iYacht - Computational Fluid Dynamics for Marine Vessels

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.

The good news is that running extensive CFD isn't always necessary for optimizing a boat, especially in cases where economic factors and time constraints are prioritized over the extreme precision provided by the software.

We have built a comprehensive database from all the simulations we have conducted so far, which has significantly increased our knowledge. This allows us to achieve good approximations by integrating this knowledge to the results of analytical calculations. This accumulated expertise enables us to design boats more efficiently, often without needing to perform new CFD analyses, but instead relying on what we have learned from previous CFD studies.

What can we simulate with CFD?

The main focus of CFD calculations is on resistance. While it is also possible to analyze seakeeping performance, this adds another layer of complexity.
In addition to resistance, running CFD allows us to optimize the positioning of appendages, rudders, hull entrance angles, and the shape and position of spray rails for high-performance boats.

Recently, when studying the shape of the curved daggerboards of a 55 ft catamaran, we also calculated the lift on these foils. For some special designs, we studied the flow of air and performed aerodynamic calculations in addition to hydrodynamic ones. For instance, we conducted analyses to ensure a dry cockpit on a 52 ft planing motorboat, which included streamlining airflows around the superstructure for spray control.


A wide range of vessels

At iYacht, we handle the design and engineering of various types of boats, and this has given our team extensive experience in conducting CFD analyses on a wide range of vessels: multihulls, commercial vessels, fast planing hulls, and displacement hulls. For which of them the results were more eye-opening?

Christoph shared his personal view on this: “I have always been a fan of sailboats, so from my personal point of view, I find it really fascinating to run CFD for these kinds of boats. We analyze the boat in its real sailing conditions, such as the heeling angles, and also optimize the keel and appendages.

Multihulls are particularly interesting. We've learned a lot by studying and modeling the interactions between the two hulls. It is much more complex than monohulls, as the flow around one hull is affected by the interference of the flow around the other hull.

When it comes to superyachts, CFD can also be crucial for determining the placement of appendages like stabilizers, rudder and propellers, as well as understanding their interactions. Precise CFD analysis helps optimize performance and reduce interference.”

Recent Project: An aluminun explorer sailing yacht

CFD analysis recently played a decisive role in optimizing an aluminum explorer sailing yacht. We examined various variables, such as different hull shapes, keel shapes, and keel positions at various heel and yaw angles. Based on the results of these calculations, we iterated the design to reach the optimal configuration.

Additionally, we conducted validations on dynamic trim to determine the lift under the bow. Knowing in advance whether the bow would lift sufficiently or dive down was crucial for optimizing the shape. This allowed us to refine the design for better performance and stability.

The real goal: Guiding clients towards the best choice for the intended use

This service must be approached holistically, integrating it with the design process, and iteratively informing decisions. At iYacht, we achieve this through the collaboration of the various specialists we have in-house. Across our two offices, we rely on a team of 11 engineers, designers, and naval architects.

Above all, CFD is one of the many tools that we use to reach our goal: guiding clients toward the best choice.

It is essential to take the time to analyze the client's intended use of the boat. Early on, it's crucial to determine the specific speed for optimization. In most cases, optimizing for cruising speed is paramount, as it aligns with the client's desires

We aim to design hulls that perform well under various conditions, and for this, we run different CFD analyses in many conditions, as marine conditions are highly variable, with different seas and winds. Running CFD analysis only on certain wind and sea conditions could lead to an extreme hull that excels in some scenarios but is uncomfortable and underperforms in others. Therefore, our analyses strive to offer hulls that balance performance across diverse conditions.

Our approach is to use CFD wisely within the full iterative process of design, naval architecture, and product engineering. Integrating this method into our process allows us to predict the vessel's behavior reliably before production, ensuring optimal performance and client satisfaction.