The following formulas will give you approximate sail areas:
Mainsail = (P x E) / 1.87
155% Genoa = (( J x I ) / 2) x 1.63
135% Genoa = (( J x I ) / 2) x 1.42
100% Jib = ( I x J ) / 2
Racing Symmetrical Spinnaker = 1.5 x J x I
Racing Asymmetrical Spinnaker = 1.5 x J x I
Cruising Asymmetrical Spinnaker = 1.4 x J x I
Formula Definitions
I is measured along the front of mast from the genoa halyard to the main deck. The main
deck is where the deck would be if there were no deckhouse.
J is the base of the foretriangle measured along the deck from the headstay pin to the front of the mast.
P is the luff length of the mainsail, measured along the aft face of the mast from the
top of the boom to the highest point that the mainsail can be hoisted, or to the black band if there is one.
E is the foot length of the mainsail, measured along the boom from the aft face of the
mast to the outermost point on the boom to which the main can be pulled, or to the black band if there is one..
ISP is similar to I. It is measured from the highest spinnaker halyard to the main deck.
JSP is the length of the spinnaker pole or the distance from the forward end of the
bowsprit (fully extended) to the front face of the mast.
Py and Ey, respectively, are the luff length and foot length
of the mizzen of a yawl or ketch, measured in the same way as for the mainsail.
Iy is the measurement from the staysail halyard to the main deck.
Jy is the measurement from the staysail stay pin to the front face of the mast.
LP is the shortest distance between the clew and the luff of the genoa.
