50 PROFESSIONAL BOATBUILDERRIG DESIGN: Fractional Updatethe average chord is increased. In this
case, the aspect ratio is decreased since
aspect ratio, when properly calculated
by an aerodynamicist, is area divided by
the average chord—or, alternatively,
span squared divided by area. While
many associate increased aspect ratio
with increased e ciency, this is true
only for sailing to windward. As one
comes off the wind, the optimum
aspect ratio decreases, reaching a value
of only one, that of a square sail, dead
downwind.
So, aspect ratio needs to be con-
sidered in relation to the nature of
the planform. While the minimum
induced drag of a li ing surface is
achieved with an elliptical pressure
distribution, this does not demand
an elliptical planform. R.J. Mitchell’s
iconic WWII Spit re ghter, with its
semielliptical wing, is o en held up
as an example of optimal design.
However, its design was more com-
plex than just a choice of planform.
Beverley Shenstone, Mitchell’s aero-
dynamicist, was aware of the advan-
tage of an elliptical pressure distribu-
tion but had to employ many tweaks,
such as washout to avoid tip stall, as
the design progressed. As for Mitch-
ell himself, a practical designer, he
was quoted as saying, “I don’t give a
bugger whether it’s elliptical or not,
so long as it covers the guns.” He
wanted to keep the wing’s chord as
constant as practical close to the fuse-
lage so, for a given section thickness,
there would be space for the guns and
for stowing the landing gear. A sim-
ple way to achieve a nearly elliptical
pressure distribution is to have a
taper ratio (tip chord divided by root
chord) of 0.4, which can be seen on
lightplanes and modern square-
topped mainsails.
Returning to sails, P-32’s mainsail
of 269-sq- (25m^2 ) molded area has
an aerodynamic aspect ratio of 4.15,
not the 5.62 it would be if the sail were
truly triangular with an area of 203 sq
(19m^2 ). Note that the sailor’s “aspect
ratio” of hoist divided by the foot, 2.81
for the P-32, doesn’t properly assess
the e ect of varying a sail’s planform.lengths, and therefore area, the sail
was a “rule beater.” Wind tunnel tests
showed a drastic reduction in induced
drag even a er the sail’s area was
reduced along the leech to equal that of
a conventional mainsail. e planform
with the wider head was more e cient
area-for-area. Note that when a wider
headboard is used or roach is increased,fore-and-a mast curvature. He states
that the worst imaginable planform for
a mainsail is triangular, where the nar-
row top of the sail is in the wake of the
mast. One way of reducing this fault
was seen on Lionheart, which, while
constrained by the 12-Meter Rule, had
a bendy upper portion of the mast.
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