Dave,
As a rule of thumb, if she looks right she IS right.
I can give you the formula, but why bother. As I have said in previous threads I ALWAYS overpower my models as the wind and waves aren't scale.
As to fast patrol boats MY preference is for 2 shafts, not for power, but if it is planing with over 1/2 the length out of the water, 2 props "digging" in the water makes the hull that little bit more stable.
If you are happy with the deccaperm stay with it, but these motors reach £25 plus on eBay which is just about enough to get 2 x Motors and shafts and fast props.
But you might have to upgrade speed controller.
Bob