Pirate ahoy,
Hi Frank, out of my experience, buying a steerable Kort from a recognised manufacturer, (MMM for example) and using a suitable prop (Prop shop or Sitek) makes the world of difference, otherwise a plain tube is nothing more than a shroud for the prop. We gained at least 25% extra power using a Kort and then when using a correct prop a noticable increase in thrust, perhaps as much as 25% again. If you were to read any article on the web about Kort history and design you will certainly think carefully about fitting the best. All in all, it is a sad thing to build a beut of a vessel then for it to be let down by lack of power and maneuverability, all for the want of a few extra £££. Also consider if the original vessel was or had been converted to Kort. Although in my small opinion ( a confirmed tug aholic) what goes on inside and below the water line is my business as long as it does the job and I can smile.
Good luck
Pirate