Beyond uniform roughness: Spatial variability of hull roughness and implications for single-parameter representation

Ocean Engineering (2026)
Sang-seok Han 1, 2
1 Department of Naval Architecture, Ocean and Marine Engineering, University of Strathclyde, the United Kingdom
2 Research Institute of Marine Systems Engineering, Seoul National University, Republic of Korea

Abstract

Hull surface roughness plays a critical role in determining ship resistance. In current practice, ship resistance prediction procedures of the International Towing Tank Conference (ITTC) commonly assume uniform roughness, represented by a globally applied equivalent roughness parameter. This study examines the implications of this assumption when spatially non-uniform roughness is present.

A review and structured synthesis of ITTC documents (1933–2024) indicate that hull surface roughness has been consistently represented by a globally applied parameter in resistance prediction procedures. This suggests that the uniform roughness assumption functions as a procedural abstraction within the established framework.

To investigate its physical implications, model-scale Computational Fluid Dynamics (CFD) simulations are performed under controlled roughness conditions using two benchmark hull forms: the KRISO Container Ship (KCS) and the KRISO Very Large Crude Carrier 2 (KVLCC2). Spatial roughness distributions—including linear (steep, mild, and reverse), random, and wall shear stress-based configurations—are considered under a constant area-averaged equivalent roughness height.

The results show that total resistance is sensitive to the spatial distribution of roughness even when the area-averaged equivalent roughness height is identical. This trend is consistently observed across both hull forms and roughness distribution types. Under the most severe roughness condition, deviations in the total resistance coefficient (CT) reach approximately 12% for the KCS and 16% for the KVLCC2 relative to the uniform roughness baseline. These differences are primarily associated with variations in frictional response, together with changes in boundary layer development and wake recovery behaviour.

The findings indicate that, under spatially non-uniform roughness, resistance cannot be fully characterised by area-averaged roughness alone. Together with the ITTC document review, the results support interpreting the equivalent roughness parameter as a procedural abstraction based on a uniform roughness assumption. This representation captures the overall effect of roughness magnitude, but it does not fully describe resistance behaviour when roughness varies spatially. The results do not question the validity of existing ITTC methods within their intended scope; rather, they clarify the physical meaning and limitations of the uniform roughness assumption. They also provide physical context for the use and limitations of single-parameter roughness representation within the existing resistance prediction framework. Extension to full-scale conditions and more realistic roughness representations remains a subject for future work.

Keywords: Hull roughness; Spatial variability; Ship resistance; ITTC; Computational Fluid Dynamics (CFD)
Published in Ocean Engineering (2026)
DOI: 10.1016/j.oceaneng.2026.127352
ITTC Hull Roughness Timeline
Table 1. Emergence and procedural consolidation of hull surface roughness in ITTC documents (1933–1972).
Year Committee/Group Procedural Context Roughness Representation Spatial Variability Scope/Limitation
1933 1st ICTS Model experiments Not specified Not addressed Model surface quality control only
1934 2nd ICTS Skin friction discussion Uniform roughness implied Implicitly excluded No formal procedural adoption
1935 3rd ICTS Model experiments Local artificial roughening Not addressed Experimental device only
1937 4th ICTS Model experiments Artificial grain-based roughness Not explicitly addressed Uniform grain-based parameter
1948 5th ICTS Skin friction formulation Uniform representation implied Not explicitly addressed Trial-based roughness allowance
1951 6th ICTS Skin friction formulation Uniform representation implied Not explicitly addressed Provisional roughness allowance
1954 7th ICTS Skin friction formulation Uniform representation implied Not explicitly addressed Correlation allowance framework
1957 8th ITTC Resistance extrapolation Explicitly uniform Not explicitly addressed Single roughness allowance applied
1960 9th ITTC Resistance extrapolation Explicitly uniform Not explicitly addressed Correlation roughness allowance
1963 RC (10th ITTC) Turbulence stimulation Discrete roughness elements Not addressed Model-scale turbulence control only
1966 RC (11th ITTC) Resistance extrapolation Implicitly uniform (procedural) Not explicitly addressed Roughness via procedural allowances
1969 PC (12th ITTC) Trial-based measurements Implicitly uniform (procedural) Not addressed Surface condition documentation only
1972 PC (13th ITTC) Ship-model correlation Explicitly uniform Not explicitly addressed Roughness identified as ongoing research issue

