here is what i calculated (guessed) when making my trim tab:
displacement 219 cu ft (this is the only true volume, the rest are
area times moment arms giving units of feet cubed, for details see
john letcher’s “self-steering for sailing craft” book)
rudder area 9 sq ft
rudder arm 13’-9"
rudder volume 124 cu ft = 57% of displacement
rudder balance arm 4" to 6" (this was hardest to make an estimate)
rudder self volume 3 to 4.5 cu ft
tab arm 3 ft
tab area should be 1 to 1.5 sq ft
i built the tab on a 3/4" tobin bronze prop shaft which equals maximum
foil thickness
my tab is 44 1/2" long and has a chord of 3 3/4" including the shaft
thickness is tapered over 2 1/4" to a trailing edge of 1/8" thick
balance portion is 15" long at very bottom of tab
leading edge of balance is rounded over 3/4" dia
total foil chord at balance portion is 5 3/8"
aspect ratio is between 8.3 and 11.9, average is 10.9
area of balanced portion (lower 15") 70.6 sq in
area of upper portion 110.6 sq in
total area of tab 181 sq inches = 1.26 sq ft
using 1/4 chord estimate for center of pressure: upper area arm .56
inches aft of the shaft axis, lower area arm .657 inches forward of
axis.
moment of unbalanced portion 61.9 cu in
moment of balanced portion 46.4 cu in
total moment (imbalance) of tab 15.5 cu inches = .009 cu ft
tab volume 3.78 cu ft
these dimensions work well for me. while i had been quite skeptical
(i cannot help myself) about the pardey’s claims of light air
performance with this system, i found it to be true. this past
summer, i was able to get the gear to steer her downwind under
spinnaker and main at about 1 1/2 to 2 knots with almost zero apparent
wind. i have not tried it under heavy weather conditions, but that is
typically not a problem. the helm is considerably heavier than
previously. my initial tab was overly large (and thus had a huge
balance area forward of the shaft) and poorly faired and i had almost
unbearable vibration under power. i took great pains to fair all
surfaces and eliminate sources of turbulence and that problem is
greatly alleviated. i do have the ability to drop a pin thru the tab
tiller into the rudder head to lock the tab while powering, especially
in reverse.
john churchill