Up here in MA there is a very large reservoir that we fish for a variety of species. This particular body of water requires 25 hp max., or 50% of the hull's rated hp on smaller rigs. I've been on that body of water with 25's and with boats varying between 16' and 18'. Depending on the number of people and tackle aboard, I can guarantee you that you'll push with at least 18 mph, regardless of the hull.
The 18' Lund gets us going at 20 - 21 mph (GPS) with 3 people aboard. That's about the best that you can expect. Just make sure you get the correct shaft length on the engine. The cavitation plate (fin just above the prop) MUST be no lower than the lowest point on your transom. That's 90% of the problem most rigs have with top end speed. Prop pitch affects are quite a bit less, but by matching the correct prop to the hull, you can expect to increase your top end by a couple of mph.
EDIT: The above is based on Tiller boats btw. Heavy console boats with wind shields will deliver less top end speed - obviously.
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