"Those who fall in love with practice yet without science are like a sailor who steers a ship without helm or compass, and who never can be certain whither he is going"
Leonardo da Vinci
For all Owen Clarke Design projects, the common denominator is performance, regardless of whether the yacht is a cruising or racing design. Our designs always prove to be fast. This is partly due to the tools that we have available to us, the way in which we use them and our partnerships with some of the world’s leading engineers, designers and marine technology consultants.
As professional engineers and designers we offer our skills, experience and use of the tools at our disposal to individuals, yacht designers, boat-builders and other agencies as well as applying their use to our own designs. OCD have recruited qualified naval architects in recent years in order to expand this area of our business. We have continued to invest time and resources in the application of analytical methods such as Computational Fluid Dynamics (CFD) and Finite element analysis (FE) to fluid and structural engineering problems, expanding the use of these tools through our range of designs as the software and services have become more cost effective.
The starting point of any new design is to develop a set of hull lines which define the geometry of the boat. We’ve been using Maxsurf, the world’s leading yacht design software since drawing the lines of our first racing boat, the 35’ trimaran Fiery Cross, in 1987. Since then the product has also been developed for use in the design of commercial craft and ships. It remains however the most utilised yacht design software for designers the world over.
Maxsurf, hull, deck and appendage design
Maxsurf is a powerful three dimensional surface modelling system allowing for systematic experimentation and rapid optimisation of hull lines, appendages, decks and superstructures. With a customised interface, the various software modules use a wide variety of modeling aids and fairing tools, Maxsurf can generates hull forms to high degrees of complexity, including curves and dynamically trimmed surfaces. The highest number of surfaces we have modelled so far as part of a hull, appendage and deck model for our latest Open 60 is seventy two.
The program's deliverables include hull lines, comprehensive offsets tables and transfer files for 2D/3D drafting programs and naval architecture design suites such as HydroMax. Maxsurf analysis modes include upright hydrostatics, surface areas, hydro-static centers, moments, and offset data.
Design analysis of either a new yacht or modification of an existing design in respect of hull, appendage or rig/sail plan requires an understanding of the fundamental parameters of its geometry such as wetted surface area, waterline beam, prismatic coefficient, righting moment, etc etc. In order to undertake this work we transfer hull, deck, superstructure and appendage files to another program in our yacht design suite called HydroMax, a high angle stability analysis program.
Hydromax, large angle hydro-statics
Owen Clarke Design uses HydroMax to study a yacht’s static behaviour at defined angles of heel up to 180 degrees. The programme takes hull lines generated by MaxSurf and produces stability curves and other hydrostatic information for a given loading heel angle and free trimming condition. It produces righting and flooding curves that are required by racing boat regulatory bodies, by the Marine Safety Agency and for the Recreational Craft Directive (RCD) for safety purposes.
The information created by Hydromax can be read by VPP (velocity performance prediction) programs and be used for the adjustment of certain characteristics of the yacht. For instance the addition of lead in a keel bulb, or water ballast in a wing tank can be calculated as a result of the changes to a yacht’s displacement, center of gravity and righting moment calculated by the program.
The design circle is closed when Hydromax calculates the effect of changes on the hydrostatic characteristics of the yacht, and feeds this information back into the VPP software. The relative negative/positive effect on drag of wetted surface area, waterline beam etc, against the righting moment/stiffness (which can be equated to thrust) is analysed by the VPP program.
WinDesign, Velocity Performance Prediction (VPP)
The prediction of a yacht’s behaviour using this software is applicable throughout the design process from preliminary candidate studies, analysis of tank/wind tunnel/CFD results, to final hull, appendage and rig selection.
OCD are in a privileged position regarding the development of WinDesign and have access to data and use of the program related to the performance of canting keel and open class yachts not available in the public version. The reason for this is that Clay Oliver, former Farr YD and Emirates Team NZ and the creator of WinDesign (used by over 350 design offices worldwide), has worked with us since 2007 as co-designer on the development of our Class 40 and Open 60 yachts. Experimental results from sixteen swing keel yacht models (1/7th and 1/3rd scale) since 2000 have been used in the modification of this VPP and have created a unique tool for us to work with.
For the sailor, the output from the VPP program includes not only target boat speeds but also data such as sail crossovers, optimum heel and sailing angles as well as a variety of other deliverables. In addition to working from internal course models WinDesign can analyse candidate designs utilising different weather scenarios with the help of our own meteo database or from material supplied by the project team. This allows us to design a cruising yacht for a circumnavigation, or optimise a race boat for either an inshore, Atlantic or round the world race. The direct benefits for the sailor of different design choices are easily recognised by referencing the outputs which can be displayed as seconds per mile or in minutes/hours over an ocean course.
