here’s wally

From the Fabulous Forums…
Reading the “Wally” article on the Ockham site was intriguing, we often race on narrow water where there are short lived shifts and Lioness is not nimble, so we often grit our teeth and ride the header in preference to trying to tack every 5 degree header asI would in a laser or a J24 knowing we are forced to take the tacks at the shoreline.

Reviewing the Exp log files in < 8 kts, we never get to full target speed before we have to tack at some wind directions on sub minute legs.  The engineer in me is trying to figure out how to model the drag to determine the trade off of tacking vs sticking with a header, and the relative value of pinching per the “wally” method vs sailing to Tgt wind angle at a larger angle to the mark.

Taking the available wind energy as the square of the speed is a simplification, neglecting shear and presuming correctly trimmed sails. Using that, I can get a reasonable fit with a 4th order polynomial, but I cringe at the loss of “physicality”. I would expect linear and quadratic terms for hull skin drag and turbulence, as well perhaps as an apparent wind linear and quadratic for rig/sail drag. Playing around in Excel with “solver” I can get a decent approximation with arbitrary units based on kts of wind speed using linear and quadratic terms for hull vs boat speed and rig vs Apparent wind speed. I added an arbitrary stab at wavemaking “HullSpeed”, using 1/(max-BSP) to force it to get very large approaching hull speed.

Any pointers to either where the drag coefficients are hidden in the US Sailing performance package, or if there’s a simple analytical expression for wave making resistance?

Once I have a SWAG at straight line, flat water drag then the losses in a tack become more interesting, fitting actual deceleration/acceleration results vs TWS and rudder angle etc.

Jump in the thread.