December 7, 2015
Quantum Expert Answer
Probably a little bit of both. Once you decide on a target area, reduce both proportionally. As you reduce the LP, the clew will have to rise for the lead to stay on your track. Designed clew height should not get too high (3'-4' max if track allows) as this will begin to compromise upwind ability. Reducing luff length works well, especially in the interest of maintaining a reasonable aspect ratio (height versus width). If the sail gets too tall and skinny, it is hard to control the twist and overall shape. The downside is that the luff is the most efficient part of the sail in terms forward driving force (the back of the sail creates more sideways force).
As in many things, balance is the answer. A Quantum sail consultant in your area will be happy to look at your actual measurements and sail sizes to help you determine the geometry that best balances the trade-offs. Based on your description, it sounds like the right solution might have an LP of around 85%, and a luff length shorter by 2-3'.