Seems to me that the thin rear would generally have less drag than the fat rear.
Not necessarily at all.
Thin rears tend to be fitted to tall hulls and this in turn has a strong relationship to a large wetted surface area. Wetted surface area gives rise to a significant drag component as well.
In boat design you can never look at one specification and say with certainty wether it makes the boat faster or slower. The true answer is always to be found in the combinations of all specifications.
Wouter
