tool can be used to set first category of weights. When you launch the tool for first time (with ship instance selected), FreeCAD-Ship initialize ship weights with Lightweight ship (equal to ship displacement) that is placed on ship geometry centre of gravity X coordinate, and at design draft height. Usually you have at least, 2 relevant weights:
 * Structure.
 * Main engine (or several of them).

So we will change it. Doing double click over each cell we can edit the value, seting this weights:
 * Structure, 15000 kg, (-0.1, 0, 1.25) m
 * Starboard engine, 5000 kg, (-6.5, -0.65, 0.5) m
 * Port side engine, 5000 kg, (-6.5, 0.65, 0.5) m
 * Emergency engine, 2500 kg, (0.2, 0, 2.5) m

Weights definition 3D preview.

Weights position are shown at 3D screen view. This annotations will be removed when you ends with the tool, so don't take care about this. When you press Accept weights will stored at your ship instance.

Cisterne
Cisternele/rezervoarele trebuie să fie create pe partea de sus a geometriei solide, așa cum este cazul navei, astfel încât primul pas este să creați două rezervoare de arc (câte una pe o parte a navei) geometrii solide pe care le vom lua în considerare (de obicei navele au o mulțime de rezervoare pentru combustibil, apă dulce, sare apă, încărcătură etc.).

Generarea geometriei
Pentru a genera rezervoare, încărcăm Part module și creăm o cutie solidă.

We need to edit the box, so we select it at Atributes and tags tree, and change from view to data tab. Uncollapse Placement, and into them Position, and set x to 1.5, and z to -1. We want to change box lenght too changing it for 5.0 (note that units can be in mm, don't take care about this).

Tank geometry will be common part of created box and ship geometry, so we can hide Ship instance, and show s60_IowaUniversity geometry. Selecting box and s60_IowaUniversity we can use Common operation generating our starboard tank geometry.

Generated tank geometry.

We can perform port side tank selecting our starboard geometry and executing mirror tool, selecting XZ as mirror plane.

In order to convert geometry into a ussual solid shape our tanks, and recover our s60_IowaUniversity geometry, we can load Draft module, and with starboard tank geometry selected execute Upgrade, and repeat with port side tank geometry. We can rename geometries as:
 * StarboardTankGeom
 * PortTankGeom

We can delete created Box, that we don't need anymore.

Generarea exemplelor de rezervoare
Dacă reîncărcați FreeCAD-Ship module încă o dată, putem găsi unealtă generator de variante de rezervor.

Tank instance generation tool icon.

Now we can select StarboardTankGeom and execute tank instnace generation tool, where some data must be provided. We will set 40% of filling level, and 925 kg/m$$\mathrm{m}^{3}$$ (fuel approach). When Accept is clicked a new tank instance called Tank is generated. We can rename it as StarboardTank, and hide StarboardTankGeom.

We can repeat the same process in order to generate PortTank.

View of generated weights.

Figure shown our ship result that we will compute.

Calculul curbei GZ
FreeCAD-Ship furnizează un instrument pentru a calcula cu ușurință o curbă "GZ".

GZ curve computation tool icon.

With Ship instance selected, we can run the tool. The first thing that we can see at opened dialog is a list with all tank instances found at active document. We want to use both of them, so we click over the tanks that are remarked with a diferent background.

Pentru a cunoaște deplasarea și schița rezultată a navei, putem apăsa Actualizați deplasarea și schița, luând un timp pentru calcul. Primim următoarele date:
 * Deplasament = 37505,5 kg
 * Pescaj = 0.818664 m

So we are in a unloaded situation, where draft are sightly lower than design draft. Ussually lower drafts imply lower ship stability, the draft depends on loading condition, so if we really expect than ship can be operated in this loading condition we can consider implement ballast tanks.

We can also automatically compute ship trim, operation that can take around one minute, retrieving that our ship have 0.95 degrees of trim angle (positive by stern). In this example we will work without trim angle (0 degrees).

Tool request roll angles considered too. In this case we want to know all ship behaviour, so we can set:
 * 0 degrees starting roll angle.
 * 180 degrees ending roll angle.
 * 46 points. One for each 2 degrees. GZ computation can take some time, so take care about the number of points requested.

When we press Accept tool starts the computation. If you runs FreeCAD from terminal you can see work progress. In a couple of seconds we will receive GZ curve.

This tool use pyxplot and ghostscript too. You can see where gz.dat output file has been placed at the report view (View/Views/Report view), and load it with datasheet software (for example libreOffice). Nearby data file several auxiliary files has been created too:


 * gz.dat: Computed GZ curve data.
 * gz.pyxplot: pyxplot layout in order to plot the curve.
 * gz.eps: EPS image version.
 * gz.png: PNG image version.

This files will be overwritten if you executes the tool another time.

Resultate
Resultant GZ curve.

GZ maximum value is placed over 30 degrees (45 degrees), getting 0.25 m at 30 degrees (0.2 m is the minimum). Up to 30 degrees the area below GZ curve is 0.065 m·rad, up to 40 degrees we have 0.092 m·rad, being the area between 30 and 40 degrees of 0.027 m·rad. So our ship don't meets the IMO requeriments. The solution is place ballast tanks.

Pe de altă parte, nava, în această stare proastă, are valori pozitive ale "GZ" până la un unghi de rotație de 95 de grade, dar nu a fost suficient pentru cerințele de stabilitate ale OMI, arătând cerințele grele impuse acestui element.

Desigur, acest exemplu nu este real (mai întâi pentru toate cisternele de combustibil nu pot fi plasate în structura dublă a fundului, sau folosind structura cocăi), dar este un bun exemplu pentru a învăța să folosești FreeCAD-Ship.