The program is very flexible and we have also used it on a consultancy basis many times. For the 2001 America’s Cup Jubilee Regatta we designed a new foil package for USA 61 (Tom Blackaller’s Freemantle 12 metre with the twin rudder system), after analysis using the vpp with ten years averaged weather from the East Solent. More recently we looked at sail damage scenarios for Mike Golding in order to minimise the sail inventory onboard his open 60 Ecover. This ensured that the right number and range of off wind sails were carried to anticipate loss/damage during the various ‘stages’ of the Vendee Globe.
Router, race modelling
In addition to analysing designs using the internal tools within WinDesign on our most recent projects we have been iusing Router. This is a race modelling program that uses yacht polars from the VPP and historical weather data in the form of GRIB files. As of 2010 OCD have access to historical weather data covering all the Volvo Ocean Race legs and the Vendee Globe for all years since 1990. For future projects we'll be expanding our weather library to include Trans-Atlantic events and ocean/offshore races such as the Transpac and Fastnet, as required.
The output data from running the program is primarily the distance sailed and the time spent sailing by the yacht in a matrix of true wind angles and true wind speeds during any given race. Router can be used for the design of new yachts, modification of existing designs, sail plan and sail wardrobe development.
Router software was developed by Michael Richelsen of North Sails for the Ilbruck Volvo project. It has since been developed over several versions, is now available commercially and was used by most teams in the last Volvo Ocean Race. We are the first design office to run Router with weather files in GRIB format which improves the speed of input and the quality of the output data from the program.
Among the advantages of using Router over the way we have undertaken race modelling in the past is that we are now in complete control from start to finish, no longer reliant on a third party and all work can be carried out in-house which makes the process more flexible and transparent to clients.
Tank and Wind Tunnel Testing
When not using test data, velocity performance prediction programs are restricted to using their internal mathematical algorithms to calculate values of aero/hydro lift and drag. In broad brush terms VPP’s do this very well and for similar hull forms and rig types they provide good indicators of comparative performance. However there is a limit to the accuracy of any generic VPP that relies on an internal algorithm for the calculation of results. One can imagine that the analysis for a moderate displacement IRC 45 which has a relatively standard hull form (not greatly dissimilar from those used in the Delft series of tank model tests from which most VPP internal mathematical algorithms are derived) is quite different from a light weight canting keel Volvo 70 or Open 60, with dagger-boards, ballast tanks etc.
A design office that uses model testing has access therefore to more reliable data for input into any VPP analysis. The result is that the likelihood of error is eliminated when comparing different designs, and outputs such as speeds, sail cross-overs etc are more accurate. In the bigger picture, and contrary to what one might think, model testing allows a designer to be more adventurous and creative with their ideas in the first place in the knowledge that the test results will prevent them from incorporating a ‘bad idea’ and ‘adding slowness’ into a good design. Model testing helped Owen Clarke Design introduce many innovative design ideas into the IMOCA 60 class for the first time with confidence, many are now common to all Open 60's, including, central ballast, three forestay sailplan (Kingfisher), the masthead rig (Ecover 2) and also the interceptor (Ecover 3) which we’re sure would have been taken up except it was so effective the class banned it for later boats.
Tank and wind tunnel analysis is not only useful for race boat designs. The mitigating risk factor is an important one, especially for production yachts or large one off custom cruising boats. Where the cost of model testing is a fraction of the project budget then model testing can be considered money well spent (especially for non standard designs/design features) to review areas such as: powering/engine selection, sail plan and/or specific sails, and hull design.
Computational Fluid Dynamics (CFD)
CFD is a branch of fluid mechanics that uses numerical methods to solve and analyze flow. Computers are used to perform the calculations required to simulate the interaction of fluids with surfaces, in the case of yacht design; appendages, hulls, sails and spars. With the advent of high-speed and lower cost computing CFD can be applied to many more problems and on smaller budget projects than ever before.
Owen Clarke Design has been using CFD as a tool since the design of Kingfisher in 2000. Since that project there hasn’t been one of our large race boat programs that hasn’t included CFD analysis, nor one of our smaller race boat designs that hasn’t benefited from that research. The illustration below shows the pressure distribution on Kingfisher’s hull and appendages modelled using a commercially available RANS code based software called CFX. For this project although the whole hull and appendages were modelled, compared and calibrated against the results from tank testing, the primary aim of the investigation was the interaction of the three appendages, distribution of side/lift forces and the optimisation of their attack angles. One of the advantages of CFD over model testing is this ability to look at the lift forces and centres of pressures on individual appendages and sails, as well as providing a visualisation of the results. This is something that has been impossible or difficult to do using models. On the other hand in broad terms it is true to say that CFD’s weak point is the calculation of drag which model testing is very good at.
