After you have inserted your marker point and set up your grid, you are ready to start manually fitting surfaces to your marker files. For this you may need to manually add surfaces, or you can select a similar vessel from the Maxsurf Sample Designs directory and resize that as a starting point. When you are doing this, it is important to think about the Surface Topology.

Surface topology is the way that surfaces are organised in order to form a 3D Shape. A number of hullshapes and the recommended surface topologies are given in the following section.

To a large extent, the surface topology recommendation depends on what you want to do with the surface model. For example: if you only want to do hydrostatic calculations you generally do not need to worry about the surface topology too much, whereas in case you need to calculate plate expansions and deliver production information, the surface topology is critical.

Deciding on a surface topology is one of the most difficult parts in surface modelling and requires a certain level of experience in surface modelling. Unfortunately the surface topology is also something that has to be decided before starting a surface model because it cannot be changed easily later on in the modelling process.

When determining the surface topology it is important to recognise hullshape features:

Knuckles and chines

Knuckles and chines in the hullshape are very easy to recognise. They can either be: partial knuckles or full-length knuckles or chines. Especially when a structural model has to be created you will want to use bonding along the chine edge. Alternatively you can use compacted control points.

Deck step-ups

It is important to recognise deck step-ups. In these situations, extend the hull surface beyond the step-up and then trim the hull back.

Deck step-up at the forecastle.

Transom

To model a transom you can do two things:

o        In profile view, simply end the hull surface at the transom and then either:

§          Don’t model the transom. Only when the transom is a vertical plane.

§          Bond the transom surface to the aft hull surface edge.

o        Extend the hull to beyond the transom and use a trimming surface to trim the hull.

Maindeck

To a large extent Decks are very similar to Transoms and the same options apply. Do not model ‘tween decks if you need a hydrostatic model or set the ‘tween deck to internal structure.

Bulwarks

Bulwarks are generally not included in the surface model when the model is intended solely for hydrostatic purposes. The reason for this is that the hydrostatic sectioning routine cannot identify whether the bulwark surface encloses a volume or not and will sometimes include the volume between the bulwarks as part of the hull volume. This will affect hydrostatic analysis such as large angle stability and damaged stability analyses.

When the model is used for production information only (not for hydrostatic analysis) and the bulwarks are part of the hull plating you can simply model them by extending the hull upwards from the deck to the top of the bulwark. If the bulwark has step-up in it, you can use a trimming surface to trim it back, same as for deck step-ups. (To simplify modelling for hydrostatics, you could include a weather deck which you would trim the topsides back to for Hydrostatic analysis.)

In case the model is intended for both hydrostatic analysis as well as production purposes, you have the following options:

§   Bond a separate bulwark surface to the top of the hull and set it to an internal structure surface type. That way it will not be includes in the hydrostatic section.

§   Offset the bulwark surface inwards by the plate thickness and add a top-plate. This is to make sure that the hydrostatic sections will be closed.

The bulwark of this tug has been offset inwards
and closed at the top to form one closed section line.

When determining surface topology, it is highly recommended to read: When to Use Trimming on page 109 and When to Use Bonding on page 115.

After you have determined the surface topology, it is time to do a rough manual fit of the surfaces to the marker cloud data. The order in which you should fit a surface to markers is strictly from top to bottom:

§   Corner control points

§   Edge control points

§   Internal control points

The corners of the surface will usually be marked with a marker point. A useful command to set the position of the corner control points is the Snap Control Point to Markers. To automatically snap a control point to a marker:

Ø  Select the marker in any design view

Ø  Select the control point whilst holding down the Shift key.

Ø  Markers | Snap Control Point to Marker

The control point will be made coincident with the marker. In addition, the marker's properties will be updated so that it is now linked to the surface to which the control point belongs and, if the control point lies on a corner or edge, the marker will be linked to the corresponding corner or edge, otherwise the control point will be linked to the surface's interior.

Edge fitting - concepts

Often, for best results, it is recommended to do the edge fitting manually. This gives you the opportunity to get a feel for the required surface stiffness and number of control point columns/row along that edge and preparing the surface appropriately before fitting the surface automatically.

The Fit Edge to Markers command will automatically fit the selected edge to the selected markers. This command requires an appropriate number of control points and surface stiffness along the edge direction to have been defined.

Edge fitting - procedures

Ø  Select the markers that mark the surface edge shape

Ø  Select the surface edge or one control point that lies on the surface edge (not a corner point) while holding down the shift key

Ø  Select Markers | Fit Edge to Markers

This will bring up the following dialog:

Usually, you will want to leave the corner control points where they are,
because you should have manually fitted those in the previous step.

The next dialog allows you to specify the marker order:

The Sort options allow you to override the order in which the markers have been selected.

For a successful fit, the order in which you select the markers is important and it is therefore recommended to select the markers in the correct order manually. If you are fitting a longitudinal edge (control point row), then you must select the markers from left to right. If you are fitting a transverse edge (control point column) then you must select the markers starting at the centreline outwards. In the fitting process the control point in the first row or column of the edge will go to the first selected marker and the control point in the last row or column of the edge will go to the last selected marker.

When a surface edge has been fitted to markers, the markers are associated with that particular surface edge. Once markers have been associated with a particular surface edge, you can fit the surface edge to its markers by simply selecting a control point in the edge (other than a corner control point) and selecting Markers | Fit edge to markers. This is very useful as it allows you to easily change an edge’s properties (number of control points and stiffness) and refit the edge to obtain the best possible fit to the markers without having to always reselect the markers. It also makes it very easy to add or remove markers to those used for the edge fitting.

