OK Meowy84, here goes.

Usually on a mono hull, with the prop under the boat, the prop has approx 1/4" clearance from the hull.
If the prop is behind the transom as shown in the pic, it is adjustable and you need to use the old try it and see method.
No matter if if the shaft is solid or flex, if it is under the boat it is submersed all the time unless the boat is airbourne.
A flex shaft gives you the advantage of you being able to bend it to change the angle of attack by the prop.
This helps to fine tune your boat.
If your boat bounces on the water at full speed, simply bend the shaft down, this will push the bow down.
If your boat won't come on the plane, you can bend the shaft up, this will push the bow up.
With a solid shaft the only way to adjust the props angle of attack is to remove and re-fit the shaft after some filling of the hull exit point. "Messy".
Also a flex shaft is supported along the full length of the shaft tube but a solid shaft can have a bearing either end of the shaft tube and is unsupported between these bearings, this allows the shaft itself to bow in the middle which causes vibration, vibration causes friction, friction reduces performance.
And that is all the advice I can give at this point.

Regards Brettos.