Hull Geometry

Hull geometry is read directly from the Maxsurf design file. The number and location of sections for the hydrostatic calculations is under user control up to a maximum of 200 sections allowing accurate calculation of hydrostatics for vessels of all geometries.

Monohull and multihull vessels and even asymmetrical hulls can be analysed with equal ease in Hydromax.

---

Model definition

In the following sections the method for defining a vessel model in Hydromax is described.

Geometry

Hull geometry is read directly from the Maxsurf design file. The number and location of sections for the hydrostatic calculations is under user control up to a maximum of 200 sections, allowing accurate calculation of hydrostatics for vessels of all geometries.

Monohull and multihull vessels and even asymmetrical hulls can be analysed with equal ease in Hydromax.

Non-watertight geometry can be included in the Maxsurf design (for example the superstructure in the model below), in which case, it will be ignored when the sections defining watertight envelope of the hull are calculated.

^{Perspective view of trimaran with hydrostatic sections superimposed}

 
^{Bodyplan view of hydrostatic sections for the same trimaran}

Critical points

The deck edge and margin line are calculated automatically from the watertight envelope of the hull but they may also be modified if necessary. In addition any number of downflooding points or other key points may be defined. All data for these critical points is entered numerically but is also displayed graphically.

 
^{Heeled vessel showing margin line, deck edge and two downflooding points;
immersed key points are drawn in a contrasting colour.}

Compartments and tanks

Within Hydromax there are several types of spaces that can be defined in the vessel:

Compartments:

Spaces in the vessel that may be flooded during analysis of the damaged vessel.

Tanks:

Tanks in the vessel that can contain a fluid and are filled to a specified level in a loadcase. Like compartments, they may be flooded during analysis of the damaged vessel.

Non-buoyant-volumes:

These are spaces in the vessel that are permanently flooded (even for the intact vessel). For example moon-pools, bow thruster ducts and waterjet ducts. As with tanks and compartments, non-buoyant volumes may have a permeability specified; for example, to account for the impeller and shaft in a waterjet duct.

Tanks are defined numerically by their bulkhead and deck locations. Tanks are then automatically trimmed to the hull surface tanking into account shell plating thickness if desired. Complex tanks of virtually any shape can be defined by NURB surfaces defined in Maxsurf. These may then be selected as additional tank boundaries. In the example below, this technique has been used to define the non-buoyant volumes for the two waterjet and the bow-thruster ducts as well as the two spherical and cylindrical tanks.

^{Hydromax trimaran model with a number of tanks and non-buoyant volumes shown;
the compartments have been hidden for clarity. Non-buoyant volumes have been used to define the two waterjet ducts and the bow-thruster duct.}

^{Hydromax trimaran model with compartments shown.}

Load cases

Loadcases are used to define the mass and its distribution for some of the hydrostatic analyses (large angle stability, equilibrium and longitudinal strength). The format is much the same as a spreadsheet, however the defined tanks are automatically listed; when the weight or volume of a tank is edited, Hydromax automatically updates the tank and vessel centre of gravity and mass distribution. For each tank, the free surface method to be used may be specified (Hydromax also has the ability to calculate the actual position of fluids in tanks as the vessel heels and trims during the analysis. See below for further details.)

Fixed weights are defined in the loadcase. In addition the format of the loadcase can be customised with the addition of subtotals and blank lines as desired. The total weight and centre of gravity, adjusted for free surface moments are automatically calculated as tank loadings are changed or fixed weights are added or removed.

^{Typical loadcase - fixed weights have been manually added at the top, with tank loads automatically added at the bottom. Sub-totals and blank lines have been added manually to provide additional information and improve readability.}

Damage

Damage definition is simply achieved by adding damage cases to the model and selecting the tanks and compartments that should be damaged in each case. Any number of damage cases may be defined. The list of compartments and tanks is automatically generated from the compartment definition.

^{Damage case definition.}

^{Vessel compartmentation (damaged compartments shown in red).}

^{Vessel equilibrium response to specified damage.}

---

Hydrostatic analysis

The following sections describe the analyses that can be performed in Hydromax.

Upright hydrostatics

The upright hydrostatics analysis mode calculates the vessel hydrostatic parameters through a specified draught range at specified, fixed trim. Upright hydrostatics can be computed for the intact and damaged vessel.

