The analysis mesh for the slender body analysis can be displayed by ticking “SB Mesh” in the Display menu.

Slender body mesh (orange grid) for MORC sample model

If the vessel type (monohull, multihull) has been correctly setup in Maxsurf, the geometry should be correctly interpreted by Hullspeed. The mesh is a series of sections and waterlines forming a rectangular grid that is symmetrical about the hull’s centreline. For multihulls this means that there is one such mesh for each individual hull. This mesh is symmetrical about the local hull centreline; for example, a catamaran’s slender body mesh consists of one mesh that is symmetrical about the demihull centreline and mirrored about the catamaran’s centreline. This means that each individual hull must be symmetrical about its own centreline, but the overall model can be asymmetrical (e.g. a proa).

If the hull has an immersed transom, an appendage is added to the slender body mesh which blends all the waterlines back into the local centreline; this can be seen in the image above. See Modelling the Transom on page 25 for more information.

 View a video showing how to set up a model of a Catamaran in Maxsurf, so that it is correctly interpreted for the Hullspeed Slender Body geometry mesh definition.

In some cases it may be necessary to edit the slender body mesh. This is done by selecting “SB Analysis Geometry” from the analysis menu. A dialog is displayed with a table containing one row for each mesh group; you need one group for each individual hull in the model:

Dialog for definition of slender body groups

This dialog is the same as that used for definition of the Shipflow groups in Hydrolink. Of main interest for Hullspeed are the following columns:

Colour:

The colour the mesh is drawn

Surfaces:

The surfaces to be used to calculate the offsets. Double click on the surfaces cell to select which surfaces should be used. It is best to choose only the surfaces that define the sections, this is especially true for multihulls. When you close the surface dialog you will be asked if you want to automatically set the bounding box to the selected surfaces. Clicking “Yes” will set the bounding box to the rectangular extents of a box containing the selected surfaces.

Selection of the surfaces to be grouped for measuring the slender body mesh

Num. of contours:

The number of contours for the mesh; the number of waterlines is chosen automatically to match the number of sections. The greater the number of sections the better the accuracy of the analysis (but the analysis will also take longer).

Trans. origin:

This is used as the local hull centreline. For monohulls this should be zero, for multihulls this should be the local symmetry plane / centreline of the individual hull. For example, for a catamaran this is the transverse position of the demihull centreline.

Aft, Fwd, A port, A starb, A top, A bott, F port, F starb, F top, F bott:

The last 10 columns define a longitudinally prismatic (or tapered) box which defines the boundary of the mesh. The definition of the bounding box is similar to the way tanks are defined in Hydromax: the Aft and Fwd columns define the longitudinal aft and forward extents of the box, the A port, A starb, A top and A bott columns define the port, starboard, top and bottom extents of the box at the aft plane. The corresponding four columns prefixed with F, can be used to define a tapering box with different port, starboard, top and bottom extents on the forward plane.

Open and Save buttons:

The data in the dialog can be saved and retrieved using the Save and Open buttons in the dialog. This can be useful if you have customised the slender body mesh definition and want to be able to retrieve it easily.

Further information on this dialog can be found in the Hydrolink manual in the Shipflow export section

The slender body mesh is created by calculating the hull surface offsets on a regularly spaced grid of sections and waterlines. Mesh points which fall off the hull surface are given an offset of zero and remain on the hulls centreline. For the slender body theory, the mesh must start and finish with waterlines that lie on the centreline, i.e. the bow and stern sections must have all points at zero offset. (It is possible to remove this requirement for the stern by applying a transom correction).

Hullspeed deals with transoms in a special manner; the mesh is brought back to the centreline plane behind the model by applying a “virtual appendage” which is smoothly attached to the transom. This method was found to give good results for monohull and catamaran forms with transom sterns (Couser 1996, Couser et al 1998).

Mesh layout for the stern of a canoe-bodied hull without transom – Mesh follows hull

Mesh layout for a hull with a transom– Artificial closure of mesh behind transom using a smoothly attached “virtual appendage”

The “virtual appendage” can be removed by making the aft extent of the mesh bounding box end just after the end of the transom – see below:

With the aft bounding box of the mesh terminating just aft of the transom, the “virtual appendage” is removed

Note:

The virtual appendage is not included in the wetted surface area calculation. It is only used to artificially close off the numerical model to calculate the wave resistance.

Note that removing the virtual appendage will affect the free-surface wave pattern and wave resistance calculation. The calculated free surface wave pattern with and without the virtual appendage can be used to judge whether this appendage should be added or not. From the work of Couser 1996, Couser et all 1998, it can be seen that adding the virtual appendage gives good results for monohulls and multihulls with transoms sterns.

Hullspeed is able to compute the resistance of multihulls (catamarans, trimarans, pentamarans etc.) using the theoretical slender body method. There is no limit to the number and position of the individual hulls, but each hull must have transverse symmetry about its local centreline plane. To use this capability, it is important that the vessel type (see the Maxsurf manual) is set up correctly. There should be one mesh group (one row in the SB Geometry dialog, see page 23) for each individual hull of the model. Some example mesh definitions for different types of multihulls are shown below:

Catamaran

If the vessel type is correctly defined in Maxsurf, Hullspeed will automatically generate a symmetrical mesh that is centred on the local demihull centreline.

Vessel Type dialog setup in Maxsurf for a catamaran

Catamaran has a single mesh mirrored about the vessel centreline.
The mesh is symmetrical about the local demihull centreline

Trimaran

Vessel Type dialog setup in Maxsurf for a trimaran

Two meshes will be required, one of the main hull and one for the outer hull (referred to as “ama”). If the vessel type is correctly defined in Maxsurf, these two meshes will be automatically defined. However it may be necessary to change the longitudinal extents of the meshes, especially if the hulls have transoms. It may also be necessary to select which surfaces define the main hull and which define the ama. The simplest way to do this is to double click in the Surfaces cell, select only the surfaces required for the specific mesh and then click OK. Then click Yes to set the bounding box extents to the selected surfaces.

Correct Mesh definition for trimaran sample above, Aft extent for Mainhull is –77.17, but only –64.88 for Ama

These files may be found in: Program files\Maxsurf\Sample Designs\Multihulls\Trimaran\Trimaran.msd

The mesh definition file is: TrimaranHSmeshDefn.hyd

Proa

It is possible to model asymmetrical vessels provided that each individual hull is symmetrical about its own centreline. In the example below, the main hull is a symmetrical surface, whilst the outrigger (referred to as ama) is made up of two surfaces that are:

§   asymmetrical about the vessel’s centreline (there is no ama on the other side of the centreline)

§   mirrored about the ama centreline

Two mesh groups are required. The one for the main hull is defined as if the main hull were a monohull. For the ama, the transverse origin is specified as the transverse offset of the local centreline of the ama hull, and only the starboard side of the ama hull is used.

Slender body mesh definition for main hull and ama

Wave pattern calculated for proa model