Note: Early conferences appear under varying official titles in contemporary proceedings. For clarity, all early meetings are referred to here using the normalised designation International Conference of Tank Superintendents (ICTS). Committee designations also vary across ITTC periods, including changes in naming and scope. For consistency, committees are referred to using standardised abbreviations (e.g. RC and PC), irrespective of variations in official titles. The descriptors summarise the documented treatment of hull surface roughness during this period.

Table 2. Evolution of hull surface roughness representation within ITTC procedures (1975–2011).
Year Committee/Group Procedural Context Roughness Representation Spatial Variability Scope/Limitation
1975 RC (14th ITTC) Resistance prediction Other (equivalent roughness) Identified outside scope No prescriptive roughness model
PC (14th ITTC) Hull roughness effects Implicitly uniform (procedural) Not explicitly addressed Roughness pattern (single-parameter)
1978 RC (15th ITTC) Resistance extrapolation Other (velocity-shift framework) Explicitly unresolved No prescriptive roughness model
PC (15th ITTC) Resistance extrapolation Implicitly uniform (procedural) Not explicitly addressed Standard roughness assumption
1981 RC (16th ITTC) Hull roughness effects Other (equivalent roughness) Identified as unresolved No universal roughness correlation established
PC (16th ITTC) Roughness allowance Implicitly uniform (procedural) Identified as unresolved Single-parameter roughness limitation acknowledged
1984 RC (17th ITTC) Hull roughness drag Other (multi-parameter roughness) Identified as unresolved Single-parameter approach considered inadequate
PC (17th ITTC) Roughness allowance Implicitly uniform (procedural) Explicitly challenged Single-parameter approach limitations identified
1987 RC (18th ITTC) Hull roughness drag Other (multi-parameter roughness) Identified as unresolved No conclusive roughness drag predictor available
PC (18th ITTC) Roughness allowance Implicitly uniform (procedural) Explicitly challenged Unsuitable for in-service assessment
1990 RC (19th ITTC) Hull roughness drag Other (multi-parameter roughness) Explicitly considered Spatial distribution effects explicitly identified
PC (19th ITTC) Roughness allowance Implicitly uniform (procedural) Explicitly challenged Single-parameter roughness limitation retained
1993 RC (20th ITTC) Experimental techniques Not addressed Not addressed CFD capability limited to smooth hulls
PC (20th ITTC) Performance prediction Implicitly uniform (procedural) Questioned but unresolved Single-parameter allowance procedurally retained
1996 PC (21st ITTC) Sea trial controls Implicitly uniform (procedural) Inspection-level awareness Roughness measurement recommended; not standardised
1999 RC (22nd ITTC) Turbulence modelling Other (CFD roughness modelling) Recognised but not developed High-Reynolds-number roughness modelling immature
2002 RC (23rd ITTC) Hull roughness drag Implicitly uniform (procedural) Explicitly challenged Texture parameter required for correlation
SC (23rd ITTC) Roughness measurement Implicitly uniform (procedural) Explicitly challenged Single-parameter applicability limited for Foul Release
2005 SC (24th ITTC) Roughness allowance Implicitly uniform (procedural) Acknowledged but limited Empirical validation limited; single-parameter inadequate
2008 SC (25th ITTC) Roughness allowance Implicitly uniform (procedural) Considered; not standardised Roughness-correlation separation formalised
2011 RC (26th ITTC) Scaling & extrapolation Implicitly smooth (CFD) Explicitly acknowledged CFD-correlation-roughness inconsistency identified
RP (26th ITTC) Performance prediction Explicitly uniform (single ks) Uniformly assumed Single-parameter roughness formalised

Note: Committee designations are standardised as RC (Resistance Committee), PC (Performance Committee), SC (Specialist Committee), and RP (Recommended Procedures and Guidelines), irrespective of minor variations in official titles across different ITTC periods. The terminology reflects the procedural context of the referenced documents and serves as an interpretative summary rather than a literal physical classification.

