It's very simple. Always go with the max HP rating of the rig. If you can afford it. You will NEVER be disappointed. You can get away with 75% of the max HP if you are on a tight budget but it's really a pain having to move people around in the boat just so you can get it up on plane. It's actually dangerous when I had to leave the steering wheel and take a few steps out towards the bow just to get my first 30 HP (brand new Merc 4 stroke) on my 14.9 up on plane. The new 40 HP pops it up on plane in 2 feet now. Nothing is worse than taking like 400 feet to get the rig up on plane and then some yahoo in another boat cuts in front of you and then you get to do it all over again

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Also, these small boats are really a pain as soon as you put a 3rd or 4th passenger on board. And the extra weight makes boating more dangerous when it 's underpowered. HP's usually average about $1000 for every 10 HP's you move up. But if you shop around you can easily cut that in half (I did ).
You won't read many "I'm unhappy with my rig cause it has just the right power" threads here. You mostly just see threads where some boat salesman in order to close a deal sells underpowered rigs to people that don't know any better and get caught up with having to bring home their shiny new rig.
You can tweak an underpowered rig but for the extra 2 mph on the top speed for all the time and effort it would have been smarter to just go with the max on these smaller rigs (been there, done that already).
Probably, on rigs 18 feet and up one can go with the 75% of the max. But a lot is dependant on the hull style and weight of the outboard (2 stroke vs 4 stroke).
Lunds have a certain hull design that makes them feel like you are in a larger rig. But the trade off is they need more HP's to get them onto plane than say an Alumacraft style.
When it comes to boating and safety if one has to cut corners than they should think twice about getting into boating in the first place . . . It ain't a cheap sport no matter how you slice it