_{Upright hydrostatics.}

_{Form coefficients.}

Specified condition

The specified condition analysis will calculate the vessel's hydrostatic properties for a user-specified condition. The user may specify any combination of draughts, heel, trim, displacement and centre of gravity. This analysis is particularly useful for the analysis of inclining experiment data, for example. The hydrostatics for the specified condition can be computed for the intact and damaged vessel.

^{Specified condition analysis parameters: specify heel, trim and immersion with a combination of draughts, displacement and centre of gravity.}

KN cross-curves

Cross-curves of stability can be produced for a range of displacements either for fixed trim or free-to-trim. In the case of the free-to-trim analysis, the LCG is specified directly or derived from a specified initial trim. In addition, an estimate of the VCG can be given for free-to-trim calculations so that the effect of VCG is better taken into account providing more accurate KN values for VCGs in the region of the estimated VCG. The cross-curve of stability can be computed for the intact and damaged vessel.

^{Cross-curves of stability (KN)}

Limiting KG

This analysis calculates the maximum possible vertical centre of gravity (limiting KG) that satisfy specified stability criteria for a range of displacements. Calculations may be done with fixed initial trim or free-to-trim with the LCG specified directly or derived from a specified initial trim.

In batch analysis mode, the maximum KG can be computed for each criterion individually, though this can be considerably slower depending on the number of criteria to be analysed. The limiting KG can be computed for the intact and damaged vessel.

^{Maximum KG for which stability criteria are passed.}

Floodable length

The floodable length analysis calculates the maximum length of compartment that can be flooded whilst still passing certain stability criteria. These criteria must include either deck-edge or margin-line immersion, but may additionally include minimum values of GM (transverse and longitudinal) and maximum angle of trim.

Floodable length can be calculated for a range of displacements and a range of compartment permeabilities.

^{Vessel profile view during analysis, showing flooded compartment in red.}

_{Compartment length plotted against compartment centre for each displacement.}

Large angle stability (righting arm)

The large angle stability analysis calculated the vessel's righting arm for a selected loadcase and damage case. Calculations may be performed fixed- or free-to-trim. The effect of fluids in tanks may be dealt with by the traditional approach of adjusting the vertical centre of gravity, with various options (including IMO) for computing the free-surface moment. Alternatively it is possible to perform the calculation using the actual position of the fluid in the tanks taking into account the effects of trim and heel. The righting arm can be computed for the intact and damaged vessel. If desired, the righting arm may be computed for the full 360deg. range of heel angles.

_{Righting arm and heeling arm curves.}

Once the righting arm curve has been computed, any number of stability criteria may be evaluated. Hydromax comes pre-programmed with a wide range of stability criteria from many authorities, but allows users to define their own custom stability criteria from a range of more than 50 basic criteria calculations. Full details of the evaluation of the stability criteria, including intermediate values may be displayed; alternatively, the compact display option may be chosen, in which case only the pass/fail status and the achieved value are displayed.

_{Stability criteria status (compact format).}

_{Stability criteria with calculation details shown.}

Equilibrium

The equilibrium analysis simply balances the vessel for the selected load- and damage-case. Additionally grounding and waveform may be specified. The equilibrium analysis is useful for verifying "what if?" scenarios or the computed VCG from an inclining experiment, for example.

_{Vessel equilibrium response to specified damage.}

_{Body-plan view of damaged vessel in equilibrium.}

_{Equilibrium water plane with damage shown in red.}

Longitudinal strength

The longitudinal strength analysis uses simple beam theory and treats the hull as a simple girder to calculate the bending moment and shear forces experienced on the hull for the selected load- and damage-case. Additionally grounding and waveform may be specified.

_{Profile view showing vessel orientation and grounding with longitudinal strength graph below.}

Tank calibration

Tank calibration provides tabulated and graphical data of tank capacity etc. Sounding intervals and the sounding pipe geometry may be specified by the user. The tank calibrations may performed for the vessel with even keel or with specified trim.

^{Tank calibration graph.}

---

Environment definition

Environmental parameters such as grounding, wave form and fluid density can be specified.

_{Specification of grounding parameters.}

^{Specification of wave profile}

_{Equilibrium in static wave.}