Table 3. Recent advances in hull surface roughness modelling and spatial considerations within ITTC documents (2014–2024).
Year Committee/Group Procedural Context Roughness Representation Spatial Variability Scope/Limitation
2014 RC (27th ITTC) Roughness modelling Explicitly uniform (equivalent ks) Explicitly recognised Non-uniformity recognised; uniform framework retained
RP (27th ITTC) CFD wall modelling Explicitly uniform (equivalent ks) Not resolved Smooth-wall baseline; roughness via ks wall function
2017 RC (28th ITTC) Ship surface roughness Dual (smooth model / ks at full scale) Recognised; not standardised Complex roughness identified; single-parameter retained
RP (28th ITTC) Performance prediction Explicitly uniform (equivalent ks) Uniformly assumed Global single-parameter roughness allowance
2021 RC (29th ITTC) Ship surface roughness Procedurally uniform (ks-based) Explicitly challenged 3D resistance effects and CFD wall-function integration
2024 RC (30th ITTC) CFD roughness modelling Roughness-function framework Explicitly analysed Best practice unresolved; ΔU+–R+ relationship unclear
PC (30th ITTC) Performance prediction Roughness-function framework Explicitly analysed Full-scale validation scarce; spatial variability unresolved

Note: The terminology used in this table summarises the procedural treatment of hull surface roughness in the referenced ITTC documents and does not imply a physical classification of surface condition.

Selected Figures
Bottom-view spatial roughness distributions for the KCS and KVLCC2 hulls under the B20% severity level
Figure 1. Bottom-view spatial roughness distributions for the KCS and KVLCC2 hulls under the B20% severity level.
The figure compares the representative spatial roughness distributions applied to the KRISO Container Ship (KCS) and KRISO Very Large Crude Carrier 2 (KVLCC2) hulls. The cases correspond to uniform, linear–steep, linear–mild, linear–reverse, random, and wall shear stress-based distributions, respectively. All cases preserve the same area-averaged equivalent sand-grain roughness height, allowing the effect of spatial variability to be isolated. The bottom-view presentation clarifies how the imposed roughness fields extend over the flat-bottom region of each hull.
Relative change in total resistance coefficient for the KCS hull under different spatial roughness distributions and fouling severities
(a) KCS hull
Relative change in total resistance coefficient for the KVLCC2 hull under different spatial roughness distributions and fouling severities
(b) KVLCC2 hull
Figure 2. Relative change in total resistance coefficient, CT, for the KCS and KVLCC2 hulls under different spatial roughness distributions and fouling severities.
The figure compares the relative change in the total resistance coefficient (CT) with respect to the uniform roughness baseline for the KCS and KVLCC2 hulls. Results are shown for four representative spatial roughness distributions—uniform, linear–steep, random, and wall shear stress-based— under three fouling severity levels (S20%, M20%, and B20%). For both hull forms, the linear–steep distribution produces the largest reduction relative to the uniform case, whereas the random distribution remains close to the baseline. The shear stress-based distribution has only a minor effect for the KCS, but produces a small increase in CT for the KVLCC2.
Boundary layer development along the KVLCC2 hull under different spatial roughness distributions at the B20% severity level
Figure 3. Boundary layer development along the KVLCC2 hull, visualised through axial velocity slices, under different spatial roughness distributions at the B20% severity level.
The figure compares boundary-layer development along the KVLCC2 hull for four representative spatial roughness distributions: uniform, linear–steep, random, and wall shear stress-based. The annotations show the relative changes in the total resistance coefficient (ΔCT) and boundary-layer area (ΔABL) with respect to the uniform baseline. The linear–steep distribution produces the largest reductions in both quantities, whereas the random case remains close to the uniform reference. In contrast, the wall shear stress-based distribution increases both the boundary-layer area and total resistance, indicating delayed near-wall flow recovery.