The number 135 refers to the percent of the boat's fore-triangle area that the genoa occupies. In other words, your 135 genoa is 35% larger in area than the area of the fore-triangle of the boat. The fore-triangle would be measured by the area of the triangle formed by the distance from the mast base on deck to the top forming one side, the distance from the mast base to the jib tack on the base side, then the distance from the tack to the top of the mast (or to the point where the jib stay intersects the mast) for the third side. Those measurements I think are in our owner's manuals, but any decent national sail maker would have them since its such a popular boat.
Actually, the area of your sail is probably easily obtained by contacting Catalina or a reputable sail maker.
(and if I can remember my High school geometry the area of a triangle is 1/2 base X height (assuming a right triangle.)

I did not do that well in geometry, I am so confused.
Is it a right triangle or starboard triangle? If you looked at it from the other side would it be a left triangle or port triangle?

Our encylopidia show standard rig J and I dimmensions to be 14.4 and 44.9. 14.9 X 44.9/2 = 323.28 + 35% = 436.43 square feet. I think.

A right triangle is one that isn't wrong. Big collective groan!

I don't have time to make a drawing showing all this, but a 100% jib actually has more sail area than 100% of the foretriangle. Similarly, a 135% Genoa has more than 135% of the foretriangle area.

The reason is that the percentage refers to the Luff Perpendicular - a line from the clew to the luff that is perpendicular to the luff and how that number relates to the J measurement (base of the foretriangle). If you draw it out and do the math, you can prove this for yourself.

Another related fact is that a high-clew headsail will have the clew come back farther aft than a deck-sweeping sail, assuming they both have the same LP. A drawing clearly shows that the high clew's perpendicular occurs higher on the luff, and the luff is angling aft as it attaches to the mast. You gain sail area abaft the mast, but lose some at the foot forward of the mast and the sail area is about the same.

I knew I would get corrected when I made that post. but now my head hurts.
:rolleyes:

All of these calculations to figure out the cost of cleaning a sail. That'd why as a MKI owner, I just plug scuppers in the cockpit fill it with water a little soap and some bleach. I then put the sail in there, take off my shoes and do the Macarana or Watusi on the sail. Drain, rinse and hang to dry. No mental exercise required.