December 8, 2015
Quantum Expert Answer
The biggest effect of clew height is on the relative performance while reaching versus upwind. Obviously, total area has an impact on the horsepower available in light-to-moderate air. Also, one could argue that raising the height of the center of that force upward might have an impact on heeling moment, but this is relatively subtle and hard to quantify. A low clew keeps the sail closer to the centerline as the sail loads. This helps upwind. A second possible effect, if the clew is low enough, is to allow the addition of material (foot round) so that the sail actually touches the deck. This creates an "endplate," which minimizes the escape of air from one side to the other and the disruption of the pressure differential that makes the sail work. This is why pure racing genoas typically have a clew height of less than half a meter off the deck.
The downside of a low clew is the effect on sheeting angle as the sail is eased out for reaching and running. The angle of the sheet quickly changes if the clew is low, so that the sheet is pulling in, not down. This allows the clew to rise, and the sail "twists" or opens up along the leech, causing the top of the sail to luff well before the rest. With a higher clew, the angle remains more constant, and the sheet pulls down even when the sail is eased. Higher clewed sails setup better on a reach. Specialty racing headsails, ("jib tops" and "blast reachers") have higher clews for just this reason.
The only other issue to consider when selecting clew height is a staysail. If using a headsail in combination with a staysail set on an inner forestay, the clew is often raised to keep the two sails from interfering with each other and to make the sail easier to tack.