Hi Manus ..Fenlander is bang on the button as usual...with his suggestion to try a smaller prop
I did a recent test with a Honwave T38 airfloor as a couple of guys I know asked if it would plane with a 6HP. I said it would..most said..no chance !!!
I borrowed the boat and put my standard Yamaha 6HP four stroke on it and within a couple of meters she was happily on the plan. (I give the thumps up in the video when it goes on the plane.. around 10 meters after starting.)
It gave similar performance as my wee F Rib 2.75 with the outboard..example planed easily and went approx 15mph. I weigh 200lb ..so much same as yourself. The boat was empty except transom wheels..big bow dodger ..front seat and 10 litres of fuel.
The owner then tried and he struggled to get it on the plane. He weighs 50lb more than me. It did get on the plane after a long run but we could both see it was struggling and it was in fact labouring the outboard. I put a smaller prop on and he tried again. This time he got much the same performance as myself except he could only reach around 12 mph top speed.
For info..the prop I used for myself with the 6HP is the bog standard 7.1/4 x 8” pitch. It was running at 5150 revs to give me around 15 mph tops. (Max recommended is 5500 for my OB) The smaller pitch that gave my friend better performance was a 7.1/2 x 7” pitch and it too reached good revs with the smaller prop.
You are using an 8” pitch on a 5HP ..so you should get better performance with a 7” pitch but not as good a top speed. But as others have said.. it will be marginal performance and as soon as you get a few fish on board..it possibly wont plane the way it did on the way to get the fish.
Im pretty certain you will get it on the plane with trial and error and perhaps the smaller prop..but it could well be a bit disappointing with its performance.. however that is entirely up to you if all you want is to get it to plane with nothing else on board..