Savitsky Planing Resistance

What Is the Savitsky Planing Method?

The Savitsky method is a semi-empirical approach used to predict the hydrodynamic performance of planing hulls operating at high speeds. Unlike displacement-based resistance methods, Savitsky’s formulation focuses on lift generation, trim angle, wetted geometry, and pressure-induced resistance that dominate planing regimes.

Historical background

The method was developed by Daniel Savitsky in the 1960s and later refined through experimental studies on prismatic planing hulls. It was originally formulated to predict running trim, wetted length, lift coefficient, and resistance for hard-chine monohull craft. Today, Savitsky’s method remains a cornerstone of high-speed craft design and performance analysis.

Planing hydrodynamics overview

In the planing regime, a vessel’s weight is supported primarily by hydrodynamic lift rather than hydrostatic buoyancy. As speed increases, the hull rises, wetted area decreases, and resistance becomes dominated by frictional and pressure-induced components rather than wave-making effects.

The Savitsky method captures this behavior by explicitly accounting for trim angle, deadrise, wetted length, and dynamic pressure acting on the hull bottom.

Core formulation

In simplified form, the total resistance of a planing hull may be expressed as:

R_T = 0.5 · ρ · V² · S · (C_f + C_p)

where ρ is water density, V is vessel speed, S is the effective wetted surface area, C_f is the friction coefficient, and C_p represents pressure- or lift-induced resistance associated with trim and deadrise.

Resistance components explained

• Frictional resistance – computed using the ITTC-1957 friction line, typically evaluated over the wetted length of the planing surface and adjusted for surface roughness.
• Pressure (induced) resistance – associated with lift generation and trim angle, increasing with the square of trim and influenced by deadrise angle.
• Wetted geometry effects – wetted length and beam directly affect effective wetted area and thus resistance magnitude.

Key input parameters

The Savitsky method is sensitive to several geometric and operating parameters:

• Speed and corresponding Froude number based on wetted length
• Trim angle, which governs lift and pressure drag
• Deadrise angle, influencing lift efficiency and resistance
• Wetted length and chine beam defining the planing surface
• Water density and surface roughness

Typical applicability range

The Savitsky method is applicable to hard-chine monohull planing craft operating at moderate to high speeds. Typical applicability includes:

• High-speed patrol craft and fast workboats
• Planing leisure boats and motor yachts
• Trim angles typically between 2° and 6°
• Deadrise angles commonly between 10° and 30°

Engineering significance

The Savitsky method is primarily used during early-stage design to estimate resistance, running trim, and power requirements of planing vessels. It allows designers to explore the influence of hull geometry, trim control, and surface roughness on performance without resorting to full CFD simulations.

Simplified implementations, such as this quick mode calculator, are particularly useful for sensitivity studies and preliminary feasibility assessments.

Limitations and correct use

• The method assumes steady-state planing in calm water.
• It does not account for porpoising, ventilation, or dynamic instabilities.
• Hull flexibility, spray effects, and appendage lift are neglected.
• Accuracy decreases for very low speeds or transitional regimes.
• Final design decisions should be validated using model testing or CFD.

Related calculators

Savitsky-based resistance estimation is typically combined with the following tools during high-speed craft design:

• Wetted Surface Area (WSA) – used to estimate frictional resistance contributions.
• Effective Power (PE) – converts resistance into power demand at speed.
• Brake Power (PB) – includes propulsive and mechanical efficiency effects.