Aerodynamics of carrying water

Since in theory the point of carrying ballast is to
improve overall speed....I am curious as to the actual
performance improvement. The shift of the polar with
added ballast is rather straightforward, but not the
reduction in climb rate. Assuming something like
a 25% time of flight in climbing mode, my back of the
envelope calculations for a fully loaded modern ship
in strong condtions would gain somewhere around 7-8%
overall task speed improvement? Anyone smarter then
I care to refine this number?

It isn`t. It`s more complicated because interthermal speeds doesn`t
vary with the square root of % extra weight. In condor is a nice tool
which shows this very clearly.
Reality shows that filling a glider meens flying "a bit" slower than
sqrt(% extra weight), increasing range and the change for a real good
thermal.
The extra gain depend almost fully on the diameter of the thermal, with
1 m/s and huge thermals you can fly a racing glider almost full. And
what about the gain running cloud streets? Also not linear.

For huge thermals and weak cloudstreets of 1 m/s actual gain for an
Diana would be (theoretically) 40%.

Assuming 3 m/s in typical European (small) thermals shows a gain of
only 8%.

Life`s complicated ;-)





l/d max occurs when induced and pressure drag are the same, not at 75%
induced. (interference drag is (per definition) negociated)

When 75% of the drag is induced you`r flying at Vy-min (min sink)

One other point: the higher stall-speed is worth mentioning because of
difficulties with landing, thermalling, manouvrebility etc.