I have my very own 44ft MLB engineer EN3/MKCS William Collette Ret.who has very kindly agreed to help you with your engineering questions.
That's about the best speed you're going to get! She's a bathtub with power-screws rather than speed screws. She'll get you there safe and sound...but not quickly. It's not an engine & prop problem, it's not a problem...it's a design.