ok many questions - but if that's what it takes....;-)

HEre are the answers:

COG - 240mm from the back, so it is a wee bit too far back i think no? i think it should be 40:60 rather than 60:40 no?
I'm using 1300mAh long max lipos 20C rating
29mm Graupner k series prop.
Turn fin is completely flat (super sharp but not curved at all - will take your advice there)

An alternative to change the whole thing could be to add another 2s....that would x2 the rpm and change the COG to something nearer what i want...

thing is i don't think the motor could handle the load.

The distance you mention for the motor is from the front right? not from the back as mine is.....

As regards speeds i think it was around 30km/h...yes i know that's slow - but as i said - prop was way too low in the water and was nowhere near reaching a respectable rpm....

Cheers,
Andrew