Aerodynamic and Hydrodynamic Performance
design software for Powerboats

How To Evaluate LatStab Index:

The Dynamic Stability Index (DSI) uses a unified rating scale to predict how a hull will behave under real-world operating conditions.

Greater value is better. DSI > 1 is more stable, DSI < 1 less stable. [see Graph #69, DSI>1 is better].

• GOOD: DSI > 1.2 → Good dynamic stability; hull exhibits well-damped roll response.

• MARGINAL: 1 ≤ DSI ≤ 1.2 → Marginal stability; hull may feel sensitive but remains controllable.

• UNSTABLE: DSI < 1 → Poor dynamic stability; risk of oscillatory or divergent roll behavior; onset of chine-walk, track rolling, hook.

NOTE: Stability indicator results are not ONLY value-based, see the 'trend-based' ASSESSMENT provided in 'Performance Summary Report' and the 'TRAFFIC LIGHT' symbol on this graph. CLICK on 'TRAFFIC LIGHT' symbol for DETAILED STABILITY REPORT. 

Read on for explanation of these analytical indicators...

Advanced Methodology
The presented (AR®) analysis technique introduces a dynamic, energy-based method for evaluating lateral stability across the full operating speed range. Developed through extensive hydrodynamic research, modeling and full-scale testing, it captures the complex interactions of roll forces, damping, geometry, and pressure fields within a single, unified framework. 

By separating Restoring Authority from Damping Effectiveness and identifying which physical mechanism governs stability at each speed, the method accurately predicts instability onset before it manifests on the water. Implemented within TBDP/VBDP software, the approach provides designers with a practical, physics-based tool to assess real-world handling behavior and apply targeted design corrections early in the design process.

TBDP©/VBDP© Lateral Stability Analysis even accounts for dynamically changing damping effects of vee-pads, center-pods, multiple Lifting Strakes, etc.

"This new dynamic lateral stability analysis by AeroMarine Research delivers a major step forward in predicting a boat’s true stability behavior across all speeds, from the first moments of planing to the highest performance speed ranges.  Despite the depth and sophistication of the method, the resulting 'Dynamic Stability Index' makes it amazingly easy for users to assess stability quickly and confidently." [PBDesign magazine, Dec 2025]

Making it Easy:
TBDP©/VBDP© presents a reporting format that makes the results and recommendations easy to understand...

TBDP©/VBDP© presents a full range of reporting information that makes the results and recommendations easy to understand.

For each of the Stability measures, the software does ALL the work behind the scenes, and gives both DETAILS and also gives the 'GOOD/NO GOOD' summary of all considerations.  [Check out 'Easy Results View' here]

What Is Lateral Dynamic Stability?
Lateral dynamic stability describes a hull’s ability to resist growing roll motion when subjected to small disturbances such as waves, steering inputs, or asymmetric lift. A hull is laterally stable if these disturbances decay with time, and unstable if they grow from cycle to cycle. Therefore, lateral stability cannot be determined solely by static equilibrium.

Consequently, lateral dynamic stability cannot be fully characterized by static equilibrium methods alone. While a hull may possess positive geometric restoring stiffness at rest, it can exhibit dynamic instability at high velocities if hydrodynamic roll energy amplification exceeds the system's damping capacity. This physical threshold marks the onset of non-linear roll oscillations, commonly known as 'chine walking'.

Dynamic lateral stability is governed by three interacting factors: ·

• how much roll energy is generated by disturbances,

• how effectively that energy is dissipated each cycle,

• how strongly the hull restores itself as heel develops.

Understanding stability therefore requires evaluating motion over time, as well as forces at equilibrium.

Why Traditional Methods Fall Short

• Savitsky-Based Analysis - Savitsky theory evaluates planing hull behavior using static equilibrium of forces and moments. Lateral stability is inferred from restoring stiffness and lift distribution at individual operating points, implicitly assuming that roll disturbances decay naturally. Roll inertia, energy persistence, and speed-dependent amplification mechanisms are not modeled. As a result, Savitsky-based methods cannot predict when or why lateral instability will develop as speed increases.

• CFD Software Analysis - CFD software tools provide detailed pressure and force distributions at specific speeds, but these results are typically isolated snapshots . They do not dynamically evaluate whether roll motion grows or decays over successive cycles, nor do they efficiently capture regime transitions across a speed range. Even 'high-end' add-ons for CFD software are computationally expensive and often fail to translate directly into real-world handling predictions.

A Dynamic Energy-Based Stability Framework

This presented (AR®) method treats lateral stability as a dynamic energy balance problem. Instead of simply testing whether restoring forces exist, it evaluates whether roll energy introduced by disturbances is removed faster than it accumulates. Two complementary stability mechanisms are both necessarily evaluated: · 

• A-DSI (Amplitude-Limited Stability Index) - measures the hull’s inherent restoring authority and ability to resist and limit roll motion as heel amplitude increases. This is an advanced modeling indicator (developed by AR®) that more accurately represents higher velocity stability behaviors. This indicator represents a hull's dynamic 'Restoring Authority' - the hull’s ability to generate corrective roll moments as heel amplitude develops - or 'how strongly the hull shape itself pushes back against rolling as it leans'.  It reflects hull geometry, lift distribution, and effective roll leverage.

• C-DSI (Cycle-Limited Stability Index). - based on the classical damping-based stability formulation traditionally used for planing hull stability assessment and is most representative when damping mechanisms dominate the stability response.  This indicator measures how effectively the hull dissipates roll energy over time instead of allowing oscillations to build  It captures the combined influence of roll inertia, hydrodynamic damping, and flow interaction.  

Both mechanisms are evaluated continuously across the operating speed range.

Dynamic Regime Identification (Λ)

The Dynamic Regime Indicator, Lambda (Λ), identifies which physical mechanism governs stability at each operating condition by comparing available restoring leverage to overturning leverage. Lambda determines whether stability is governed primarily by restoring authority (A-DSI) or damping effectiveness (C-DSI).

The AR® analysis technique evaluates all of the conditions:

• Λ > 1 — Restoring Authority-Dominated Regime (A-DSI) - Restoring authority controls stability. Geometry and lateral lift engagement are the primary stabilizing mechanisms.

• Λ < 1 — Damping-Dominated Regime (C-DSI) - Stability depends primarily on energy dissipation. Roll inertia and lift concentration dominate, and instability onset becomes more likely.

• Transitional Regime (A-DSI and C-DSI) Both mechanisms contribute. Stability is highly sensitive to speed, trim, and loading, and small design changes can have large effects.

This regime awareness is extremely helpful to the designer because it explains not just whether a hull is unstable, but why.