Which Calculation Conditions Must Remain Identical to Compare Propeller Nozzle 19A and Propeller Nozzle 37 Objectively?
Author: Jeroen Berger • Publication date:
A comparison between propeller nozzle 19A and propeller nozzle 37 only acquires decision value when the calculation conditions under which the comparison is performed have been defined unambiguously in advance. The central question is not which profile appears more favourable, but whether the observed difference arises solely from profile geometry and not from shifting assumptions, boundary conditions or interpretation.
For shipping companies, shipowners and technical managers evaluating a design or investment decision on the basis of such a comparison, that distinction is decisive. Once a single element in the calculation setup changes, not only does the outcome shift, but also the meaning of the comparison itself.
Objectivity therefore begins with defining one fixed vessel configuration, one unambiguous comparison basis and one consistent calculation framework applied identically to both profiles.
Identical Vessel Configuration as the Starting Point
Hull form, aft ship geometry, propeller and rudder must remain identical. Only the propeller nozzle profile varies. This includes the same propeller diameter, blade geometry, pitch distribution, hub geometry and identical positioning of rudder and skeg.
Mutual distances form an integral part of this condition. Tip clearance, axial distance to the rudder, radial distance to the hull and centring around the shaft line must correspond exactly. Once these geometric boundary conditions shift, the comparison no longer evaluates a profile difference but a difference in installation geometry.
The same discipline applies to the representation of the propeller in the CFD calculation. If one variant uses a different modelling approach, resolution or propeller representation than the other, the comparison shifts from profile behaviour to model behaviour.
The Same Operating Points Within the Same Operating Profile
Both profiles must be evaluated at exactly the same combinations of vessel speed and propulsion loading. It must also be explicitly defined which quantity remains constant: rotational speed, shaft power, thrust or vessel speed.
This is where comparisons frequently fail. A comparison at equal vessel speed answers a different technical question than a comparison at equal loading. If 19A is implicitly assessed on one basis and 37 on another, the underlying question changes unnoticed.
The selected operating points must therefore be representative of the dominant operating profile and applied according to exactly the same definition for both profiles. Objectivity here means that not only the design point but also additional loading or manoeuvring conditions are calculated under identical assumptions.
One Fixed Comparison Basis
The comparison basis must be defined clearly in advance. Lower shaft power at equal vessel speed answers a different question than higher thrust at equal rotational speed. The same applies to comparisons at equal loading, equal propulsion performance or equal propeller operating condition.
What is decisive is not the absolute magnitude of the difference, but the consistent application of the same comparison basis across all operating points. Once this definition shifts between conditions or variants, the result loses its clarity and therefore its comparative value.
In formal processes, for example in relation to a classification society or internal technical review, this comparison basis should therefore be explicitly documented in the comparison protocol. Only then does it remain traceable which technical question has actually been answered.
Identical Scale and Environmental Conditions
Scale approach and environmental conditions have a direct influence on propeller nozzle behaviour. The choice between model scale and full scale must therefore be identical for both variants, as must water depth, free-surface modelling, vessel condition and any asymmetry conditions such as small rudder angles or limited drift angles.
If one profile is evaluated under different environmental conditions, the comparison no longer concerns geometry alone but effectively represents a different physical situation. Interpretation then shifts from profile behaviour to context behaviour.
The comparison thereby loses its methodological integrity, even if the results initially appear convincing.
Identical Interaction Conditions with the Rudder
Because the propeller nozzle influences the flow towards the rudder, rudder conditions form an integral part of the comparison. If only straight-ahead operation with zero degrees rudder angle is considered, that condition must be identical for both profiles.
When additional evaluation includes limited rudder angles or slight asymmetry, these conditions must also be applied using the same definitions and numerical setup. Only then can differences in rudder loading, flow structure or required power be attributed to the profile itself.
If rudder angle, asymmetry or manoeuvring condition differs between variants, the comparison no longer evaluates only the propeller nozzle profile but a different interaction within the propulsion system.
The Same Numerical Setup
Turbulence modelling, wall treatment, mesh generation, solver configuration and convergence criteria must remain unchanged between both calculations. Even subtle differences in mesh quality, wall modelling or solution strategy can influence the pressure distribution around the nozzle and blade tip.
The risk here does not lie in the selection of a specific numerical model itself, but in modifying that setup between profiles. Once the numerical configuration differs, part of the observed difference becomes a consequence of the calculation framework.
The comparison then shifts from profile behaviour to model behaviour, even when the geometry itself remains identical.
Documentation as a Condition for Objectivity
An objective comparison requires not only identical assumptions but also explicit documentation of those assumptions. Only when vessel configuration, operating points, comparison basis, scale and environmental conditions, rudder interaction and numerical setup are defined identically and documented in a traceable manner can the difference between propeller nozzle 19A and 37 be attributed to profile behaviour within the same vessel context.
This may appear self-evident, yet in practice it is not always documented consistently. Uncertainty often arises not from the outcome itself, but from the question of what the outcome actually compares.
Within a methodologically sound comparison, a profile difference only gains decision value when it remains demonstrable afterwards that both variants were assessed under exactly the same assumptions.
This Article Within the Series
Within Propeller Nozzle: Design and Performance Validation, this article builds on the preceding article Which manoeuvring and loading conditions are relevant to evaluate propeller nozzle behaviour with CFD. Where that article establishes which operating points and asymmetry conditions are required to make nozzle behaviour visible, the focus here shifts to which calculation conditions must remain identical between variants to ensure that a comparison between propeller nozzle 19A and 37 remains objective.
The next step in the series shifts from comparison setup to quality assessment of the analysis itself. In How Can You Identify That a CFD Comparison of Propeller Nozzles Is Methodologically Inconsistent, it is elaborated which signals indicate that an observed difference may originate from the calculation framework rather than from profile behaviour.
For shipping companies, shipowners and technically responsible parties who wish to translate these methodological principles into a concrete project assessment, the page Propeller Nozzle for Ships forms a logical continuation. There, geometry, operating profile, reference profiles such as 19A and 37 and project-specific design alignment come together in a traceable nozzle configuration for newbuild and retrofit.