Planing Hydrodynamics

Savitsky Method for Planing Hulls

Empirical regression method — D. Savitsky (1964 & 1976)

The Savitsky method is the industry-standard empirical approach for predicting the hydrodynamic performance of prismatic planing hulls at high speed. It establishes equilibrium between hydrodynamic lift, hydrostatic buoyancy, hull weight and thrust to solve for the running trim angle and wetted geometry.

Lift Equilibrium
W = N + B

Where N is the hydrodynamic normal force and B is the buoyant force. The method iterates trim angle τ until vertical and moment equilibrium are achieved simultaneously.

Hydrodynamic Lift (N)

The planing lift coefficient CL is a regression function of deadrise angle β, mean wetted length-to-beam ratio λ, and beam Froude number FnB:

Planing Lift
C_L = C_L0 − 0.0065β · C_L0^0.60

CL0 is the flat-plate lift coefficient. Deadrise correction reduces lift for warped-section hulls with β > 0°.

Skin Friction (F_f)

Based on the ITTC-1957 correlation line using mean wetted area. A roughness penalty (type 1 or 2) is applied over smooth-plate friction to capture real-surface effects on high-speed craft.

Validity Range

The method is valid for beam Froude numbers FnB ≥ 0.6, deadrise 0°–30°, and length-to-beam ratios 2–7. Below FnB = 0.6 the hull is in the displacement or semi-displacement regime and this method diverges.

Output Breakdown

Performance Metrics

Five independently computed result groups

Froude Vessel Specs

Beam and volumetric Froude numbers, displacement volume, and hull mass confirm the planing regime and loading state.

Attitude Equilibrium

Running trim angle τ (deg) and heave zwl characterise the hull's dynamic sinkage and pitch posture at speed.

Geometry Wetted Shape

Keel and chine wetted lengths, mean λ ratio, transom draft, and wetted bottom area — the instantaneous underwater footprint.

Forces Force Breakdown

Hydrodynamic lift, skin friction, flap contribution, and net force vectors in the body-fixed frame.

Power Propulsion

Required thrust magnitude, effective power in kW, and equivalent horsepower for propulsion system sizing.

Interactive Tutorial

Simulation Guide

Operational Control Workflow

1

Hull Geometry

Set overall length, beam, deadrise angle, and LCG/VCG positions. The 3D canvas renders a prismatic planing hull that updates live as you adjust sliders.

2

Trim Flap & Sea State

Configure trim flap chord Lf, deflection δ, propulsion angle ε, and significant wave height Hsig for added resistance in waves.

3

Analyse Results

Switch to Result Plots to see force component bar charts, or open Data Table for the full numeric output and CSV export.

Savitsky · Planing Hull Dynamics

Planing Hull Solver

Hull Geometry
Trim Flap & Propulsion
Sea State
Wetted Length Method
Roughness Penalty
Performance Summary
Fn (Beam): —
Trim τ: — °
Wetted Area: — m²
λ (mean): —
Thrust (N): — kN
Eff. Power: — kW
Horsepower: — HP
READY
L/B: — · β: —°
L/B: 3.33 | β: 15.0°