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Ship ResistanceBlountFox

From FreeCAD Documentation

Ship ResistanceBlountFox

Menu location
Resistance → Resistance Blount and Fox prediction
Workbenches
Ship
Default shortcut
None
Introduced in version
-
See also
None

Description[edit | edit source]

Calculates ship resistance by the Blount and Fox method. This method was formulated by Donald L. Blount and David L. Fox in 1976 [1], with application to fast vessels in a pre-planing regime. It is based on a Savitsky [2] correction at low speeds.

Plots the resistance curve, the equilibrium trim, the power prediction and the resistance coefficients. It also exports the results to a spreadsheet.

Resistance graph[edit | edit source]

The resistance is represented in kN, as a function of speed in knots. It includes the total resistance, the total resistance and the pressure resistance. The dialogue includes a checkbox to include both, Blount and Fox and Savitsky resistance curves in the graph, this allows the user to compare both methods resistance easier.

Trim graph[edit | edit source]

The equilibrium trim is represented in degrees, as a function of speed in knots.

Power graph[edit | edit source]

The power is represented in kW, as a function of speed in knots. It includes the effective power and the break power.

Coefficients graph[edit | edit source]

The resistance coefficients are adimensional, they are represented as a function of speed in knots. It includes the effective power and the break power.

Exported Spreadsheet Data[edit | edit source]

The spreadsheet contains the values of different variables for each evaluated speed, separated by columns.

Variable Unit
Speed kn
Trim °
Total resistance kN
Friction resistance kN
Pressure resistance kN
CF ×10³ -
CP ×10³ -
CT ×10³ -
EKW (Effective Power) kW
BKW (Brake Power) kW

Application limits[edit | edit source]

It must be taken into account that the obtained results represented in the graphs must be inside the application limits of the method.

The application limits of the Blount and Fox correction method are:

FΔ1.0

LCGLP0.46

Where:

  • FΔ is the volumetric Froude number, defined by the following expression:
    FΔ=Vg13
  • LCG is the longitudinal center of gravity of the vessel.
  • LP is the projected keel length of the vessel.

These limits extend the application of the Savitsky method to low-speed conditions.

Usage[edit | edit source]

This tool does not have a strict dependency on a Ship instance(see Ship CreateShip), which means that it is possible to use it introducing the necessary ship data values in the dialogue.

Using ship geometry[edit | edit source]

In order to execute the method, select a Ship instance and invoke Resistance → Resistance Blount and Fox prediction.

The ship data will be obtain from the ship and the user only needs to enter the boundary conditions. Data obtained from the vessel can be edited.

Using ship data[edit | edit source]

In order to introduced the ship data manually, invoke Resistance → Resistance Blount and Fox prediction and fill the dialogue fields.

Tutorials[edit | edit source]

References[edit | edit source]

[1] D. L.Blount and D. L.Fox, “Small-craft power prediction,” Marine Technology, Vol. 13, No. 1, Enero 1976.

[2] D. Savitsky, “Hydrodynamic design of planing hulls,” Marine Technology, Octubre 1964.