NavCad Case Studies: Real-World Applications in Ship Design and Retrofit

Mastering NavCad: A Practical Guide for Ship Resistance and Propulsion Analysis

Introduction

NavCad is a widely used engineering tool for predicting ship resistance, powering requirements, and propeller performance. This guide gives a concise, practical workflow that helps naval architects and marine engineers use NavCad effectively to evaluate hull performance, size propulsion systems, and explore optimization opportunities.

1. Prepare input data

• Geometry: Enter principal dimensions (L, B, T), displacement, block coefficient (Cb), prismatic coefficient (Cp) and longitudinal center of buoyancy. If available, import lines/hull-form details.
• Weight and loading: Provide lightship and payload to define operating displacements and trim.
• Operating profile: Define service speed(s), sea state assumptions, and typical operating RPM/load points.
• Propulsion arrangement: Specify shafting layout, gearbox ratios, installed engine power and propeller options (diameter limits, blade number).

2. Choose resistance method and settings

• Select empirical method: For preliminary studies, use Holtrop-Mennen or Delft methods for total wave and frictional resistance. Use model-test data if available for the most accurate calibration.
• Frictional resistance: NavCad applies ITTC 1957 friction line with a form factor; verify or adjust the form factor (1 + k) based on hull roughness or calibration against experiments.
• Appendages and protuberances: Add appendage data (rudders, skegs, bilge keels) and estimate their drag contribution.

3. Calibrate with model or full-scale data

• Apply correlation allowance: Use model test results or sea-trial data to set correlation factors (CF) and scale model data to full scale.
• Tune form factor and roughness: Adjust k and roughness/frictional correction to match known resistance points.

4. Propeller and wake modeling

• Wake fraction: Input wake distribution or allow NavCad to estimate wake using empirical hull-geometry correlations.
• Propeller selection: Define candidate propellers (diameter, pitch/diameter ratio, blade area ratio, number of blades).
• Open-water curves: Use manufacturer open-water curves or empirical series; verify required thrust and torque across operating points.

5. Shafting and engine match

• Shaft losses: Set shaft and gearbox efficiency values; include thrust deduction and effective power (EHP vs. SHP).
• Engine map: Enter engine power and specific fuel consumption vs. load, or choose generic engine curves.
• Propulsive efficiency: NavCad computes overall propulsive efficiency (eta0 = etaHetaR * etaS); inspect components to find loss sources.

6. Performance analysis

• Speed-power curves: Generate required power vs. speed curves to confirm installed power meets service speed with margins.
• Load cases: Run multiple cases (lightship, full load, ballast) and environmental conditions (head seas, added resistance) to ensure robustness.
• Fuel consumption: Estimate fuel use over operational profiles and compute range/endurance.

7. Optimization and sensitivity

• Propeller optimization: Iterate diameter, pitch, and blade number to maximize propulsive efficiency while avoiding cavitation or overloading the engine.
• Hull modifications: Test small changes in Cb, bulbous bow presence, or appendage shapes to see resistance impacts.
• Parameter sensitivity: Vary roughness, wake fraction, and correlation allowance to understand uncertainty in predictions.

8. Troubleshooting common issues

• Unrealistic wake or thrust values: Check hull geometry inputs and ensure the center of thrust/wake assumptions are consistent.
• Propeller cavitation warnings: Reduce pitch/diameter or increase diameter; consider more blades to lower loading.
• Mismatch between EHP and SHP: Revisit shaft losses, gearbox ratio, and propeller open-water data.

9. Reporting and documentation

• Export plots and tables: Use NavCad’s export features for speed-power curves, efficiency breakdowns, and open-water graphs.
• Document assumptions: Clearly list correlation factors, roughness, sea-state, and propulsion architecture used for each case.
• Present margins: Provide recommended safety margins for powering and fuel estimates.

Conclusion

NavCad is a powerful tool when fed accurate inputs and calibrated against tests or trials. Adopt a disciplined workflow: prepare clean geometry and loading data, select appropriate resistance methods, calibrate with data, model propeller and shafting carefully, and iterate for optimization. This practical approach yields reliable resistance and propulsion predictions useful through preliminary design to sea-trial verification.