Very high speed prediction

[Jimboat]

Interesting thread!

Savitsky's model was breakthrough in 1964 for empirical understanding of Lift/Drag. When we applied to high-performance planing hulls, we found that these were more specialized cases. The strong interaction of several lifting forces – all with interaction with each other – required specialized solutions. We spent several years doing wind tunnel and tow-tank research that was specific to high performance planing powerboats (Vees and tunnel hulls and modified vee hulls). Correlation to Savitsky’s pure model is not really applicable, since so many other forces are in action.

[Yellowjacket]

Racing, by its very nature, sometimes finds ways to bend the laws of physics, as we know them. It can’t break the laws, but it tends to find any loopholes that the theoreticians overlook. Small racing hulls are clearly are going faster than Stavitsky predicted, probably for a myriad of reasons and interactions. In addition to perhaps trapping more air under the hull due to the aforementioned surface effects, aerodynamic lift is going to be present on the aft flanks of these hulls. Getting 20 or 30 pounds of aero lift could make a big difference in a boat weighing less than 500 pounds, and unless you had surface pressure measurements all over the hull, or a CFD aerodynamics analysis, you'd never know it. Prop lift could be a big factor too, and these are but three factors of probably a half a dozen things that could make Stavitsky predictions off by enough to be useless.

I was hoping that I would find some info on the web that would be a guidance in determining wetted surface width at the transom for a Class C/D kind of runabout. The Savitsky analysis results indicate that you can reduce the drag at 45 knots by almost 32% if you brought the bottom width down from about 34 inches to around 20 inches. I would suspect that the ride would improve too, so there is a big incentive to do that. But the question is, would the drag be higher for the wider hull at 70 mph, or would the narrower hull be faster up there? I suspect that the only way to know for sure is to build both boats and test and find out. Not sure that surface effects from even small chop could be done with any accuracy in a CFD analysis.

Padded V hulls (which we knew nothing about 40 years ago when the C/D utilities were in their heyday) with much higher planing surface loadings are running at pretty impressive speeds. As Jim noted, you can't expect these hulls to obey Savitsky predictions since just the aspect ratio of the pad is so long in addition to all the other things going on. But I am curious as to whether any analysis tools that are out there could predict what a 500 pound 17 inch tall, 13 foot padded V could do compared to what the flat bottom C/D hulls are doing with 50 hp?

[Jimboat]

good stuff. BTW, our software tools accurately model and predict the kind of differences in design and performance that you're referring to, Yellowjacket. The forces (for recreational or race hulls) generated from vee surface hydrodynamic Lift & Drag, hydrodynamic Pad Lift & Drag, aerodynamic surfaces Lift & Drag, drive/lower unit Lift & Drag, deadweights, etc. And yes, the deadrise and aspect ratio of the planing surfaces does make quite a difference to the L/D generated. As for the benefits of aero Lift, any amount of aero lift is a big benefit, since it reduces the ultimately required hydrodynamic drag, so effects are quite dramatic. The simultaneous solution of all these forces is somewhat complex however.