PY Numbers (again)

Originally posted by Daniel Holman


   
Originally posted by Daniel Holman


Graham is bang on regarding disparity in sailing ability being far more telling than even a large (30 pt) reduction in PY. I guess until Steve Cockerill is cloned to sail all these boats against himself in a variety of conditions it'll always be tricky.
Below is a graph for hiking S/H boats that I put together, with actual PY against predicted according to the formula:
Predicted PN = 
(0.4*(3198*(Length^-0.7713)) 
+(0.05*(849.37*((Tot Displ/Length^3)^0.3367)) 
+(0.4*(2658*((RM/Tot Displ)^-0.4516))) 
+(0.15*(947.7*((SA/L^2)^-0.2454)))
Where the RM is calculated by an assumed sailor height / weight pertaining to each class, and some biomechanical data.
Most of the scatter, esp in the faster boats can be attributed to waterline beam, i.e skinny boats like canoes, RS300, non foiling moth, will be quicker than their parameters would suggest as the L/Bwl will be higher, so lower drag for most of the speed ranges dinghies operate in. Accurate Bwl data is pretty hard to come by tho..
Other causes for scatter include: Design excellence - i.e within these parameters you could have 2 boats that would appear identical on paper, one is shaped like a matchbox or has a highly inefficient rig, the other one is a more optimal design. Also the "PN effect" - rarer designs/less widely sailed designs/less generally well sailed designs will not show as well from the PN returns, hence will have actual PNs suggesting they are slower than they are. Lasers are the converse of this - to the left of the line despite being quite crude in many ways. 
The streaker is to the right of the black line, suggesting that its actual PY is a bit lenient, and a 30pt drop would bring it near the black line.