Max rated engines are for max rated passegers capacity as stated on tech plate. That rib is rated for a 75 HP "max" engine to transport 9 passengers. If transporting 2-4 adults a 50/60 Hp will suit you well, you can even go for a prop maximization to deliver near top rpm if required for better overall rib performance.
For instance my rib is rated for a 40 HP to transport 7 passengers, currently using a 18 HP to transport neatly just 3 passengers throughout the whole throttle range.
Happy Boating
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