Once you have fitted the surface's edges, the interior control points, displayed by turning on the surface net, may be very irregular. This command uses a three dimensional smoothing procedure to smooth the surface's interior control points to follow the shape defined by the surface's edges. This will give you a much better starting point for faring.

Smoothing is done on the current surface; select a control point in the surface to make it the current surface. The net must be turned on for this command to become active.

Before smoothing, having fitted the edges

After smoothing the interior

It is a good idea to “help the Genetic Algorithm” and provide it with a good starting point prior to set it off running. This means that it will pay off if you spend some time manually adjusting the shape of the internal sections to fit to the markers. To do this effectively, you should have used the Generating a Grid from Markers command and automatically linked the Markers to the sections, or – if you prefer to do this manually, you should read the section on Linking Markers to a Section.

The Fit Surface to Markers command in the Markers menu uses a Genetic Algorithm search to minimise the measured error between the Markers and the Surface by modifying the shape of an existing surface by changing the position of position of its control points.

Genetic Algorithm – Concepts

The Genetic Algorithm (GA) is an advanced optimisation method based on a simulation of the process of evolution. Although slow, GAs are excellent for solving problems with large numbers of dimensions and constraints. The surface fitting problem falls into this category, as it is not sufficient to simply create a surface that is a close fit to the data points provided, it is also desirable that the surface be fair and that the control point net be smooth and regular.

Genetic Algorithms start by creating a population of individuals with a small amount of random variation. In the case of surface fitting, a population of usually 25 to 50 surfaces is created based on an initial example surface. Each individual in the population is made unique by randomly moving one or more control points a small distance.

Once the initial population has been created the system simulates evolution by repeating the following steps until a solution is found that meets the fitting criteria sufficiently well.

Measure the fit of each individual surface to the data points and give it a score based on its quality of fit, fairness and net regularity. Rank all the individuals from best to worst.

Repeatedly choose two individuals as parents, with individuals being chosen in proportion to how highly they were ranked in step 1. This means that the most successful individuals produce the most offspring in the next generation.

Create members for the next generation by choosing some surface control points from one parent and the remainder from the other parent. Occasionally include a small amount of random control point movement to provide some mutation from generation to generation.

The result is a population that progressively gets closer to an ideal solution of the problem, with poor solutions being eliminated. The highest ranked individual in the final generation is the best solution.

Genetic Algoritm – procedures

Before you can fit a surface to markers it is necessary to link the markers to a surface, see Linking Markers to a Surface.

To start fitting a surface using the Genetic Algorithm:

Ø  Select Fit Surface to Markers command from the Markers menu

You will see the following dialog appear:

Ø  Specify Surface

At the top of the dialog you can specify which surface you want fitted to the available markers. This requires that (a selection of) the markers have to be linked to the surface you wish to fit. The markers do not need to be ordered in any specific way. If your design only has one surface than no options will be shown in the Surface pull down menu.

Ø  Specify Time limit

You can set a time limit for the search. It is recommended to first search for approx 5 – 10 minutes and see whether the initial surface definition is capable of being fit to the markers. If, for example, the markers require a very flexible surface with a lot of control points columns and you have used a very stiff surface with only a few control point columns, you will never get an accurate fit even if you let Maxsurf search for 24 hrs.

The four corner control points of the selected surface are always assumed to be in their correct locations and Maxsurf will not move them during the fitting procedure. You can use the Snap Control Point to Marker function in the Markers menu to help you fit the corners.

Ø  Lock edges

If you do not wish Maxsurf to attempt to fit the surface edges, select Lock Edges. You would do this if you were happy that the surface is already adequately fitted to the edges of the data points or when you are using the Fit Surface to Markers command to fit a developable surface to the marker points generated from the ruling lines. You can use the Fit Edge to Markers function in the Markers menu to help you fit the edges.

The two remaining options constrain the surface so that bottom concave and tumblehome at the hull side are not permitted. If you know that your design contains these features then one or both of these boxes should be unchecked to minimise the search option and get better and faster results.

Once you have specified these options it is simply necessary to click on the Fit button. Once Maxsurf has fitted the surface it will add values to the RMS Error Achieved (see Measure Surface Error) and Generations Tested fields.

Once you have clicked the Finish button you will be returned to the Maxsurf viewing windows where you will see the new fitted surface.

This command is only available if you own a Prefit Licence.See Activate/Deactivate Prefit on page 204.

This command measures the distance between the marker and the closest point on the surface with which it has been associated. After measurement, the distance for each individual marker is entered in the Markers table, and summary data is displayed in a dialog.

Max error: is the maximum error between the marker that is further away from the surface. This marker is displayed in brackets behind the error value and can be easily located by sorting the marker table in the Markers Window by error (see also image below).

RMS error: is the Root Mean Square. This is the square root of the sum of the squares of all of the errors between every data point and the fitted surface. It is a good measure of overall fit, and it should be regarded as an average error rather than a worst error value.

Mean error: the average error of all errors.

The error of each marker associated with a surface will be calculated and displayed in the Error column of the Markers Table.

The Markers window displays the surface error for each marker.

Measure Error can be called up with any of the drawing windows or the markers window topmost. Note that the error is calculated using the current surface precision setting.