I used to get about 5 to 6 mph on the Great Ouse but the speed limit was lower than that in places so id didn't really matter. I had a Norman 23 with a 9hp Tohatso two stroke. Just remember that if you need to get somewhere fast go by road but if you want to enjoy getting there use the boat
Our 23 with an old-style Honda 9.9 would do a bit over 4 mph as measured by a handheld GPS but I found that anything above about 3.5 mph mostly resulted in way more noise, vibration and fuel consumption for very little gain.
It's not about speed, it's about being nowhere in particular.
Your boat would probably go a bit faster on a river as straight cut edges and shallow water tend to pull the boat down and hold it back. I would have thought 5-6mph should be no problem.
I thought 5-6 should be possible, but it's the same as Ian described. I thought I might have a problem with the motor but apparently not. Oh well, anybody want a cheap pair of water skis? Jud.
Mine's a Honda 9.9 4 stroke and about 3 months ago I towed a broken down 48' narrowboat on a rope for about a mile with no problems, so the thrust is there but the boat won't get near 5mph. Must be the water I suppose. Jud.