what happens if the floats are too short?
mariner,
Your ST 51 with an 11x6 should be loads of power and the floats sound long enough.
You said in your post that you have
"the step located 3/4 in front of the CG "
The step should be be behind the CG, about 3/4" is fine. About 4" of the floats should be ahead of the prop.
If the floats are are sensible shape and are rigged with their top flat surface (and planing bottom) parallel to the aircraft's datum line, which is usually parallel to the tailplane (H stab) then it should plane well and take off no trouble. The only other common problem is that the floats throw up so much spray into the prop that the engine gets drowned and either stops or is way down on power.
It is important to fit spray strips to the lower inside edges of the floats from the front back to the step such that they will deflect the water out and down - away from the prop.
I have made them from three layers of 1/16 x 1/4 balsa stuck on at the edge, overlapping slightly. Each layer overlaps the previous one a little, i.e. it sticks out. When the glue dries you carve them into pointed triangles and then glass them. I apply them on both sides of each float back to the step and they deflect the water out and down, keeping the spray away from the prop and helping to lift the float.
Don't hold on full up elevator during the take off run!
Keeping the tails of the floats down in the water causes drag, so when it starts to plane let the aircraft ride level to pick up speed, just skimming on the centre of the floats just ahead of the step. Then after speed has built up rotate to take off.
Alasdair