Model a displacement hull

Use DisplacementHull for a low-speed craft that crosses a free surface. It is a CompositePart: each quadrature point contains an ordinary PointBuoy and anisotropic DragSurface. Consequently, the model works unchanged with a flat ocean, Earth/SeaWaves, spatial currents, and every supported backend.

from manta import Craft
from manta.parts import DisplacementHull, Mass

boat = Craft("boat")
boat.add(Mass(
    "structure",
    mass=95.0,
    moi=(18.0, 24.0, 32.0),
    mount_offset=(0.0, 0.0, -0.08),
))
boat.add(DisplacementHull(
    "port_hull",
    dimensions=(1.20, 0.22, 0.30),
    displacement_volume=0.052,
    hydrostatic_offset=(0.0, 0.0, 0.015),
    drag_coefficients=(0.20, 0.85, 1.0),
    sample_resolution=(5, 2, 8),
    mount_offset=(0.0, 0.36, 0.0),
))

For a catamaran, add a second instance at the mirrored lateral mount. This is geometry and physics composition only; steering and surface-trajectory policy belong outside Manta.

Calibrate it

dimensions defines the distribution envelope. By default its ellipsoid volume is the full-submersion displacement. Prefer a measured or CAD-derived displacement_volume when available; it intentionally may differ from that idealized volume. Shift hydrostatic_offset to match the measured centre of buoyancy. Keep the Mass parts at measured centres of mass: their separation from the wet sample centroid is what produces the righting arm.

The default drag reference areas are the ellipsoid's projected frontal, lateral, and planform areas. Supply reference_areas=(Ax, Ay, Az) when tow tests or a better geometric estimate are available, then fit or calibrate the three drag_coefficients separately. Offset drag points see the local omega × r velocity, so they also produce roll, pitch, and yaw damping.

At the desired calm draft, check that

rho_water * displaced wet volume = craft mass

hull.displaced_volume_below(z) is a hard-cut, flat-water calibration helper. Runtime physics instead samples the world's smooth and possibly moving fluid boundary.

Check resolution

sample_resolution=(axial, radial, circumferential) controls the product quadrature. Total displacement and drag area are conserved exactly at every resolution, but draft and righting curves are discrete approximations because the set of wet sample centres changes at the surface. Increase resolution until draft, roll/pitch restoring moment, and damping change less than the vehicle's calibration uncertainty. The practical default (5, 2, 8) creates 80 buoy/drag pairs.

The fluid field's surface smoothing must also be physically appropriate. A very sharp boundary with sparse vertical samples produces force steps; an excessively wide boundary smears the waterline and alters draft.

Limits

This model covers low-speed displacement behavior. It does not model planing, slamming, dynamic wave radiation, mesh collision, or CFD interaction between multiple hulls. Add separate Manta parts for effects such as added mass; do not fold vehicle control policy into the hull.

For a modular slender hull, author sectional hydrodynamic inertia beside the module geometry but assemble it into one rigid-body tensor before evaluation. Computing an isolated-body Munk moment for every joined module is generally not physical: concealed interfaces and pressure-flow interactions mean arbitrary body hydrodynamics are not additive. Strip-theory transverse sections are a useful approximation; axial and end effects need a whole-assembly correction.