Computational Fluid Dynamics for marine vessels

The term ‘CFD optimized hull‘ is becoming a buzzword like ‘innovative’ or ‘sustainable boating.’ When scientific words become overused for marketing purposes, their real meaning starts to dilute. But what does it really mean to run a CFD calculation? When is it worthwhile to perform it, and what are its advantages?
At iYacht, we offer a comprehensive range of services, including CFD optimization.
Before delving deeper into our process, we would like to bring some clarity on what a CFD really is, what are its implications in terms of time and costs, and in which cases it makes sense to invest into a full optimization and in which cases is more cost effective to run some checks.

CFD vs tank testing

Tank testing used to be for decades the preferred method to predict performance of a new design. At iYacht, we had experience with this type of testing for some projects: the last time we went with a model in a tank test facility was around 7 years ago, for analyzing the hull shape of a commercial vessel.
In the meantime, CFD programs have been refining, and computational capacity has been constantly increasing. Nowadays, the results we get from running a software simulation are very comparable, in terms of accuracy, to the results of a time-consuming and expensive tank testing. Adding up the pros and cons, in most cases, the software outweighs the physical testing. Tank testing, compared to running a software simulation, requires in-person presence, building the model, and organizing the facility: activities that consume a lot of time and resources.

Christoph Braun, one of the naval architects who runs CFD at iYacht, explained clearly how quickly the software and the computational capacity have evolved.
“When I was at the university, it was possible to run CFD only on a very powerful computer, we had to reduce the hull shape down to the bare minimum and even so, the calculation required many days.
Now, it is possible to perform the CFD on one of the workstations installed here in Hamburg in our office, and the wide availability of programs make this once-time consuming analysis suitable also for relatively smaller companies.
Even the most complex and complete CFD of a hull with several appendages lasts not longer than approximately two days”

With many options on the market, we periodically conduct in-house research to determine which program to adopt. We want to ensure that we are still using the best software toolbox  suitable for our needs. Currently, our choice is the Simerics Multi-Phase plugin for Orca 3D, which allows us to perform naval architecture calculations within the same design software we use for 3D modeling the boat and its components.

As a one-stop shop, one of our promises is to offer a streamlined process to our clients. Relying on seamlessly integrated programs is a significant advantage for both us and our clients in terms of reducing costs and achieving final results in a short span of time.

The integration with the design software is important because the CFD of the hull is run quite early in the design phase, allowing us to adjust the hull design based on the calculation results. The verification of other details, such as the position of appendages, can be optimized in a later design phase

Is your design CFD optimized or CFD validated?

Most boats nowadays are marketed as ‘CFD optimized,’ but this is not always true. It’s important to distinguish between full optimization and merely running a few tests to validate the design or check specific details.

Popularized by F1, streamline images are now widely recognized by the public. These colorful lines around a 3D model make many non-expert people immediately assume that the design has been scientifically proven and optimized, but true CFD optimization is much more complex. To genuinely optimize a hull with CFD, numerous tests must be conducted under various conditions and with different hull designs. The outcomes are then analyzed, and decisions are made for the next design iteration based on these results. This process involves designing many hulls and running numerous tests—an extensive undertaking.

While it is possible to run two or three tests to check specific elements, such as the position of appendages using streamline analysis, we prefer to call this CFD validation rather than optimization.

When is a full CFD optimization really worthwhile?

Understanding that complete CFD optimization is a significant task, the next question naturally arises: when does it make sense to undertake it?

For recreational crafts, we recommend CFD optimization for series production, where the volume of boats being manufactured justifies the initial investment, or when high performance is required. In the latter case, even if the boat is a one-off, running CFD can be worthwhile to achieve the desired performance.

For commercial vessels, which operate for many hours, optimizing the hull to reduce running costs and fuel consumption is highly beneficial.