John,
Please read this section of the class relues regarding how to measure a Spi.
Box rule And in particular:
1.14 The gennaker
1.14.1 The maximum distance measured from the top of the main beam to the highest theoretical point to which a gennaker sail can be hoisted is referred to as the "gennaker hoist height". The gennaker hoist height shall not be more then 7.50 m. (= 7500 mm)
1.14.2 The distance from the top of the main beam to a mast gate may be used for the gennaker hoist height measurement when no part of the gennaker can be hoisted past the distance measured.
1.14.3 The gennaker must satisfy the following two shape and size conditions:
[color:"red"] SMG > 75% * SF (= shape condition) [/color]
Max. Gennaker area = SF * (SL1+SL2)/4 + (SMG-SF/2) * (SL1+SL2)/3 = 17.5 sq m (= size condition)
Where:
* SMG is the width at mid-height, which shall be taken between the mid point of the luff and the mid point of the leech.
* SF is the length of the foot of the sail measured around the edge of the sail, between the lowest points of the luff and leech.
* SL1 is the length of the luff of the sail measured along the edge of the sail, from the highest point of the sail to the lowest point of the sail along the luff.
* SL2 is the length of the leech of the sail measured along the edge of the sail, from the highest point of the sail to the lowest point of the sail on the leech.