There is no direct correlation between horse power and speed. That is to say you can't put an exact number because of all the other variables. I have seen a car with about 75 hp do well over 150 mph and my farm tractor with 72 hp only has a top speed of 30 mph. There is an old saying "how fast something will go depends on how much money you want to spend".
8-15
50 mph
Depends on how it is geared.
70
A 3HP motor can reach speeds up to at least 30 mph depending on what you are using it for.
4-5 mph depending on wind, tides etc..
around 70-75 depends on gearing and if you run a can or pipe
About 10 to 12 on plane
Put a 12 hp motor on it.
You will notice on the throttle handle that there are three positions on the legend. "Really Slow", "Start" and "Slow".
A 3.5hp outboard will give you 8mph tops, which would be a normal walk to a slow jog.
I have a 14/36 flat boat with a 15 and it goes 26mph with only me and minimal gear.