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[essex2visuvesi]

Im a little confused about this

Planning on building a hydro and looking at brushless motors and batteries

surely it would be better to have a higher KV motor and a lower cell count eg 4000Kv on a 2 cell lipo than a 2000Kv on a 4cell as the 2 cell pack will weigh less than a 4 cell of the same spec (eg 4900Mah 20C)

As an example this setup
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=17728
http://www.hobbyking.com/hobbyking/store/__7299__Rhino_4900mAh_4S1P_14_8v_20C_Lipoly_Pack.html
weighs in at 639g

compared to
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=17702
http://www.hobbyking.com/hobbyking/store/__7653__Rhino_4900mAh_2S1P_20C_Lipoly_Pack.html
which is 411g

The above is just an example comparing like for like and not what I'm planning on using

Granted the ESC will need to be beefier for the 2s but wouldn't the reduction in battery weight still be a bonus?
Or am I missing something obvious?

[Subculture]

Lower voltage means higher current for equivalent power. That puts more of a burden on the batteries. You could go for a smaller size capacity pack for the 4S setup, which will offset the weight penalty.

[essex2visuvesi]

so if I went for even lower KV motors and 6S then I could probably get away with even lower capacity cells?

he reason I ask is I have some 3000mah 6S lipos from my 500 heli :)

[Subculture]

Yep. The controllers tend to get more expensive for those higher voltages though.