  We have searched for corotations (CR) in three Southern grand design spiral
galaxies: NGC 1365, NGC 1566 and NGC 2997. We have also introduced a method of
quantifying errors in the phase diagram used to detect CR. We established the
m=2 pattern CR at 12.1 kpc, 9.4 kpc and 7 kpc, for NGC 1365, NGC 1566 and NGC
2997, respectively. By using published rotation curves, we could determine
spiral pattern angular speeds of 25.0 km/sec/kpc, 12.2 km/sec/kpc and 17.6
km/sec/kpc, respectively. A three armed component has been detected in NGC
2997, with the CR placed at 8.7 kpc with a pattern angular speed
Omega_{CR_3}=12.7 km/sec/kpc. An m=1 component was detected in NGC 1566. We
warily locate the CR at 7.1 kpc, with a pattern angular speed Omega_{CR}
approx. 16.6 km/sec/kpc. This pattern does not present ILR. Ages have been
determined by studying the radial density profile of the m=2 Fourier components
in g (newly formed stars) and i (perturbing SDW supported by the disk of old
stars), aided by the global aspect of the real spiral pattern in comparison
with numerical simulations. The pattern is approx. 1200 Myr old in NGC 1365,
approx. 800 Myr old in NGC 1566 and younger than 80 Myr in NGC 2997.
