Wetted Surface Area (WSA)

What is the Wetted Surface Area (WSA)?

The wetted surface area (WSA) is the total area of the ship’s hull that is in contact with water at a given draft. It includes the underwater hull surface up to the waterline and is a key parameter in resistance and power estimation.

Definition

Wetted surface area is commonly denoted as S and represents the geometric surface over which viscous friction acts between the hull and the surrounding water.

Unlike displacement or waterplane area, WSA cannot be determined from a single coefficient alone and is therefore typically estimated using empirical formulas during early-stage design.

Holtrop–Mennen method

The Holtrop–Mennen method provides an empirical formulation for estimating wetted surface area based on principal ship dimensions and hull-form coefficients, including LWL, BWL, draft, block coefficient (CB), midship coefficient (CM), and waterplane coefficient (CWP).

This method is widely used in preliminary resistance prediction and is considered suitable for conventional displacement hulls operating in typical commercial speed ranges.

Simplified displacement-based estimation

For very early design stages or rough feasibility studies, wetted surface area may be approximated using a simplified empirical relationship of the form:

S ≈ 1.7 × ∇^2/3

where ∇ is the submerged displacement volume. This approximation ignores detailed hull-form effects and should only be used when limited data is available.

Engineering significance

Wetted surface area directly influences viscous resistance through the frictional resistance component. Larger WSA values generally lead to higher frictional drag and increased power demand at a given speed.

Accurate estimation of WSA is therefore essential when calculating total resistance, effective power, and fuel consumption.

Speed and friction coefficient

When vessel speed is known, the friction coefficient can be estimated using the ITTC ’57 formulation, which relates Reynolds number to frictional resistance. In such cases, WSA acts as the primary scaling surface for the friction term.

Limitations and correct use

• WSA estimates are draft-specific and must correspond to the same loading condition.
• Empirical methods are valid primarily for conventional displacement hulls.
• Simplified formulas should not be used for detailed resistance predictions.
• Final design values should be confirmed using hydrostatic models or CAD-based surface calculations.

Related calculators

Wetted surface area is typically evaluated together with other hull-form, hydrostatic, and resistance parameters:

• Block Coefficient (CB) – overall hull fullness indicator.
• Midship Coefficient (CM) – midship section shape parameter.
• Waterplane Coefficient (CWP) – normalized measure of waterplane shape.
• Displacement Calculator – converts principal dimensions into submerged volume.
• Holtrop–Mennen Resistance – uses WSA to estimate total calm-water resistance.
• Effective Power (PE) – derived from resistance and vessel speed.