Hi Victor,
If the new motor revs significantly higher than the stock motor, you'll suffer more propwalk and have to steer to keep tracking straight, I've explained what the consequence of that will be.
The FT 007 has a driveline, that runs clockwise, seen from the rear, which is opposite to what's custom in the boating world, so chances are that your heli motor and 29 mm hydro prop will rotate counterclockwise (again seen from the rear).
The propwalk will make your boat veer off to the right, counteracting it means offsetting the drive train (motor and propshaft) to the right making the boat unsuitable to run the stock motor and prop again.
The 10 - 13 % is the length of the hull, from the tip to the transom (not including any overhang at the transom if there is any).
The smaller the boat the shorter this distance can be without compromising the running attitude (too much).
If you power the hull beyond the stock setup, you can't cheat too much without running in to trouble...
Behind the transom of the boat a 'dent' forms in the water, as the boat picks up speed; at a certain speed this dent will sit in the spot where the prop is, this causes the prop to lose grip, the revs go up and the speed goes down.
The dent disappears and the prop has grip again and the game starts all over.
On an existing setup you can overcome this by using a larger diameter prop(with a lower pitch if available), but the motor (and ESC, battery and wiring) has to be able to cope with the higher ampdraw without overheating.
The angle of a surface piercing drive has to be as shallow as possible; when the boat sits even keel on a flat surface, the rear bearing has to sit 1-2 mm (more if the hull is larger) above this surface.
Too steep and the prop will be too submerged, drawing way too much amps and pushing the bow down, too high and the prop will loose grip as soon as the boat gets on the plane, as the prop gets lifted out of the water.
A picture to show the angle:
Getting the angle right, along with the distance between the prop and the transom, often requires a longer shaft, as simply moving the motor towards the rear compromises the angle.
Regards, Jan.