That is the idea.
This was the first flapped airfoil on a 15 meter class glider of that
design to shift the C of P forward at high cruise. I can go as far forward
as 28% chord in the normal operating range. It was the insistence of G.
Waibel that one could carry laminar flow over 90% chord at the bottom
surface over a sealed hinge. The rest was up to L. Boermans to figure out.
Variation of that airfoil design are in used on all kinds of modern
gliders now. The side benefit is, that the normal H stab down load is
reduced or even contributing to the over all lift, when cruising faster.
The DU89-134/14 in its zero line configuration (#2 flap setting in an
ASW27) looks more like an airfoil for a tail less glider.

When you fly with out water you fly with the C of G back ( weaker
conditions) when adding water
(stronger condition) the C of G moves forward, this matches the design
outcome, hence, I personally think ,that lead to the idea of no tail tank
required initially.

As to stick force i.e. trim load on the stick and how they exactly relate
with water I do not know. I fly with no water till now because I fly
mostly in weak condition. I have my C of G back in the 80% range and with
that I have normal trim loads, Trim all the way back no stick load in a
steady 40 deg. bank and I can cruise with out stick load with trim
forward.
Udo

Schleicher seem to have an interesting slant on this. I have a 27
without
a fin tank. Although ballast does move the cofg forward they say this is
no bad thing as you want to be nose heavy for running ridges and on good
days and tail heavy for weak thermals. Therefore, they argue that you
put
water in for good days moving the cofg slightly forward, and drop it
when
thermals are weak moving cofg further aft.

They see no need for pilot adjustment of balance through a fin tank.
Some
owners obviously disagree as they pay extra for the tank.

I just add / remove tail lead as I feel for the day / season / type of
flying (within limits of course)

Jim