At BMT Specialised Ship Design (formerly BMT Nigel Gee), the process for vessel resistance and speed calculation is broadly divided in: parametric optimisation phase, hull form optimisation phase and when suitable to the project, experimental testing.
During the parametric optimisation phase, often taking place pre contract, BMT primarily uses its large model test database to support parametric optimisation studies and derive suitable hull form characteristics and associated resistance data. As needed, empirical methods such as Holtrop, Savitsky, etc., are used to align model test results to new hull form particulars (Beam, Displacement, etc..). This has proven a reliable approach over the years.
When client requirements are best served by novel or unusual hull forms, available model test data or resistance calibration methodology are not adapted for accurate resistance and ship speed prediction. In such case the speed margin may be increased however this does not necessarily make for a better design. This is why BMT decided a year ago to start integrating CFD calculations into its preliminary design process with the aim to improve resistance accuracy for such cases.
As a first step, BMT started a CFD validation project to establish the accuracy of CFD results against model test data results. The CFD calculations were run using FINE™/Marine for a variety of designs including displacement, semi displacement and planing hulls across monohull and catamaran hull forms. The runs were carried out at full scale and model scale.
The validation campaign demonstrated that the CFD resistance results compared to model test resistance run at full scale showed differences ranging from 0 to 7% when using standard C-Wizard settings. The CFD resistance results were usually showing lower value. Importantly, the differences were relatively constant for given Froude Number ranges allowing to reduce the overall uncertainty against model test results.
The validation example below shows the bare hull resistance of a 100m+ yacht evaluated by means of CFD simulations in calm water conditions compared to the model scale test.
Figure 1 - 100m+ yacht resistance comparison CFD vs scaled model test
Since the scaling required to define full scale resistance from a model test introduces a level of uncertainty when comparing with full scale CFD results, CFD was also run at model scale for a subset of the hulls reviewed. When run at model test scale the CFD results tended to correlate very well with the model test result with average variation in the order of +/-2%.
In addition to the more established hull form optimisation studies carried out in contract, and following the positive outcome of the first CFD validation campaign, BMT Specialised Ship Design now uses FINE™/Marine as part of its parametric optimisation phase to provide greater certainty at an earlier stage of the design.
Figure 2 - 100m+ yacht hull form illustration
BMT Specialised Ship Design (formerly BMT Nigel Gee) part of BMT, is a leading independent naval architecture and marine engineering design consultancy providing services for advanced and specialised vessels, from initial concept through to detailed design and production. The company has an established track record in the design of Yachts, Commercial, Offshore Energy and Defence vessels.
Principal Naval Architect