Savitsky method error low deadrise / planing station geometry troubleshooting

Background

The Savitsky method predicts resistance and running trim for hard-chine planing hulls. It requires accurate values for deadrise at two reference stations, chine beam, and chine height below waterline. Errors in these parameters particularly very low or zero deadrise produce physically unrealistic predictions and can cause convergence failures in the equilibrium planing calculation.

Common Causes of Savitsky Errors

• Incorrect deadrise: Manual entry of geometric deadrise rather than effective deadrise. The effective deadrise of the planing bottom (), which differs from the measured geometric angle at any single point, particularly for warped-bottom hulls.
• Zero or near-zero deadrise: Very flat-bottomed hulls can produce numerical instability. A minimum practical deadrise is typically around 2–3 degrees for most Savitsky implementations.
• Chine position errors: Incorrect chine height below waterline affects the dynamic waterline calculation. Chines reported too low or too high will produce incorrect wetted area and lift predictions.
• Station positioning: The two reference stations are positioned forward of the transom. If station positions are outside the planing wetted length range, the interpolated parameters will not represent the operative planing geometry.

Recommended Approach

Hull CAD mport is strongly recommended for planing hull geometry. The import process extracts deadrise and chine positions at the defined station cuts directly from the 3D hull geometry, using the geometric projection of the bottom shell. Manual entry should be used only when no CAD geometry is available.

When manual entry is required, the deadrise input should reflect the effective slope of the planing bottom averaged across the anticipated wetted planing length not the maximum slope at any single section. The NavCad deadrise estimate utility (accessible from the hull data form) allows entry of station offsets to calculate an effective value.

Checking the Prediction

After setup, run the Savitsky prediction across the full speed range and inspect the trim angle curve. A physically reasonable result shows trim increasing at low planing speeds (hump) and decreasing through the planing range to moderate trim at top speed (typically 2–5 degrees for well-designed planing hulls). Trim angles outside this range, or a non-convergence at any speed point, indicate a geometry or data entry problem rather than a prediction method limitation.

Propeller Tunnels

For hulls with propeller tunnels, deadrise input must account for the effective bottom geometry in the presence of the tunnel. See the related article on tunnel hull modeling in NavCad.

Related Articles

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• How do I model a planing hull with propeller tunnels in NavCad?
• How do I choose a resistance prediction method in NavCad?
• How do I use NavCad's Method Expert to check hull form validity?