Keel / centerboard area ?

Just a small question / verification.

For such boats (http://users.win.be/ws102917/microclass/rules/fig1.gif , www.microclass.org ).

18 ft loa (5.5m), 3 ft 7 in (1.1m) max draft , sail area 200 sqft. (18.5 m²)

The keel/CB area should be around 200 * 3.5% = 7 sqft. (.65 m²)
3.5% comes from principle of yacht design.

Kell span = 3ft 7 - 10 in (canoe draft) = 2 ft 9 in (85 cm)
Mean chord = 7 / 2ft9 = 2 ft 6 in (76 cm)
That should give an AR about 1.1.

Is the assumption of 3.5% incorrect, or is the draft very limitative for this class ?
Or any other thing I missed ?
(Even with 140 sqst sail (SAD = 20) , I dont get more than AR 1.5).

 

Transverse pressure on sails = righting moment / heeling arm. If we assume windage on hull topsides and lift generated by hull underwater are equal, then transverse pressure on foils (keel + rudder), aka lift, = transverse pressure on sails. Lift is speed dependent, so the boat will make more leeway before it gets to full speed, and less once you have it churning along nice.

Estimate the wind speed, the resulting boat speed (the churning along nice one), the heel angle, and the righting moment at that heel angle. Then use the lifting line formula to solve for leeway angle. You want it to be 2.5 to 3 degrees. With 1.5 to 2 degrees of weather helm, your rudder should be operating at an avr. angle of incidence of about 4.5 degrees.

Faster boats and boats with higher aspect ratio keels generally require less foil area, and visa versa.