Some time ago I decided to do some testing with different size outboards on my 10' Portabote.
I posted the results in the forum.
I was having problems with my old 4hp outboard and was looking at buying a new one.
Here is a copy of my post:
"Well guys,I finally found out the cause of my thumping transom.
Went out on the river today with Jamie (The forum member with an engineering background, Thanks Jamie)
We tried a couple of his outboards and I was very pleased with the result.
Let me first state that with my 4hp 2 stroke engine I am able to attain 20kmh (12mph) but it has an intermittent thump.
I weigh around 100kg and I also had a small 12 volt car battery on board but that's about it other than 5 litres of fuel.
First of all we tried Jamie's 5hp 4 stroke (2 years old) and the maximum speed I was able to attain was between 17 and 18 kph (10.2 mph and 10.8 mph) a little slower than my 4hp 2 stroke.
Next we tried Jamie's 7.5 hp 2 stroke and I'm sorry to say I couldn't get it to plane.
I couldn't get it back down onto the water to plane - It was flying! (Just joking about not planing).
It planed very well and I didn't have to move from the rear seat to the middle seat to do so.
I was able to go 26kph with the 7.5hp 2 stroke even though it's a 1980 something Johnson. That's 15.6mph with a standard transom.
(Thanks again Jamie.)"
I have settled for a 5.8 hp Parsun Outboard which, technically is a little over powered for my 10' Gen IV bote.
The maximum power rating according to my compliance plate is 5hp.
According to Portabote the maximum recommended safe hull speed is still 15 mph even for the new Alpha series.
It also appears that the maximum hull speed is, as indicated by Portabote, 15mph (24 kph or 13 knots).