CFD software varies widely in both its cost (be that rental, purchase, licence fee) and application. The more expensive RANS based software is only applied to very complex tasks, such as problems which involve a free surface between two fluids (such as between water and air). Solving for free surface affects, waves, wake simulation etc requires complex solvers, while calculating solutions for a single fluid is simpler. Rudders, keels and bulbs can be dealt with by less time consuming and cost effective panel code software such as X-foil. When looking in detail and in isolation at section analysis for a mast tube, or rudder for instance then we would use a two dimensional software such as MSES. MSES was used recently to develop a new daggerboard section for our latest Open 60 after post-analysis of data from a parallel program of third scale tank testing and CFD. Since we were looking primarily for appendage solutions and not concerned with viscous drag and wake simulation we were able to utilise the simpler panel code software REVA, reducing process time and costs, thus allowing some of the r+d budget to be devoted to other areas of investigation.
CFD was first used in the aerospace and aeronautics industries and for naval architects it is not only a useful tool for the development of hulls, keels, rudders and dagger-boards. Through a long association over a number of years with high technology suppliers such as Southern Spars and North Sails France our clients have benefitted from the use of CFD based aero software packages such as Flow and Membrane. Most of our 60’ rig and sail packages have been developed using this software and it has been used to help define sailing loads in rigging, sheets and halyards since first used for the design of Ecover 2 in 2003. The cost of using this software has reduced over time and allowed us to use it as a tool during the rig development phase of our third generation Class 40 design in 2009.
While it's good that CFD programs have become faster and easier to use, it is important to make sure that their use is appropriate to the solutions being sought. We are in a constant process of reviewing the technology by attending design seminars and conferences, referring to professional journals and consulting with organisations specialising in CFD such as the Stevens Institute and the Wolfson Unit. One of our design partners, Clay Oliver earlier in his career wrote CFD code for both the US Navy and was the first to apply it in the Americas Cup and together these associates are an invaluable source of information and advice as to what is working and is not working in the world of CFD.
We have a great deal of experience and faith in the results generated in our field of yacht design when it comes to appendage and rig development. A cautionary word with regard to its use in hull development however: Because CFD produces such great graphics, is computer based and seen to be at the cutting edge of technology it is marketed by some as a panacea, a solution and remedy of all problems. However despite what might be said as part of a sales pitch those who use CFD at the 'sharp end', be it in Americas Cup, Volvo world or elsewhere still in the end turn to tank test modelling to be the final arbiter of truth when it comes to hull design. It is a fact that no Americas Cup program ever built an IACC 90 that had not been model tested. Despite several decades of analysis, tens of thousands of design hours and what must be several hundred millions of dollars spent in research there was still difficulty consistently modelling accurately what happened in and around a single displacement ‘low speed’ IACC hull and its wake. The fundamental question being; what is the drag force? What then the chances that in the space of a few short years, with far less resource than in the Cup that the same programs can be applied to lighter, planing variable displacement hulls with moving appendages, all of which so markedly affect the running trim (bow up, bow down) and hence drag of Maxi, Volvo 70 and Open 60 yachts ? At this moment of time we’d caution, none. We're not there yet.
That doesn’t mean that in the hands of experts CFD cannot be used to analyse and separate out candidate hulls for more in-depth analysis in the same way we use our VPP. However, this requires the use of RANS code software to simulate boundary and wake effects. The time spent meshing and the cost of running this type of analysis is at present equivalent or higher than model testing. Until costs are reduced and process times improved, because of our VPP/testing database for the moment we believe that for hull development purposes our present methodology is still the most efficient. For us it’s efficiency that counts, processing the maximum number of design candidates within the time and budget available.
We continue to monitor developments in CFD, invest in it and when this balance changes and the right project comes along OCD will be running a parallel RANS code/model testing hull development program. That time we understand is close at hand with shared time on a series of linked super-computers cutting processing times and cost, bringing affordable hull force solutions within the reach of mainstream as well as grand prix projects. In the meantime we will continue to use CFD for the design of rigs, sail plans, appendages and other applicable fluid/flow problems such as the drag associated with propulsion systems and appendages.
For more information regarding our naval architecture services email: NAVAL ARCHITECTURE
For an overview of services we've provided on past projects go to: CONSULTANCY
For information regarding engineering services we offer go to: ENGINEERING