It is very important to note that all hull offsets input to Michlet, or output by Michlet are for the underwater portion of the hull only. You can, however, specify a value for the sinkage to raise or lower the hull with respect to the undisturbed free surface (see the section on Trim and Sinkage earlier in this manual).
Hull offsets in comma-separated format can also be read from file. For example, if the input line for the first hull contains only the value -1, then Michlet will read offsets from the file useroff1.csv. If the offset input line for the second hull was specified in the same way, Michlet would read offsets from the file useroff2.csv. Similarly for hulls 3, 4 and 5.
Let X(x) = 4x(1-x), with 0<= x <=1.
Then the non-dimensional offsets are given by
0 if X = 0 or if Xf2 <z, otherwise Y(x, z; f0, f1, f2) = 1/2FGf1.
Here F = Xf0, G = 1 - z2X-2f2 and 0 <= x, z, f0, f1, f2 <=1.
At any given length and draft, Michlet adjusts the beam by uniform scaling of the non-dimensional offsets given by the above formula, so as to achieve the desired displacement.
In the above formulation, the first parameter, f0, controls the shape of the waterlines, the second parameter, f1, determines the cross-section shape, and the third parameter, f2, controls the shape of the keel-line. If, for example, f0 = 0.0, then the waterline shape is rectangular, if f0 = 0.5 the waterline is elliptical and if f0 = 1.0, the hull will have parabolic waterlines. And similarly for cross-sections and keel-lines.
The shape parameters for some common shapes are given below. Remember that you can toggle the view from sections to waterplanes by pressing the v key once the new hullform has been displayed.
Parameter values other than 0.0, 0.5, and 1.0 can be used. For example, the hull defined by the three parameters 0.25, 0.25, 0.25 is somewhere between a spheroid and a rectangular block.
This hull series includes and extends the first series. The first three parameters (f0, f1 and f2) are the same as for Series 1, but are relevant to the forebody only; the next two parameters are for the afterbody. A cross-section parameter is not required for the afterbody because the two bodies must have the same cross-section for a clean join. The maximum value of the first five parameters is 3.0; cusped shapes are possible with this series.
The last two parameters specify the relative lengths of the forebody and the afterbody. If the sum of the last two parameters is less than 1.0, then the two bodies will be joined by a length of parallel middle body. Note that if the sum of the last two parameters is greater than one, then there is no parallel middle body, and the length of the afterbody is reduced so that the total is exactly equal to 1.0.
The cut-off ratio determines how much of the afterbody will be removed. If f7=0, then nothing is removed and the hull has a pointed stern with stern offsets all equal to zero. If f7=1, then all of the afterbody is removed and the hull has a transom stern which has the same cross-section as the forebody (or parallel middle section). If f7 is between 0.0 and 1.0 then the hull will have a transom stern smaller in area than the forebody.
Note that the parameters for this hull series cannot be changed on-screen in the current version of Michlet.
The following is a short example of the spline format. Comments in the file begin with the # character in the first column. The file must not contain blank lines.
# Example of Spline format
3
BEGIN
0.0
0.00, 0.0
0.10, 0.0
0.50, 0.0
0.95, 0.0
1.00, 0.0
END
BEGIN
0.15
0.00, 0.0
0.25, 0.16
0.33, 0.19
0.60, 0.28
0.75, 0.35
0.87, 0.36
1.00, 0.37
END
BEGIN
0.45
0.00, 0.0
0.20, 0.21
0.45, 0.37
0.85, 0.48
1.00, 0.50
END
BEGIN
0.70
0.00, 0.0
0.26, 0.13
0.51, 0.23
0.66, 0.32
0.88, 0.36
1.00, 0.38
END
BEGIN
1.0
0.00,0.0
0.15,0.0
0.45,0.0
0.95,0.0
1.00,0.0
END
#The last line must be EOF
EOF
The first number (3 in the example above) signifies that Michlet will use cubic splines (the only valid option in the current version).
The last line of the file must be EOF.
Data for each station is enclosed between BEGIN and END statements. (They do NOT have a # symbol before them.) The first number on its own is the station ordinate. 0.0 is the bow. Always use 0.0 for the first station, and always include at least 5 pairs of ordinates for any station.
Stations must be in ascending order starting at the bow and ending with the stern.
The pairs of numbers (separated by a comma) are the z-ordinate (0.0 = baseline), and the offset at that z-ordinate. The waterlines must be in ascending order from baseline to the top of hull. Values can be in dimensional, or non-dimensional form. Michlet will scale the hull to the length, draft and displacement that you specify in the in.mlt file.
Negative offsets resulting from interpolation are clipped to 0.0. Any offset greater than 99999.9 is set to 99999.999.
To use your own spline offset file, save it to sploff1.csv and use -2 for the Offset Type in the in.mlt file. That is, use
Offsets
-2
Hull offsets for the NPL hulls in the examples use the cubic spline format. See the npl_sploff1.csv file in the example directory.
For multihulls, use sploff1.csv for the first hull, sploff2.csv for the second hull, etc.
Note that an injudicious spacing of nodes can lead to unusual bulges in the hull surface when using the spline method. It is often best to use an equal spacing of nodes. For flat horizontal surfaces, or sharply-curved portions use a larger number of nodes than you would for vertical surfaces.