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Computing Resistance |
  
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Computing Resistance |
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Toolbar |
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Menu |
Orca3D > Speed/Power > Holtrop Analysis |
Command |
OrcaHoltropAnalysis |
Orca3D can compute the bare hull resistance of a displacement hull for a user-defined range of speeds. The total predicted resistance, total effective power, and total propulsive power can then be calculated using a user-defined design margin and propulsive efficiency.
The process of computing the drag on a planing hull surface can be summarized as follows:
| 1. | Select the planing surface or polysurface for the analysis. Note: the selected surface or polysurface: |
| - | MUST ONLY select external hull surfaces that are “wet” at the static waterline for which the analysis is being performed. Do not include appendages such as rudders. |
| - | MUST ONLY represent half of the hull |
| - | MUST have a transverse coordinate of 0 on the centerline (for example, a multi hull ship must be positioned so that the centerline is at Y=0) |
| - | MUST be oriented with forward in the negative X-direction and up in the positive Z-direction |
| - | Integrated skegs may be included as part of the hull surface |
As a general statement, there is not a lot of difference between adding a skeg's volume and wetted surface as part of the "hull", versus independently treating it as an appendage. The viscous component of the skeg's drag is larger when evaluated as an independent appendage due to a shorter viscous length and Reynolds number, and corresponding higher frictional drag coefficient (plus a small form drag). On the other hand, there will be an increase in the predicted bare-hull wave-making drag when adding the skeg's displacement into the "hull". (The increase in wave-making drag is often proportionally many times greater than the increase in displacement.) So, what you gain in one, you lose in the other. The magnitude of the viscous and wave-making differences will depend on hull geometry and speed, but fortunately, the overall difference between the two approaches is rarely more than a few percent.
| - | MUST have a surface normal direction pointing outward into the water. The surface normal direction can be verified by selecting the surface, then selecting Direction from the Rhino Analyze menu. |
| 2. | Type the command: OrcaPlaningAnalysis, select Planing Analysis from the Orca3D menu, or select the Planing Hull Analysis icon from the Orca3D toolbar to initialize the planing analysis. |
| 3. | Input the following values into the Orca Planing Analysis dialog box: |
| - | Mass and geometry properties. |
| - | Range of vessel speeds. |
| - | Margins and efficiencies. |
| 4. | Select the OK button and the results will be displayed in report form in a separate window. |
IMPORTANT: Select ONLY the external hull surface(s) to be included in the analysis. The program will use all surfaces selected in its calculations of various parameters. If any other surfaces are selected, you will receive incorrect results.
Input Dialog
Once the Holtrop analysis is started from the command line, Orca3D Menu, or Orca3D toolbar and the surface is selected, the Holtrop Speed/Power Analysis dialog box will open. If you have a Design Condition already defined, it will be used as the Mass/Geometry source, and most of the input fields will be completed automatically. You will only need to input the desired speed range and increment, and specify your Resistance Design Margin and Propulsive Efficiency.
If you have not already defined the Design Condition, you will be prompted to do so. If you don't want to, the dialog will open, but now the Mass/Geometry source will be "Manual Override." Also, even if you do have a Design Condition, you can always switch to Manual Override to change any of the Mass and Geometry parameters:
Once you have entered all of the parameters, click OK to run the analysis. The output data will then be presented in the Report Viewer.
Mass and Geometry
Displacement: the weight of the vessel at the desired condition, in the units specified.
LCG (from origin): the longitudinal center of gravity of the vessel measured from the world origin in the units specified.
LWL: The length of the waterline at the desired condition
BWL: Waterline beam at the desired condition.
Tx: Draft at the station of maximum area at the desired condition.
Half Entrance Angle: The angle between the design waterline and the centerline in planview at the desired condition
Stern Coefficient: A coefficient used to describe the cross-sectional shape of the afterbody. It is used in the prediction of form factor and wake fraction.
Extreme V-shaped buttock-flow (such as barges) = -25 to -20
V-shaped buttock-flow = -10
Normal diagonal-flow = 0
U-shaped waterline-flow = 10
AWP: Area of the waterplane at the desired condition
Wetted Surface: Wetted surface area of the hull at the desired condition. Transom area should not be included in this value.
Ax: Area of the station of maximum wetted cross-sectional area at the desired condition
Transom Area: The area of the submerged portion of the transom
ABulb: Cross-sectional area of the bulb at the forward end of the waterline
ZBub: The vertical coordinate of the centroid of the bulb area at the forward end of the waterline
Speeds
Enter the minimum speed, maximum speed, increment speed, and design speed of the vessel in the units specified for this analysis.
Margins and Efficiencies (see About Effective Power)
Resistance Design Margin: the margin added to the bare hull resistance to calculate total resistance and effective power. This margin can be used to account for appendages, wind, waves, shallow water, etc.
Propulsive Efficiency: the ratio of effective power to propulsive power. This efficiency can be used to account for losses in the propeller, shafting, transmission, etc and will thus determine the true definition of total propulsive power.
