Proposed National Keelboat Rating System
Special Interest Groups!! (Final – Part 5)
Having already covered the displacement keelboat’s, we now look at the planing sportboats and bigger asym rigged sleds, and how to rate them!
These boat’s perform with the same profiles of regular keelboats upwind, but downwind all the rules change. Two issues stand out, the terminal hull speed and secondly the apparent wind pressure issue(s).
The terminal hull speed occurs on sportboats when the planing hull’s resistance equals the opposing driving force of the sail area. In a simple world, one could have just used the power formula times the waterline speed and end up with a residual downwind speed number. Unfortunately, the hull’s resistance rises at an exponential rate, and peaks before the theoretical downwind speed on many sportboats.
The system has therefore used a formula to apply the hull speed against the immersed DLR # to generate an estimate as to when the hull speed will max out! This works very consistently from lightweight day racers through huge asymmetrical rigged sportboats, all of which plane at substantial downwind speeds.