How water moves around Sunwake

Computational fluid dynamics (CFD) tests how the two hulls make waves, settle into a running attitude, and resist motion. This completed fine-grid case covers one condition: bare hulls at 8.15 kn in calm water. It provides evidence for the next design decision, not a final powering number.

Explore the force history or watch the solution settle.

Plan view of the computed water surface around both Sunwake hulls at 8.15 knots, with red wave crests and blue troughs — The fine-grid solution at 36,000 solver iterations. Red marks a crest, blue marks a trough, and the black lines mark the computed waterline.

Condition8.15 kn4.193 m/s

Displacement12,700 kghalf load

Fine grid2,848,785cells

Bare-hull resistance1,885 Nwindow mean [ENG]

Tow power7.90 kWbefore losses [ENG]

Running attitude26.8 mmdown, 1.157° bow-up [ENG]

The question this work answers

Sunwake's hulls sit 5.0 m apart, or one third of the 15.0 m waterline length. The earlier analytical wave-resistance model predicts that their wave systems partly cancel at 7.0–7.5 kn and reinforce at 8.5–9.0 kn. That result affects passage speed, energy use, and hull spacing, so the CFD campaign tests it before the lines freeze.

This case shows

A stable result at one speed

The 8.15 kn case produced a coherent wave field, stable force averages, and a bounded running attitude on the fine grid.

The campaign still needs

A comparative speed and spacing sweep

One operating point cannot confirm the predicted cancellation and reinforcement. The production sweep remains open.

The resistance settles into a narrow band

The chart uses the preserved solver history. Move the slider to inspect the whole-boat pressure and viscous forces at 200-iteration intervals. The shaded region marks the accepted averaging window from 28,800 to 36,000 solver iterations. The dashed line marks its 1,885 N total mean.

Preserved force data

Fine-grid force convergence

Total Pressure Viscous

Solver history

Iteration36,000

Pressure954 N

Viscous905 N

Total1,859 N

The averaging window, not the last sample, produces the reported result: 978.4 N of pressure resistance plus 906.8 N of viscous resistance equals 1,885.2 N total. The medium grid at its own running attitude produced 1,798.5 N. The fine-grid result is 4.82% higher, which shows that mesh choice still matters.

Watch the wave pattern settle

Animated sequence of 15 computed water-surface states as the CFD solver converges around the two hulls — **This is a solver-convergence sequence, not water moving in real time.** The 15 stored frames span 9,600 to 36,000 solver iterations. The method uses local time stepping, so frame spacing does not represent physical seconds.

The bow waves and the inner-hull wave become stable before the accepted force window begins. The smooth pattern and the lack of reflected waves at the domain edges support the force result. They do not validate its absolute value.

Pressure explains the running attitude

Outboard, inboard, and bottom views of computed pressure on the starboard hull, from minus 2.15 to plus 1.63 kilopascals — Red marks pressure above still-water pressure. Blue marks pressure below it. The black line is the computed waterline, and the dashed line is the design waterline.

What the colors mean

The bow pushes; the shoulder pulls

The red patch at the stem shows the bow pushing water aside. The blue band along the forward shoulder shows lower pressure pulling the hull down. That pressure pattern produces the calculated 26.8 mm sinkage and 1.157° bow-up trim.

The pressure drag in the force chart is the fore-and-aft imbalance of this field. The displayed scale covers the full wetted range from -2.15 kPa to +1.63 kPa, so the image clips no wetted pressure value.

The mesh supports the calculation

A CFD result depends on where the grid resolves the flow. This case puts small cells around the free surface, each hull, and the wake. Three prism layers follow the hull skin to support the wall-function calculation of viscous resistance.

Four cutaway views of the CFD grid showing concentrated cells at the water surface, around each hull, and through the wake — Grid sections show the refined water-surface band, the hull refinement shell, the wake box, and the three near-wall prism layers.

Map of y plus over the wetted hull showing most of the area inside the target band and small orange patches at the stem and keel — y+ measures how close the first grid cell sits to the hull in flow terms. The target band covers 99.2% of the wetted area. The 0.8% outside the band lies mainly at the stem and keel.

What the case leaves out

The solver models smooth, bare hulls in calm water. It does not include:

Hull surface roughness or fouling.
Rudders, skegs, propellers, or shafts.
Air drag from the hulls, bridgedeck, or house.
Wind, waves, or the motions that waves cause.
Propeller interaction with the hull and wake.
A converged fine-grid attitude or a validated numerical uncertainty band.

The reported 7.90 kW is bare-hull tow power. It is not shaft power or DC bus power. Review the broader assumptions on the performance page and in the hull form study.

Reproducible record

Item	Recorded value
Case	`half_8p15kt_s5p00m_s26p8mm_t1p157deg_fine`, fine-grid attitude iteration 3
Condition	12,700 kg displacement, 5.0 m hull-center spacing, 8.15 kn, calm water
Mesh	2,848,785 cells, 190,670 hull faces, three prism layers, 3 mm first layer
Solver	OpenFOAM 14 `incompressibleVoF`, k-omega SST, 16 ranks
Field record	Iteration 36,000, written 2026-10-01 at 08:25 UTC
Force window	Iterations 28,800–36,000, 7,201 samples; gate passed
Evidence	CFD visualization packet in pull request 70, source commit `f4bba2f`

The page uses the preserved SUN-85 iteration 3 fields and force history. The rendering command checks the force window against the accepted evidence record before it publishes a view. The page changes no geometry, resistance baseline, weight, vessel energy budget, or construction decision.