While news on the North Sails Moth developments are a little slow (Testing is still ongoing with our Version 6 designs shipping to Melbourne next week) I thought I would show moth sailors a little bit of what we have been working on with larger yachts.
While this may not seem applicable to Moths, in general Mothies are also techies and so maybe this will be interesting. Also, in the very near future we will have the VPP setup to rank moth sail designs and trims in a similar manner. What is interesting about making a VPP for a moth is that it may actually be a little easier to tune and acheave accurate results. Mainly because if we concentrate on foiling conditions, it is easy to calculate the foil drag as they produce a known amount of lift (Yacht and Crews mass minus any lift from the sail due to weather heel). Therefore with a known amount of lift, the drag can be established easier for a given boatspeed than a traditional yacht which has more variables.
What follows is a bit of background put together for the division of North Sails known as the Design Services Group (NSDS). NSDS is where I work, in a nutshell we provide designers and yacht teams with answers using technology developed for the AC. There are some pretty interesting projects from rig tuning and development, to sailplan optimization, load studies, etc.
Anyway... North Sails Design Services provides yacht designers and projects access to tools for independent analysis which can be used to evaluate different options effecting performance. While North Sails is not in the business of designing yachts, North Sails does believe that a understanding of the entire yacht/rig system and how the aero forces effect the hydro side of the equation. Understanding the whole system allows us to develop faster sails and help designers improve their yachts.
The North Sails VPP System is unique in that it is the only 100% CFD based VPP which is available for designers to use. Why is a CFD based system better than standard empirical and coefficient based systems? Traditional VPP systems use either empirically based or formula based techniques to calculate hull drag and a sailplan’s forces. For example, a standard VPP may use a formula based analysis of a hull (IMS Style or Delft regression formulas) to estimate the hull drag on a yacht. This leaves quite a lot of room for error, especially when testing different hull forms or sizes which are dissimilar to the hullforms which the delft regression series arrived from.
Usually the sailplan’s aero forces are coefficients (Usually from wind tunnel measurements) that have been tuned by reverse calculating the VPP performance estimates to match real life measurements. Therefore both sides of a traditional VPP’s equation leave some room for error when scaling from one yacht to another.
By using CFD for both the Aero and Hydro sides of the VPP equation, errors from scaling & adjusting the form factor of the Hull and Sailplan are reduced. Below is a validation study where the hull and sails of the Transpac 52 Platoon (2008 J/V Design) were modeled in CFD. These results were plotted against logged data from races which the Platoon won in the 2008 Audi MedCup series.
As you can see the VPP appears to be more accurate than the onboard instrumentation of the yacht (State of the art BravoRacing package of electronics- same as used by Alinghi). Besides the wide scatter of data, the instruments also appear to have overestimated leeway (Although I am looking closer into this- could TP-52's sail with 4+deg of leeway and still win races?) this has effected the recorded TWA (Sightly wider) and results in a slower VMG. The formula used by the instruments to calculate leeway is: LEEWAY = (12 * Heel) / BOADSPEED^2 .
What should be interesting for many yacht designers should be the fact that the North Sails VPP system is capable of making accurate predictions of yaw balance and rudder angles. This is the result of testing many variables such as sail meshes, sail force calculation options, vortex modeling, hull meshing, foil analysis, etc before fully understanding how to best use Flow/DasBoot/VPP to provide the most accurate yaw balance predictions. The tests with the Transpac 52 did not have any adjustments or changes to the rig position, but if a designer wanted to test yacht balance, the VPP allows for easy adjustment of rake and rig position relative to the yacht to test different balances. The VPP would then give results of the changes including performance and rudder angles.
Flow/MemBrain is the design suite which North uses to very accuratly predict sail shapes. These shapes are then tested in CFD and forces are created. Flow is a Vortex Lattice CFD Code which is used to calculate sail forces and pressures for the VPP and the FEA solvers. MemBrain is a Non-Linear FEA model incorporating the rigs longitudinal stiffness, transverse stiffness, the JG and EA as well as the running and standing rigging’s EA’s. The stiffness properties of the sails are modeled based on the actual layouts , fiber modulus and yarn distributions exactly matching the sails that will be built.
MemBrain balances the internal stresses of the rig and sails with the external loads from Flow or other CFD programs such as CFX. Internal stresses such as rig pretensions, sheet adjustments, sail controls, etc can be adjusted as in real life to ensure that the sail shapes tested in Flow match the same trim as expected onboard. This reduces the need for estimates on sail area reduction coefficients (Reefing) or sail trim coefficients such as (Flattening) used by most VPP’s.
The above image is of the rig tune used in a recent project. Rig tune can be adjusted at an extremely high degree of accuracy. This applies to the moth rig as well.
Downwind sails can be modeled using CFX to test sails at wider AWA’s or which are likely to be turbulent. CFX can couple with MemBrain in the same manner as Flow.
North Sails design services also has access to one of the best free surface Hydro CFD programs in the world. DasBoot is a Potential Flow, Free Surface Code used by Alinghi, J/V Yacht Design for their TP-52’s & Volvo 70 development for Mean Machine. DasBoot was written by Michael Richelsen who also wrote Flow, MemBrain & the North Sails VPP. Originally DasBoot was written for the Illbruck IACC Challenge and later refined during the design of the Volvo 70 Mean Machine (Which unfortunatly ran short on money). The results from DasBoot are used to generate hydro resistance files for the North Sails VPP. (DasBoot is a potential flow free surface code. Doublet & Source singularities. But unlike most hull codes DasBoot has a boundary layer model)
