How do I choose a resistance prediction method in NavCad?

Overview

NavCad organizes resistance prediction around two primary hull form categories ITTC-78 (CT) for displacement and semi-displacement hulls, and Planing for high-speed hulls with supplemental categories for submarines/SWATH, barges, and craft. Within each category, multiple established empirical methods are available. The correct choice depends on hull form geometry, speed regime, data quality, and the degree of confidence required.

Step 1 Set Scope and Configuration

Begin by setting the correct Scope (ITTC-78 (CT) or Planing) and Configuration (Monohull or Catamaran) in the Condition data table. These selections enable the appropriate data entry fields and filter the available prediction methods. For borderline semi-planing hulls, the ITTC-78 scope is generally appropriate at displacement speeds, with a Planing scope used for high-speed analysis.

Step 2 Enter Hull Data

Complete hull data entry before evaluating method selection. Method validity ranges are checked against the entered hull parameters, so incomplete data will produce misleading validity results. Hull CAD import is the recommended approach it automates extraction of form parameters such as stern shape factor and half entrance angle, reducing manual entry errors.

Step 3 Use Method Expert

Run Method Expert to check which methods are valid for the entered hull form. Method Expert evaluates the hull against each method's published applicability ranges and flags out-of-range parameters. Methods outside their validity range can still be used with engineering judgment, but the confidence plot will reflect the reduced reliability.

For planing hulls, Method Expert includes a hull loading check it can indicate whether the hull has sufficient power-to-weight ratio to actually plane. This is a useful first filter before committing to a planing method.

Step 4 Select and Compare Methods

For ITTC-78 hulls, . For planing hulls, Savitsky is the most widely used general-purpose method.

Running multiple methods and comparing their results when working outside well-validated hull form ranges. Significant disagreement between methods signals a need for additional scrutiny.

Step 5 Aligned Prediction (when data is available)

When model test results, CFD resistance data, or sea trial performance data exist for the vessel or a similar one, aligned prediction be technique. Aligned prediction anchors the empirical method to a validated reference, delivering accuracy closer to a new model test at a fraction of the cost. See the related article for details.

Notes

The bow shape factor in NavCad's hull data primarily affects the Andersen methods. For most other methods it has no prediction effect. The stern shape factor follows the Harvald conventionNavCad's input s one-tenth of the Holtrop Cstern value.

Related Articles

• How do I use NavCad's Method Expert to check hull form validity?
• How do I use NavCad's aligned prediction with model test or CFD data?
• How do I import hull geometry from Orca3D, PROLINES, or an STL file into NavCad?
• How does NavCad predict resistance for catamarans?
• How do I model a Submarine, SWATH, or underwater vehicle hull in NavCad?