I could see where reducing UPWIND sail area and replacing it with DOWNWIND sail area could indeed slow the overall rating down.
Think of it this way... you're spending a LOT more time upwind sailing than downwind sailing on a typical W/L course, even if the distance is identical.
That makes sense given a few assumptions:
- That a boat's downwind speed is greater than its upwind speed.
- That a boat sails the same distance upwind and downwind on a windward/leeward course regardless of sail plan.
- That additional sail area in jib or spinnaker results in the same percentage speed increase upwind and downwind respectively.
Now, I agree that (1) an Isotope generally sails faster on a downwind reach than on a beat to windward. If you assume that a catamaran both tacks and gybes through 90 degrees, then it sails the same distance upwind and down. Therefore, in a W/L round-the-buoys race, more time is spent beating than reaching. If additional sail area in jib vs. spinnaker result in the same percentage speed increase, then it would be better to increase the jib.
However, I don't agree with assumptions 2 and 3. When sailing upwind, there's a diminishing return on additional jib area. As long as a 70% self-tacking jib attaches flow to the back of the mainsail, a 130% overlapping jib won't add a correspondingly large speed increase to the boat. The Isotope jib is large for better downwind speed, not upwind speed. It isn't as effective as a spinnaker, though. Also, a sloop-rig won't sail the same course downwind as a spinnaker boat. A spin-boat can sail deeper angles, resulting in a shorter total distance. Furthermore, because a spinnaker lifts the bows, you can push a spin-rig boat harder downwind with less fear of pitchpoling.
But mainly, adding 10% to a sloop's total sail area in the jib doesn't provide nearly as much benefit as increasing the total sail area by over 90% in a spinnaker. At least I don't think so. Do you?
