How does NavCad predict resistance for catamarans?

Background

Catamarans share hydrodynamic principles with monohulls, but the proximity between demi-hulls introduces interference effects that significantly affect resistance. NavCad includes catamaran prediction for both ITTC-78 (displacement and semi-displacement) and planing speed ranges, developed from HydroComp's in-house R&D program.

Setup Catamaran Configuration

In the Condition data table, set Configuration to Catamaran. This enables additional hull data fields for demi-hull spacing and symmetry. Hull data parameters are entered for a single demi-hull NavCad applies interference corrections internally for the full catamaran system. Ensure that Displacement is entered as demi-hull displacement (half the total), not total vessel displacement.

Catamaran interference drag must be enabled explicitly in the resistance prediction settings. If interference is left off, NavCad treats the vessel as a pair of independent monohulls, which will resistance at interference-sensitive speed-length ratios.

ITTC-78 Displacement and Semi-Displacement Catamarans

Catamaran hull interference affects both resistance components:

• Viscous drag blockage effect between hulls increases local velocities, altering dynamic wetted surface and flow direction.
• Wave-making drag changes in local pressures, non-symmetric stern flow, and wave reflection between hulls alter the wave pattern. The amplitude and length of interference wave systems create the characteristic humps and hollows critical to efficient catamaran operation.

NavCad's interference model was developed from model tests, CFD analyses, and full-scale trial data. The ADVM (Analytical Distributed Volume Method) in NavCad Premium Edition provides a higher-order computational treatment, including total wave-making energy and surface wave pattern visualization useful for hull form optimization studies.

Planing Catamarans

Planing catamarans are typically longer and more slender than monohull planing craft, placing them outside the applicability ranges of traditional planing methods. extendplaning lift functions for catamarans using model test data developed for long, slender hull forms. Coverage includes:

• Standard planing catamarans.
• Fully asymmetric planing hulls (wall-sided inside tunnels).
• Foil-assisted configurations.

SWATH

NavCad includes resistance and hull-propulsor prediction for SWATH hull forms using data entry definitions suited to CAD-extracted geometry and early-stage parametric descriptions. The model includes a nose flat pressure drag prediction developed from HydroComp in-house R&D.

Propulsors for High-Speed Catamarans

High-speed catamaran hull forms are often not suited to conventional submerged shaft-driven propellers. NavCad supports waterjet and surface-piercing propeller modeling, allowing proper Vessel–Propulsor–Drive system analysis for these configurations.

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