Ship ResistanceBlountFox

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

Description

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.

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  Resistance graph obtained by Blount and Fox method.

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  Equilibrium trim graph obtained by Blount and Fox method.

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  Power graph obtained by Blount and Fox method.

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  Resistance coefficients graph obtained by Blount and Fox method.

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  Spreadsheet generated by Blount and Fox method.

Resistance graph

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

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

Power graph

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

Coefficients graph

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

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

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:

${\displaystyle F_{\Delta }\geq 1.0}$

${\displaystyle {\frac {LCG}{L_{P}}}\leq 0.46}$

Where:

• ${\displaystyle F_{\Delta }}$ is the volumetric Froude number, defined by the following expression:
  ${\displaystyle F_{\Delta }={\frac {V}{\sqrt {g\nabla ^{\frac {1}{3}}}}}}$
• ${\displaystyle LCG}$ is the longitudinal center of gravity of the vessel.
• ${\displaystyle L_{P}}$ is the projected keel length of the vessel.

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

Usage

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

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

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

Tutorials

• FreeCAD-Ship s60 tutorial
• FreeCAD-Ship s60 tutorial (II)

References

[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.