There's a theoretical maximum based on WOT engine RPM/gearbox ratio/prop pitch, the boat can't exceed this theoretical speed, because there's also "slip" to factor in.
You could buy, beg, steal, borrow a handheld gps & do some practical on-water tests. Not a bad thing to own anyway & can be picked up cheap as chips.
Or you could just make up a number, no-one is going to test it.
![Whistling](https://www.rib.net/forum/images/smilies/whistling.gif)
What licence do you need? ICC or are you taking it inland on the canals?
PS, your boat is only 10'9" so doesn't exceed 15'