Surfing's fine on the engine... these are truck engines after all, designed for low-end torque. If he's concerned about this, remind him that he tows a 3200lb boat behind his 4000lb truck with zero "give" (think about the way a prop "slips" in the water) using [virtually] the same motor as is in the boat. There is a way to calculate resistances for sure... I'd look up some equations for wind resistance, inertia, water resistance in relation to prop pitch and displacement and prove to your dad mathematically that it's not as hard on the engine as he thinks! The main issue with wakesurfing and boats is the transmission: just make sure it's fluid levels are good, have it looked at occasionally, and it should never be an issue.
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