Magnitude 80

Computational Fluid Dynamics Notes on "Magnitude 80" by EganTech's David Egan

Design of the appendage configuration for Magnitude was in many instances designing in unknown territory and the Computational Fluid Dynamic tools enabled testing the boat in the computer prior to building. The canting keel offers high stability with a much smaller fin and bulb, where as the forward dagger board can create a lot of lift when needed and conveniently retracted, saving wetted surface drag when not going to weather. The design implications of the configuration are quite overwhelming at first. Concerns of balance issues while sailing especially with transition from the canting keel to the Center Board. Or more simply where should the appendages go and how big should they be? Although the smaller bulb and the canting keel are an obvious improvement, these boats go very fast even in light air so what are the effects do the bulb and fin so close to the surface have on wave drag? At the project start there were many questions so a fast CFD (Computational Fluid Dynamics ) analysis scheme was implemented to quickly work through all the various options of canards, rudders, bilge boards and the design trade–offs. Helm balance was a major concern and many full configurations were analyzed with the results then incorporated in VPP analysis to determine the net benefits of the various options. Very quickly the Dagger Board option became the favorite for drag especially with the ability to retract it down wind! Looking at the underwater image of Magnitude 80 we see, with the color coding by pressure (the hot colors are high pressure and the cold colors are low pressure), that the rudder, fin and centerboard are all working to reduce leeway.

If we exaggerate the scale a bit we see interestingly enough that the three appendages are all working in different flow regimes, which led to the development of three different foil sections, each designed for minimum drag at their design operating condition. Help here came from Ian Howlett in the UK in sorting through the masses of possible foil selections available today. With these foils sections chosen, the CFD and VPP were again used to match the correct plan form areas of the rudder, fin and centerboard. The correct amount of lift is maintained on each fin so that all acting together give the right balance on the helm. Care was taken here to keep the “design”as forgiving as possible.

When fully canted and heeled, both the bulb and fin find themselves very close to the water surface so a volume distribution for the bulb was chosen to minimize the drag of this effect along with fairing and strake for the fin and canting keel mechanism. The wave effect of the bulb and keel was accurately modeled and is clearly seen in the wave trough over the fin and bulb.


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