Doug Stewart from Quantum Sails looks at Genoa sizing as it relates to sheeting position and performance.
Have you ever wondered why the clew (the corner the sheets tie to) of your Genoa is so far out of reach that you cannot adjust the leech line? Did you ever wonder why the clew was so low you could not see under it? Here are the reasons why headsails are sized the way they are and what limits the finished area of a sail.
All boats come with either a dedicated track for the genoa cars to slide on or a rail thata snatch block can clip to. The location of the track as it relates to the width of the boat as well as the length will determine not only the size of the sail but also its upwind performance.
To start, lets clear up some terminology as it pertains to measurements on the boat. The J dimension without being to technical is the measurement from the front face of the mast forward to the point where the head stay enters the deck. The LP or luff perpendicular is a measurement taken from the clew of the sail to the closest (tangent) point on the luff. Taking the measured LP of any sail and dividing that number by the J measurement on the boat will then give you the size of the sail. So if we had a J of 10′ and a LP of 13′ the LP as a percentage of J would be 130%.
In Drawing 1 notice that the Clew of the blue, purple and orange sail all fall along the imaginary 110% LP line. Area of a Genoa = Luff * LP * .5 and because the LP is the same on all three genoas, the area is also the same on all three.
Sheet lead position for most headsails can generally be found by bisecting the angle of the clew. The higher the clew, the lead moves aft and the lower the clew the lead moves forward.Notice how the Orange and Blue sails sheet lead miss the track due to the clew being either to high or to low. The purples sail on the other hand is perfectly placed leaving enough room for this sail to be reefed if needed. For every roll that is taken when reefing, the Genoa car must be moved forward to ensure that the clew angle is bisected by the sheet which will in turn keep equal tension on the foot and leech. A good idea is to have reefing marks on the foot of the Genoa that can be guides to roll to when reefing.The lead position for these reefing points can also be marked on the track so the car can be moved while reefing.
Notice on the 80% jib (black sail) the clew must be considerably higher then the Purple sail in order to lead to the similar position on the track. It is not an option, based on where the track is, for this 80% Jib to have a low clew.
One question frequently asked is what the performance difference is between two sails with the same area but different clew heights.Drawing 2 represents an overhead shot of the sails in Drawing 1. Notice how the higher clew sails sit further outboard then the lower clewed sails. Generally, the higher the clew is off the deck, the further outboard the clew will fly. Just like easing the sheet out slightly when beating upwind which will result in a loss of speed and pointing ability a clew that is higher and further outboard will result in a similar performance loss.
So how does all this newfound information help you, the everyday cruiser? When trying to figure out what size Genoa to order for your boat, discuss sheet leads and clew heights with your sail maker and what impact these choices may make on the performance of your boat.