Thanks a bunch Ryan.

It's 23' long (27' if you count the strudder) and 17' wide. In the meantime i went down to the river to see how it goes. First couple of runs were hopeless - till i realised there were tons of alga on the strut. Otherwise it's quite lively. BUT nowhere near the performance i want. As regards attitude in water, the sponsons seem to be doing their job and come right out of the water but the rear end i'm not too happy with. Basicaly all the prop is still submerged, very very small rooster tail. My options at the moment are 3:

1) take up the strut
2) take the motor forwards - (at the mo it's about 1/3 of the way from the back - yes i know that's wrong - but that was as much as my only flex-shaft would let me go - didn't feel like going to the model shop)
3) use a higher kv motor - i think 3000kv is way too small to begin with....but that's what i had lying around


and 4 - all of the above - i think should be the best option

As regards the flat bit on the sponsons - about 1/3 of the way, but this is countered by the bigger angle of attack - so yes more drag on the water, but I had other constraints....a little bit the wiser for it though...