If you are going to keep the engine on the back of the rib then a 15 would be good. If you have to take it off, a lighter engine would be better.
The Tohatsu 9.8 is very powerful for its weight. I am not sure that an 8 would be enough for a 3.1 rib they are fine for 2 up in a sib.
Once you have an engine, get a service manual. The ones that cover a range of models are ok for basic servicing but I find lack the details if you really need to get serious.
2 strokes are probably best for a small rib my Avon supersport squats with a heavy 4 on the back (25Hp), but balances nicely with a 2.
Many of the new ribs coming out now have extended tubes further past the transom to support heavy 4 strokes.
He never did care for the river, did Montmorency.
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