Ship Hydrostatics/fr: Difference between revisions

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(Created page with "* Surface mouillée (WSA). * Moment d'inclinaison du bateau de 1 cm (MCT). * Position longitudinale du centre de bouyance (XCB).")
(Created page with "En tant que quantité de surface en contact avec l'eau, la WSA est fortement liée à la dynamique du bateau, y compris la résistance du bateau et la tenue à la mer. De plus...")
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* Position longitudinale du centre de bouyance (XCB).
* Position longitudinale du centre de bouyance (XCB).


As the amount of surface in contact with the water, WSA is heavily related with the ship dynamics, including both ship resistance and seakeeping. Moreover, WSA is part of the renormalization factor for many of the non-dimensional ship coefficients, like the drag coefficient:
En tant que quantité de surface en contact avec l'eau, la WSA est fortement liée à la dynamique du bateau, y compris la résistance du bateau et la tenue à la mer. De plus, la WSA fait partie du facteur de renormalisation de nombreux coefficients non dimensionnels du bateau, comme le coefficient de traînée :


<math display=block>c_\mathrm d = \dfrac{F_\mathrm d}{\dfrac{1}{2} \rho u^2 S},</math>
<math display=block>c_\mathrm d = \dfrac{F_\mathrm d}{\dfrac{1}{2} \rho u^2 S},</math>

Revision as of 19:22, 18 April 2022

Ship Hydrostatiques

Emplacement du menu
Ship design → Hydrostatics
Ateliers
Ship
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Description

Trace l'hydrostatique du bateau.

Exemple de courbes hydrostatiques

Le calcul hydrostatique est une étape critique de la conception d'un bateau, il permet de comprendre les principaux paramètres de stabilité sous-jacents de la coque.

Il s'agit en effet de données obligatoires pour que le bateau soit certifié par les sociétés de classification. Combinées aux informations sur les conditions de charge, elles fournissent les informations les plus fondamentales sur la stabilité du bateau.

L'atelier Ship trace l'hydrostatique en 3 groupes principaux. Dans tous ces groupes, la courbe Δ(T) (rapport entre le déplacement et le tirant d'eau) est représentée. Bien que de nombreuses autres hydrostatiques puissent être envisagées, elles peuvent être dérivées de celles déjà fournies, qui sont documentées ci-dessous.

Hydrostatique basée sur le volume

Il y a 3 hydrostatiques (malgré Δ(T)) inclus dans cette catégorie :

  • Surface mouillée (WSA).
  • Moment d'inclinaison du bateau de 1 cm (MCT).
  • Position longitudinale du centre de bouyance (XCB).

En tant que quantité de surface en contact avec l'eau, la WSA est fortement liée à la dynamique du bateau, y compris la résistance du bateau et la tenue à la mer. De plus, la WSA fait partie du facteur de renormalisation de nombreux coefficients non dimensionnels du bateau, comme le coefficient de traînée :

with the drag force, the water density, the ship velocity and the WSA.

The MCT plays a mayor role in the load condition planning, since it gives information about the effect of displacing any load along the ship. The actual MCT is computed according the transversal gravity center to metacenter distance, GML, obviously requiring the gravity center position. However, as it is a common practice in naval architecture, the distance of such metacenter to the buoyancy center, BML, is considered similar to such GML (). Please note that is only valid for the longitudinal direction ().

Some times the BML is prefered to the MCT. If it is your case, you just need to apply

with the length in meters and the displacement.

The XCB is obviously indicating the trim angle that is expected to get the ship depending on the weight distribution.

Stability hydrostatics

These hydrostatics are more related with the ship transversal stability. The following hydrostatics are provided by the Ship workbench:

  • Floating Area/Waterplane Area (WP).
  • Distance between the keel and buoyance center (KB).
  • Distance between the bouyance center and the metacenter (BMT)

The floating area is widely connected with the so-called hydrostatic stiffness, or in other words the resistance presented by the ship to any perturbation.

On the other hand, the KB and BMT are critical parameters to determine the transversal stability of the ship for small angles. Indeed when the gravity center is defined (it can be done with the Weight definition tool, Tank creation and Load condition definition tools) the main stability parameter for small angles can be easily computed,

That parameter is indeed required to have a minimum value which depends on the ship type and size, and will be consequently queried by the classification societies.

Coefficients

There are some coefficients that are usually considered at the first stages of a ship design to assess the quality of the ship surface, or in other words, its hydrodynamic behavior.

  • Block coefficient (Cb).
  • Floating Coefficient (Cf).
  • Main frame Coefficient (Cm).

Cb is the ration between the volume within the submerged part of the ship and the volume of its bound box, i.e. the smallest box which might hold the ship inside. Cm and Cf are its 2D counterpart, becoming the Cm ratio between the area of the main ship frame and its bounding box, and Cf the ratio between the waterplane area and its bounding box.

While large Cb values will inexorably result in inefficient ships, with more moderate Cb values it is required to combine the information with Cm and Cf. Larger Cf values indicates a large footprint in the water surface, which usually indicates a large ship resistance due to waves generation. On the contrary, the larger Cm is the larger volume of the ship body is concentrated on the center part, and thus thin shapes can be expected at the bow and stern, which is usually good for hydrodynamic purposes.

Usage

In order to compute the transversal areas curve, select a Ship instance (see Ships creation), and invoke Ship design → Hydrostatics.

The task panel is shown. You must select the trim angle as well as the range of drafts to be considered. You can also select the number of samples to be taken between the minimum and maximum draft. The larger the number of samples the longer will take the computation.

Press the Accept button when you are ready, so the Ship module will start the computation. During the computation FreeCAD will become almost irresponsive. It is however plotting the information in runtime, as well as a progress bar of the process. You can switch to a different plot tab, or stop the computation pressing the Cancel button. Just please be patient since those actions will be executed just after the next draft sample computation is finished.

Tutorials


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